LPS Transformer Voltage Question

[Bilbo]

Can anyone tell me why a 50VA transformer with a specified secondary output of 12V measures at 15.2V under a load of 500mA and why that would not be enough to drive a 12VDC output after rectification and regulation?

If the rectifier diodes cause a drop of around 1V would that not be enough for the regulator to do it's job and avoid excess heat dissipation?

I have measured 15V transformers and they output just over 18V and 18V transformers output just over 21V. 

Am I missing something?

[Addicted to music]

12VAC 

 

12 X 1.4 = 16.8V  peak

 

Voltage drop across the diode is 0.7vdc....  so....

 

16.8 - 0.7 = 16.1vdc

 

so this would not surprise me that you measured 15.2VDC under load @ 500mA.

Edited March 30, 2019 by Addicted to music

[Bilbo]

2 hours ago, Addicted to music said:

12VAC 

 

12 X 1.4 = 16.8V peak.  

 

Diode voltage drop is 0.7V so full rectification will be 1.4VdC 

 

16.8 - 1.4 = 15.8VDC....  

 

so this would not surprise me that you measured 15.2VDC under load @ 500mA.

So could I use a 12V transformer instead of a 15V to provide a 12 VDC output after rectification and regulation @ 500mA?

[mwhouston]

Yes should be fine. You may still have to drop the final voltage. 

[Addicted to music]

6 hours ago, Bilbo said:

So could I use a 12V transformer instead of a 15V to provide a 12 VDC output after rectification and regulation @ 500mA?

I’ve edited the the maths to corrected to a single diode. 

To answer this question; No.

you need to reverse the maths.

 

12vdc required.

 

12 + 0.7 = 12.7

 

12.7 /1.41 = 9v.

 

so you need a secondary winding that gives you 9vac

[Bilbo]

Now I’m really confused!

The LPS module I’m using has a full wave bridge rectifier and MJE 15034 TL072 regulator and I thought to get 12VDC out I would need at least 14VAC input to account for losses.  

[Addicted to music]

With all voltage regulators, you always require a higher voltage in,  otherwise you damage  the regulator. 

 

Ive provides the simple requirements as per requested,  but when you feed it to a regulator you must always have a higher voltage than what the regulator outputs.  Some designers will even use a diode to feed the output from the input as protection.  

So yes, a higher voltage from the LPS to the regulator is required 

[robc]

19 hours ago, Bilbo said:

Can anyone tell me why a 50VA transformer with a specified secondary output of 12V measures at 15.2V under a load of 500mA and why that would not be enough to drive a 12VDC output after rectification and regulation? 

If the rectifier diodes cause a drop of around 1V would that not be enough for the regulator to do it's job and avoid excess heat dissipation?

I have measured 15V transformers and they output just over 18V and 18V transformers output just over 21V. 

Am I missing something?

 

The LPS module I’m using has a full wave bridge rectifier and MJE 15034 TL072 regulator and I thought to get 12VDC out I would need at least 14VAC input to account for losses.  

Hi Bilbo and all,

 

Yes, you will need to account for losses in the rectifier diodes and the regulator circuit but you will also have to make sure that the regulator gets enough voltage to do it's job.

 

As a *very* general ballpark guideline, for a regulated Linear Power Supply (LPS) the secondary voltage of the transformer should be about the same as the DC voltage wanted at the regulator output. For lower voltage supplies the transformer often needs to be a little higher, as diode drops and active device resistance is a higher percentage of the voltage, and for higher voltage supplies the transformer needs to be a little smaller, mainly to reduce heat dissipation in the regulator.

 

We don't really have all the info needed to make definitive decision but for illustration purposes, and because I probably need the practice ...

 

A plain transformer, with no additional circuitry - just the metal core and windings, has secondary voltages specified in AC volts - usually at full load current (50/12=4.17Amps). With no load connected (zero current) the secondary voltage is always higher due to, at least, the resistance in the transformer's secondary winding. So, with an AC voltmeter connected directly across the secondary winding on your 12VAC transformer, with an input of 240VAC, you may see something like 12.5VAC to maybe every something as high as 14VAC.

Then there is the variation in mains voltage to consider, about 10% making the Australian mains as low 215VAC to high as 265VAC with a corresponding 10% variation in the "nominal" 12VAC secondary output voltage.

 

So, in practice the AC voltage at the output of a *bare* 12AC "nominal" transformer could be anything from 10.8VAC to maybe 15.4VAC.

 

The next thing to consider is that an AC Volts value doesn't indicate the maximum, peak, value reached by the sinusoidal AC voltage, it's only 0.707 (1/1.414) of the peak value, but this still gives it the same power producing capacity as a DC voltage of the same value. Neglecting diode drop the rectified peak voltage will, as other have pointed out, be 1.414 x VAC. This is important at smoothing capacitors attached to the output of the diode rectifier will store voltage at this peak level meaning that the regulator circuit will "see", with no load current, an input at this peak voltage.

 

With a bridge rectifier there are two diodes in series with the load so that's a 1.4Volt drop (2 x 0.7V).

 

This gives us and DC input voltage range to the MJE 15034 TL072 regulator of:

 

  [ (10.8AC to 15.4AC) x 1.414 ] - 1.4VDC  =  13.87VDC  to  20.38VDC

 

which depends upon:
 * the voltage at the power point
 * the assumption that the 12VAC rating is a full load current
 * the percentage regulation of the transformer
   - [(no load voltage - full load voltage) / full load voltage ] * 100
 * the smoothing capacitance being large enough to maintain near the peak voltage

 

Next to consider is the drop across the MJE 15034 TL072 regulator. We don't know much about this but we do know that we are asking it to produce a regulated output of 12VDC and that it's has to drop from about 1.87VDC to 8.38VDC.

 

In the 1.87VDC case this is not much voltage to work with to maintain 12VDC and in the 8.38VDC case this means the regulator would have to dissipate 8.38Volts x 4.17Amps ~= 35Watts of heat. That's a lot of heat to waste for a 50VA (Watt) load and probably a lot for a MJE 15034 transistor to channel to a large heatsink but these are worst case scenarios.

 

Yes, with enough smoothing capacitance, better than worst case transformer regulation and a big enough heatsink I'm guessing a MJE 15034 based regulator to do that job, but I haven't yet look at the heat channelling capacity of a MJE 15034. However, there could be more than one.

 

As above, some more information would be useful

 

Bilbo, what brings you to ask these questions and what problem are you trying to solve?

It sounds to me like you're replacing a transformer and you can't determine the specs of the one being replaced? I'm guessing you found one with a large enough VA rating but it's not working or your worried about trying it? Do you have a circuit diagram, or specs, for the MJE 15034 / TL072 regulator and what sort of load are you trying to drive? Does the regulator have a big heatsink? Are the any big capacitors anywhere?

 

Cheers!

Rob

[Bilbo]

1 hour ago, robc said:

Hi Bilbo and all,

 

Yes, you will need to account for losses in the rectifier diodes and the regulator circuit but you will also have to make sure that the regulator gets enough voltage to do it's job.

 

As a *very* general ballpark guideline, for a regulated Linear Power Supply (LPS) the secondary voltage of the transformer should be about the same as the DC voltage wanted at the regulator output. For lower voltage supplies the transformer often needs to be a little higher, as diode drops and active device resistance is a higher percentage of the voltage, and for higher voltage supplies the transformer needs to be a little smaller, mainly to reduce heat dissipation in the regulator.

 

We don't really have all the info needed to make definitive decision but for illustration purposes, and because I probably need the practice ...

 

A plain transformer, with no additional circuitry - just the metal core and windings, has secondary voltages specified in AC volts - usually at full load current (50/12=4.17Amps). With no load connected (zero current) the secondary voltage is always higher due to, at least, the resistance in the transformer's secondary winding. So, with an AC voltmeter connected directly across the secondary winding on your 12VAC transformer, with an input of 240VAC, you may see something like 12.5VAC to maybe every something as high as 14VAC.

Then there is the variation in mains voltage to consider, about 10% making the Australian mains as low 215VAC to high as 265VAC with a corresponding 10% variation in the "nominal" 12VAC secondary output voltage.

 

So, in practice the AC voltage at the output of a *bare* 12AC "nominal" transformer could be anything from 10.8VAC to maybe 15.4VAC.

 

The next thing to consider is that an AC Volts value doesn't indicate the maximum, peak, value reached by the sinusoidal AC voltage, it's only 0.707 (1/1.414) of the peak value, but this still gives it the same power producing capacity as a DC voltage of the same value. Neglecting diode drop the rectified peak voltage will, as other have pointed out, be 1.414 x VAC. This is important at smoothing capacitors attached to the output of the diode rectifier will store voltage at this peak level meaning that the regulator circuit will "see", with no load current, an input at this peak voltage.

 

With a bridge rectifier there are two diodes in series with the load so that's a 1.4Volt drop (2 x 0.7V).

 

This gives us and DC input voltage range to the MJE 15034 TL072 regulator of:

 

  [ (10.8AC to 15.4AC) x 1.414 ] - 1.4VDC  =  13.87VDC  to  20.38VDC

 

which depends upon:
 * the voltage at the power point
 * the assumption that the 12VAC rating is a full load current
 * the percentage regulation of the transformer
   - [(no load voltage - full load voltage) / full load voltage ] * 100
 * the smoothing capacitance being large enough to maintain near the peak voltage

 

Next to consider is the drop across the MJE 15034 TL072 regulator. We don't know much about this but we do know that we are asking it to produce a regulated output of 12VDC and that it's has to drop from about 1.87VDC to 8.38VDC.

 

In the 1.87VDC case this is not much voltage to work with to maintain 12VDC and in the 8.38VDC case this means the regulator would have to dissipate 8.38Volts x 4.17Amps ~= 35Watts of heat. That's a lot of heat to waste for a 50VA (Watt) load and probably a lot for a MJE 15034 transistor to channel to a large heatsink but these are worst case scenarios.

 

Yes, with enough smoothing capacitance, better than worst case transformer regulation and a big enough heatsink I'm guessing a MJE 15034 based regulator to do that job, but I haven't yet look at the heat channelling capacity of a MJE 15034. However, there could be more than one.

 

As above, some more information would be useful

 

Bilbo, what brings you to ask these questions and what problem are you trying to solve?

It sounds to me like you're replacing a transformer and you can't determine the specs of the one being replaced? I'm guessing you found one with a large enough VA rating but it's not working or your worried about trying it? Do you have a circuit diagram, or specs, for the MJE 15034 / TL072 regulator and what sort of load are you trying to drive? Does the regulator have a big heatsink? Are the any big capacitors anywhere?

 

Cheers!

Rob

Thanks Rob - that is certainly a comprehensive answer to my question and clarified things for me a great deal.

 

The LPS in question is based on the Studer 900 microphone preamplifier and I built it from a kit bought very cheaply on eBay.  I'm using a 50VA Nuvotem toroidal transformer with 15VAC output.  I am using it to power my SOtM SMS-200 Ultra (12V version) and it is the best sounding power supply I have yet used on the SOtM.   The thing is the kit parts are not exactly the same as the original and due to a current limiting resistor in place it struggles to provide a stable 12VDC above about 600mA.  The SOtM draws about 1.25A during boot up and then draws about 550mA whilst operating so there is not much head room available.

 

On another DIY Audio site a chap has modified this ebay kit by altering a couple of components to provide more than 3A which would be ideal for the SOtM but this may require additional heat sinking for the regulator which would mess up my build layout quite a bit.  I was therefore exploring whether a 12VAC output transformer would address the additional heat dissipation without having to mess about with the build layout and trying to fit a bigger heat sink on the regulator.

 

Here is my build.

 

Here is the modified circuit I am using as a guide.

 

The first modification I will be doing is to change the current limiting resistor (R107) on the MJE15034 emitter from 2.2 Ohm as supplied to 0.33 Ohm.  I suspect this may overheat the regulator and hence why I thought changing the transformer to a 12VAC one may mitigate the power dissipation without messing about with heat sinks and layout.

[robc]

Hi Bilbo and All,

 

Thanks for the info that helps a lot.

The Transformer and secondary voltages
I see from your build photo, nice BTW, that your using an encapsulated transformer with a single primary winding (brown, blue) and dual secondaries (black,red) & (yellow,orange). The only 50VA transformer on the Nuvotem website I could find that matched had part number 0050P1-2-015K.

 

This part's datasheet from the Nuvotem website is attached.

 

  Nuvotem Toroidal Transformer Data Sheet
  50VA Encapsulated Style, with Leads, 230V Primary, Dual Secondaries

 

The electrical specs and model number of the 2x15V are below and I've also included the 12V specs:

 

    Nuvotem       Full Load   Rated Current  No Load   DC Resistance
  Part Number     Vsec (A)   per Sec (A)     Vsec (V)   (Ohms) @ 25'C
  0050P1-2-012K    2 x 12        2.083            2 x 13.71      2 x 0.3977
  0050P1-2-015K    2 x 15        1.667            2 x 17.22      2 x 0.6411

 

Checking the Power(VA) at full load, it should be about 50VA:

 

  P12 = 2 x 12 x 2.083 = 49.99VA
  P15 = 2 x 15 x 1.667 = 50.01VA

 

Checking the voltage drop across the secondary winding (I squared R losses) at stated full load current using the stated DC resistance of the secondary winding.

 

  Vsec.drop12 = I12 x I12 x DCres12 = 2.083 x 2.083 x 0.3977 = 1.7256V
  Vsec.drop15 = I15 x I15 x DCres15 = 1.667 x 1.667 x 0.6411 = 1.7815V

 

They should roughly equal the difference between no.load and full.load secondary voltages:

 

  Vsec.Diff12 = 13.71 - 12 = 1.71V
  Vsec.Diff15 = 17.22 - 15 = 2.22V

 

Interesting, the 15V ones aren't a close as the 12V ones, but they are reasonably close. The voltage measurements carry more weight here!

 

Calculating the percentage regulation, for interest sake, using:

 

  R%   = ( (Vsec no.load - Vsec full.load) / Vsec full.load ) x 100

 

gives:

 

  R%12 = ( (13.71 - 12) / 12 ) x 100 = 14.25%
  R%15 = ( (17.22 - 15) / 15 ) x 100 = 14.80%

 

OK, that all seems reasonable. Note that this is a "per secondary winding" regulation.

 

Also on the datasheet:

 

  Primary Winding
  Input Voltage Range : 207V–253V (230V +/-10%) @ 50/60Hz
  DC Resistance @ 25'C = Approx 49 Ohms

 

Note that these output voltages are at "nominal" primary voltage of 230VAC. Where as our power in Australia is at a nominal 240VAC +/- 10% about (215 to 265)VAC. Running the transformer on 265VAC should be OK. It's just going to dissipate a little more heat. However, it does mean we need to allow for higher secondary AC voltages in our calculations:

 

  Vsec   min = 215/230 x Vno.loadAC
  Vsec   max = 265/230 x Vno.loadAC

 

  Vsec 12min = 215/230 x 13.71 = 12.81VAC
  Vsec 12max = 265/230 x 13.71 = 15.80VAC

  Vsec 15min = 215/230 x 17.22 = 16.10VAC
  Vsec 15max = 265/230 x 17.22 = 19.84VAC

 

From our original "*very* general ballpark guideline":

 

  "for a regulated Linear Power Supply (LPS) the secondary voltage of
   the transformer should be about the same as the DC voltage wanted at
   the regulator output"

 

OK, so with those 15V transformers, if your house is very close to the street transformer and it's 1am in the morning on a mild night when nobody is running heating or air conditioning and the off peak hot water heaters haven't kicked in yet you might get 265VAC on the primary of your 230VAC "nominal" transformer giving you a value of 19.84VAC on each secondary winding with no load current.

 

DC Voltage Input to the Regulator - after the Bridge Rectifier
From your build photo I see your running the 15VAC secondaries in parallel, connected to the two pin rectifier/regulator board. The percentage regulation calculated earlier was "per secondary winding", with them in parallel, this should be halved to give (R%12=7.13%, R%15=7.4%). I can also see the four diodes that form the bridge rectifier you mentioned.

 

Allowing for 1.4V drop across two bridge rectifier diodes (2 x 0.7V) and converting the RMS VAC input voltage to a DC peak by multiplying by the square root of two, 1.414, will give us the DC voltage level stored by the reservoir capacitor (also called smoothing capacitor), C102 in the circuit posted, and most probably positioned directly across the output of the bridge rectifier:

 

  VDC.in.min   = Vsec.min x 1.414 - 1.4V
  VDC.in.max   = Vsec.max x 1.414 - 1.4V

 

  VDC.in.min12 =   12.81  x 1.414 - 1.4V = 16.71VDC
  VDC.in.max12 =   15.80  x 1.414 - 1.4V = 20.94VDC

 

  VDC.in.min15 =   16.10  x 1.414 - 1.4V = 21.37VDC
  VDC.in.max15 =   19.84  x 1.414 - 1.4V = 26.65VDC

 

It's looking like the "nominal" 12VAC transformer (a figure based on 230VAC primary input voltage) should give enough "spare" voltage (16.71 - 12V = 4.71VDC) for the regulator to work at the minimum expected AC input of 215VAC input. And at 265VAC input to the 12V nominal transformer the MJE 15034 "pass" device will have to dissipate enough heat, at the load current, to drop the voltage from about 21V down to 12V. That's 9V at 500mA giving:

 

  P(VDC.in.max12)500ma = Iload x Vloss = 0.5A x 9V = 4.5W

 

which might be OK, but you'll have to try it to find out for sure. And if the load were to be continually in start up mode, drawing 1.25A:

 

  P(VDC.in.max12)1.25A = Iload x Vloss = 1.25A x 9V = 11.25W

 

which those heatsinks and chassis may well be able to get rid of for a short time but then what would happen if it sat looping in start up mode, making 11.25W continuously?

 

For the nominal 15VAC transformer you'll have plenty of voltage spare to make the circuit work. We need to find out if heat dissipation will be a problem.

 

  P(VDC.in.min15)500mA = 0.5A x (21.37 -12)V = 4.7W
  P(VDC.in.max15)500mA = 0.5A x (26.65 -12)V = 7.3W

 

and if the load were to be continually in start up mode, drawing 1.25A:

 

  P(VDC.in.min15)1.25A = 1.25A x (21.37 -12)V = 11.7W
  P(VDC.in.max15)1.25A = 1.25A x (26.65 -12)V = 18.31W

 

Perhaps the % regulation at our 500mA load will give is a little less voltage so as to help reduce the heat we need to dissipate? As the load will only be drawing about 500ma at 12VDC and your transformer will drop only 2.22V/2 when supplying 1.67A you'll only get, proportionally, about:

 

   (500mA/1.667A) x (2.2V/2) = 0.33VAC

 

drop in transformer secondary AC voltage due to the % regulation. We divide by 2 as we have two secondary windings in parallel and the above load/no.load specs were for a single winding. Converting to DC we get (0.33VAC x 1.414) = 0.46VDC, which only drops the power by a maximum of (0.5A x 0.46VDC) = 0.23Wmax, which doesn't help much but it's something.

 

So, with the existing 15V "nominal" tranny and a mains supply at 265VAC you'll need the MJE 15034 "pass" device to dissipate 7.1W. And with two of them, the chassis will be giving off about 14W of heat. Those existing heat sinks might not get rid of 7.1W each, unless a strong breeze if blowing through the chassis. Give it a go and see what happens.

 

If heat proves to be a problem then rather than buying 12VAC "nominal" transformers you could always remove the supplied heatsink and bolt the MJE 15034 to that nice aluminium chassis. The bottom has the largest surface area and some tallish feet would ensure enough airflow to get rid of the heat. I can't see clearly, but alternatively, it looks like you might be able to swing the boards around 180' and mount the MJE 15034 on the right hand side of the case, next to where the multimeter is sitting. You could spin the transformers around and adjust the wiring with minimal fuss. Of course you'll need to insulate the MJE 15034 from the chassis as it's tab is live, connected to terminal 2, it's collector.

 

BTW, I'd also run a second mains earth wire to the large plate forming the bottom of the chassis using a shakeproof washer and two nuts. The bottom plate is where all transformers and boards are mounted and that end plate may not always be in excellent electrical contact with all the other, possibly live, aluminium panels which make up the chassis. In finishing off the project bolt the white mains connector to the bottom of the chassis too.

 

Please let us know whether the heat is a problem and is so, how you get around it.

Cheers!

Rob

 

Nuvotem-Torroidal_Tranny-Encapsulated-Style-230V-Primary-0050VA.pdf

Edited April 1, 2019 by robc
Stuffed up the fomatting

[robc]

Opps, formatting fixed.

Edited April 1, 2019 by robc
Formating fixed

[Bilbo]

@robc

Thank you Rob for such a detailed analysis of the situation.  I am blown away by your knowledge!

 

I will take your advice regarding the earthing and pay particular attention to heat sinking.

When I did the build I did allocate some space to mount the MJE15034 transistor onto the base plate of the chassis if required as I was doubtful that the heat sink supplied would be up to the task of dissipating the additional heat if the current demand exceeded 1A.  From the data you have so kindly provided it looks like this will be required. I will purchase another pair of transistors to give me the leg length to mount them on the chassis.  I am aiming to achieve 12VDC and maximum current of 3A to give enough headroom for the SOtM SMS-200.

 

The transformers I used are as you noted and I have encased them further in some galvanized iron to minimize stray magnetic fields as much as possible.  They are actually down pipe straps from Bunnings flattened out then rolled into shape and secured with gaffer tape.  (I have also tidied up the fuse connections with heat shrink so there are no exposed wires).

 

 

I have read your post several times and my understanding (which is a bit limited) is that the 15V transformers are suitable as long as the heat sinking is sufficient.  Moving to a 12V transformer may reduce heat dissipation but present other problems with variation in mains supply voltage.  If I address the heat sink issue then they should be suitable for the task up to 3A.   Have I understood this correctly?  Thanks again for your advice.  

 

Edited April 1, 2019 by Bilbo

[robc]

Hi Bilbo and All,

 

10 hours ago, Bilbo said:

When I did the build I did allocate some space to mount the MJE15034 transistor onto the base plate of the chassis if required as I was doubtful that the heat sink supplied would be up to the task of dissipating the additional heat if the current demand exceeded 1A.

Great, you were already way ahead!

10 hours ago, Bilbo said:

I am aiming to achieve 12VDC and maximum current of 3A to give enough headroom for the SOtM SMS-200.

<snip>

I have read your post several times and my understanding (which is a bit limited) is that the 15V transformers are suitable as long as the heat sinking is sufficient.  Moving to a 12V transformer may reduce heat dissipation but present other problems with variation in mains supply voltage.  If I address the heat sink issue then they should be suitable for the task up to 3A.

Have I understood this correctly?

Sorry, it was a bit of a rambling post. This time I'll get your question out of the way up front.

Yes, for the existing 15VAC transformer the problem is heat dissipation, solve that and it will be fine. Using the 15VAC transformers will give the regulator more voltage to work with - especially for the worst case where the mains drops to 215VAC and your sucking 3A.

 

Moving to the 12AC transformer specified earlier would likely be fine but would limit the voltage the regulator had to work with. Using larger smoothing capacitors after the rectifier would help keep the voltage fed to the regulator high enough but at 3A draw the regulator probably wouldn't have a lot of volts spare. It looks like a good regulator so it could probably cope OK, especially as your only sucking 0.5A while listening to music.

 

Whether or not it would sound better with the 15V rather then the 12V is subjective. Given you already have the 15V trannies and the heat problem looks solvable you may as well persevere with the 15V trannies.

 

Let's check how much extra DC voltage the regulator should have with the 15V trannie in the worst case scenario, sucking 3A from your existing 15V transformer with the primary connected to a rather low 215VAC supply.

 

Using both windings in parallel, each winding will need to supply 1.5A. From the specs, which are for a single winding, the voltage will sag by 2.22VAC (17.22 -15.00) when supplying the full load current of 1.667A. Proportionally, for a 1.5A draw we can expect the sag to be only 1.5/1.667 times as bad:

 

   2.22  x  (1.5/1.667) =  2.00VAC  (secondary voltage sag at 3A with windings in parallel)

 

Earlier we found the secondary output voltage at 215VAC input and with no load was:

 

  Vsec 15min = 215/230 x 17.22 = 16.10VAC  (secondary voltage at 215AC input with no load)

 

Subtracting the voltage "sag" from the no load 215VAC primary voltage gives:

 

  16.1VAC - 2.00VAC = 14.1VAC  (secondary voltage at 3A load with both windings in parallel, 215VAC supply)

 

Which is to be expected as were running it at almost the full load and from a (worst case) 215VAC supply rather than from the specified 230VAC supply.

 

Subtracting two 0.7V diode drops because of the bridge rectifier and converting to peak DC volts to find the peak DC voltage on the regulator input capacitor we get about:

 

(14.1 - 1.4) x 1.414 = 18.0VDC

 

giving the regulator 6VDC (18-12) to work with. That's an extra 50% of the output voltage, heaps!

 

Now for the down side, the heat you need to get rid of. The power dissipation in the pass device (MJE15034) will be:

 

  P = I x E = 3A x 6V = 18W

 

That's quite a lot of heat, and this is for the worst case of input being the minimum expected value of 215VAC rather than the maximum expected value of 265VAC. And you have two of these in chassis. If your going to repurpose this regulator in the future to output 3A continuously then you'll have to revisit heat dissipation. But, for now you indicated 3A was just headroom and the load was only 0.5A continuous, and 1.5A at startup, so you should be OK. But here is only one way to find out for sure ... please let us know how you go?

 

10 hours ago, Bilbo said:

The transformers I used are as you noted and I have encased them further in some galvanized iron to minimize stray magnetic fields as much as possible.  They are actually down pipe straps from Bunnings flattened out then rolled into shape and secured with gaffer tape.  (I have also tidied up the fuse connections with heat shrink so there are no exposed wires).

Great, thanks for confirming were working with the right transformer specs. Toroids have pretty low flux leakage and your supply is in a separate box to the audio electronics so you shouldn't have a problem but I like your inventiveness with the pipe straps and it can't hurt. Any reason, other than a considerable price hike, you went with toroids rather than R-core?

 

10 hours ago, Bilbo said:

I will take your advice regarding the earthing

Good, you do want to be around to enjoy it!

 

BTW, out of interest I bought two of the regulator boards, very cheap. I'm curious about comparing them to the Jung/Didden superreg.

 

Please let us know how you go with the heat?

 

Cheers!

Rob

 

 

[Bilbo]

@robc

Thanks again for your advice - I appreciate it very much.

 

I chose the 50VA 15V Toroidal (rather than r-core) based on price, availability as well headroom for the regulator after the voltage drop from the diodes.  Had I known I could get away with 12V secondaries I now think that may have been a better choice and allowed me to avoid changing the heat sink provided.  I may yet get a pair of R-Cores to replace the toroidals once the modifications are finished and in that case I would go for the 12V output.

 

So now that I have the 15V transformers and have to deal with the additional heat dissipation to accommodate up to 3A, what is a reasonable temperature to judge whether the heat sink employed will do?  When I measure the temperature of the transistor (with the case lid on) supplying 500mA it is 80C with ambient temperature of 25C.  The data sheet quotes 150C as the maximum operating temperature but I'm not trying to boil water here.  So at what temperature would you consider the heat sink to be insufficient?  Is too hot when you can't keep your palm on the chassis above the transistor/heat sink location?

[robc]

Hi Bilbo,

Thanks for your reply.

2 hours ago, Bilbo said:

@robc

Thanks again for your advice - I appreciate it very much.

 

I chose the 50VA 15V Toroidal (rather than r-core) based on price, availability as well headroom for the regulator after the voltage drop from the diodes.  Had I known I could get away with 12V secondaries I now think that may have been a better choice and allowed me to avoid changing the heat sink provided.  I may yet get a pair of R-Cores to replace the toroidals once the modifications are finished and in that case I would go for the 12V output.

 

You're very welcome, I've got a *lot* from these stereo.net.au forums so it's nice to be able to put a little back.

 

Yeah, I thought it would be price, R-cores seem quite expensive, even on eBay from China where they don't even list the percentage regulation or winding resistance, so you really don't know what your getting. I haven't had a good look around yet so any suggestions for suppliers would be much appreciated?

 

Assuming you haven't got one, a variac can be very handy for testing these sort of things, 215VAC mains to 265VAC mains variations, and generally the ability to dial up any AC voltage you like before outlaying good money on transformers. Variac do turn up cheap second hand from time to time but I guess it depends on how much your into electronics as a hobby, rather than just a means to an end - i.e. just used to improve your audio experience.

 

Out of interest and perhaps before you outlay more money on R-Cores, have you tried pulling the car battery out of a car and powering your SOtM SMS-200 Ultra directly from that rather than the output of the 12V regulator. Fully charged a car battery should be 12.7V or less. It might be within the input voltage range specified for your SOtM SMS-200 Ultra. A car battery can push hundreds of amps, has a very low internal resistance - so you'll get little to no sag of the input voltage as the load increases - and will be a very, very quiet power supply, no junk noise from the mains, no transformer hum to worry about and for testing purposes, it's free . If you can't tell the difference between the car battery and your 15VAC toroid powered MJE15034/TL072 regulator then it's not likely swapping from toroids to R-Cores will be an improvement. Make sure you put a fuse, perhaps 5A to10A, in one of the wires you hook up to the battery and then put some electrical tape of other insulation around that terminal. If you accidentaly put a short across the battery without a fuse in place your wires will melt, or explode, pretty much instantly and molten copper is very hot.

2 hours ago, Bilbo said:

So now that I have the 15V transformers and have to deal with the additional heat dissipation to accommodate up to 3A, what is a reasonable temperature to judge whether the heat sink employed will do?  When I measure the temperature of the transistor (with the case lid on) supplying 500mA it is 80C with ambient temperature of 25C.  The data sheet quotes 150C as the maximum operating temperature but I'm not trying to boil water here.  So at what temperature would you consider the heat sink to be insufficient?  Is too hot when you can't keep your palm on the chassis above the transistor/heat sink location?

Yes, there's a pretty good rule of thumb. You should be able to hold your hand on a heatsink for 5 to 10 seconds without your hand getting too uncomfortable. I think you'll find that 80C is too uncomfortable. If you can place the MJE15034 in good thermal contact with that big large baseplate and give the chassis some nice feet to let the air flow up and around it, you will get rid of the heat much more efficiently without cooking the transistor- and the other electronics.

 

Do you really need to a heatsink for 3A when you load is usually 0.5A with a short term maximum of 1.5A, only at startup? If bolting the pass device to the chassis works for 3A heatsinking then great, but if the chassis only gives you 1.5A heatsinking then perhaps you can find another way to protect against overheating while still allowing high, 3A or more, short term peaks.

 

BTW, the datasheet 150C (Tj) value is the max temperature at the transistor junction which will be higher than the 80C case temperature your measuring. This is due to the thermal resistance from the internal silicon junction(s) to the case of the device. Don't think you have 70C to play with.

 

Cheers!

Rob

[Bilbo]

@robc

I've been looking for reasonably priced r-core transformers for a while but remain a bit suspect of Chinese sourced units, however I have some products from this company which have worked well and I was thinking of giving one a go.  They offer custom builds and so I was thinking a 40VA r-core with 12V secondaries should give me 3.3A and might do the trick in limiting heat dissipation at the same time.  I have another pair of Studer 900 kits which I was thinking of putting to use in another build using these transformers - after I have played around modifying my first build hence all my questions.  A variac is an interesting suggestion but new r-cores would be cheaper and more useful to me.

https://www.aliexpress.com/item/115V-230V-R-30-40VA-R-type-transformer-With-copper-foil-shield-R-Core-40W-18V/32876802007.html?spm=2114.search0104.3.48.15024fd7ybR4Kt&ws_ab_test=searchweb0_0,searchweb201602_5_10065_10068_10130_10547_319_317_10548_10696_453_10084_454_10083_10618_10307_537_536_10131_10132_10902_10133_10059_10884_10887_321_322_10103,searchweb201603_57,ppcSwitch_0&algo_expid=af189d0c-8186-45dd-910a-8e40e40ab899-7&algo_pvid=af189d0c-8186-45dd-910a-8e40e40ab899

 

Funny you should suggest a battery - I have tried all sorts from little 7Ah units to a 75Ah Optimia AGM which I found was the best and used this for the first 2 years I had the SMS-200.  During that time I tried a myriad of other linear power supplies (as well as the matching SOtM power supply) but none could beat the Optima for sound quality.  I put it down to brute force available from the Optima as well as the points you have noted about battery supplies.  Here is my setup from that time.

 

Now this was fine until I tried the Studer 900 and it just beat the battery for clarity and naturalness plus I didn't have to manage the recharge cycle necessary when using a battery.  What sealed the deal was whilst listening to my system on Xmas day a capacitor blew in my SMS-200 and I had to send it back to SOtM in Korea for repairs (at their request to debug the problem) and updating to the "Neo" version.  Their advice was that the repeated current surge when switching on the battery had caused the failure of the capacitor. 

 

So the battery was retired and the journey with the Studer 900 kit began with the objective of getting it's current spec up to the demands of the SMS-200 with some headroom to spare.  Your advice has been invaluable in this regard and I suppose if I sell the Optima battery I could afford two r-core transformers!  Opting for the 12VAC output version should give me what I'm chasing whilst addressing the heat dissipation issue at the same time is what I'm hoping.  

 

Edited April 3, 2019 by Bilbo

[robc]

@Bilbo

Hi Bilbo,

Thanks for suggesting an R-Core supplier. If you go ahead with the purchase please let me know how they work out, and in particular what the no-load and full-load voltages are. About a week ago I bought two Chinese 30VA R-Core transformers on eBay. $AU70 for the two wasn't too bad and I'm not going to be sucking any high currents from them so the regulation isn't that important. I'm also thinking there's a good chance they can be rewound, iff the regulation is appalling. I'll let you know how these go.

 

I'm glad to hear you've tried a big lead-acid battery but I was a little surprised to hear the Studer 900 regulator sounded better than a big battery. Without wanting to insult you, are you confident you could pick the difference every time in a double blind test? If so did you try short leads from the battery to the load, and also putting come electrolytic capacitors, ideally also complete with bypass capacitors, close to the load? Then, If the Sturder 900 still beats the (battery + short lead + caps next to the load) then I'm concerned that it's the extra voltage you have available behind the regulator that's making it sounds better and that you might loose some of that benefit if you drop from 15VAC to 12VAC transformers. Is there any way you can get hold of some cheap 12VAC transformers from somewhere, perhaps two old halogen down light transformers - they are cheap, before you spend more good money?

 

Cheers!

Rob

 

[Bilbo]

@robc

When I first started using the battery I did have short leads but I did not apply any bypass caps as I was under the impression there should be no ripple from a battery.  

 

I should mention that I am not working on the modifications to the Studer 900 alone.  A friend is currently using a 30VA r-core with 15V secondaries on one unit and a 30VA with 12V secondaries on another test unit.  I’m going over next week to have a listen to the various configurations we have built.  We also have a bunch of different brand capacitors to test as well.  I have substituted the supplied charging cap (after the rectifier) with a Mundorf and just ordered a couple of different bypass caps to put in parallel to test the effects.  

 

Between the the two of us we are tweaking this LPS with good results so far.  The issue with heat sinking is currently the biggest concern hence my original query about options to address the issue.  My friend reports that the 12V unit runs about 20C less than the 15V unit but mine runs another 10C hotter on top of that because I’m running a 50VA transformer.

Edited April 5, 2019 by Bilbo

[robc]

@Bilbo

Nice to hear from you again.

6 hours ago, Bilbo said:

When I first started using the battery I did have short leads but I did not apply any bypass caps as I was under the impression there should be no ripple from a battery. 

Yep, the ripple is the residual of the rectified sinusoidal AC from the transformer.

 

I'm looking for reasons why the Studer 900 sounded better than the big battery.

 

Although it would be small, short leads would minimise any inductance between the big battery and the load, the longer the leads, the higher the inductance. An inductor, opposes changes in the flow of current, like a sudden need for power from the load. Putting capacitors close to the load would give a source of energy from which the load could *suddenly* draw current, without having to suck it through the inductance.

 

The same idea applies with bypass/decoupling capacitors. Because film/ceramic bypass/decoupling capacitors have less inductance than the larger electrolytic capacitors they can supply energy more quickly than the electrolytics.

 

It's all about satisfying the load's short term (transient) needs for power. Notice the nice sounding Studer 900 regulator has a 470uF electrolytic capacitor at it's output, close to the load, and that's bypassed/decoupled by a 0.1uF film capacitor.
 

There has to be a reason(s) the Studer 900 sounds better than the big battery and *perhaps*, this is it. I don't know, I'm just floating the idea as it makes sense to me and having a higher DC voltage from a 15VAC transformer, rather than 12VAC transformer, *could* well help push current through to the load.

7 hours ago, Bilbo said:

I should mention that I am not working on the modifications to the Studer 900 alone.  A friend is currently using a 30VA r-core with 15V secondaries on one unit and a 30VA with 12V secondaries on another test unit.  I’m going over next week to have a listen to the various configurations we have built.

Excellent!

My musing above are just theory and what you like in practice is what really counts. Still, it does help if you can work out why one thing sounds better than another, perhaps further improvements can be made.

7 hours ago, Bilbo said:

Between the the two of us we are tweaking this LPS with good results so far.  The issue with heat sinking is currently the biggest concern hence my original query about options to address the issue.  My friend reports that the 12V unit runs about 20C less than the 15V unit but mine runs another 10C hotter on top of that because I’m running a 50VA transformer.

Congrats on the results, that must be very satisfying, not to mention fun.

 

I'd be really interested to know whether either of you can *reliably* hear the difference between the 12VAC and the 15VAC transformers, with *everything* else kept exactly the same.

 

20C less would be nice and another 10C would also be nice, particularly when the smaller transformer would be cheaper. Is your friend using the chassis as a heatsink or is this just the supplied heatsink?

7 hours ago, Bilbo said:

We also have a bunch of different brand capacitors to test as well.  I have substituted the supplied charging cap (after the rectifier) with a Mundorf and just ordered a couple of different bypass caps to put in parallel to test the effects.  

Sounds like fun, enjoy!

 

Have fun and change only one thing at a time.

 

Cheers!

Rob

 

[Gieseler Audio]

Hi Bilbo,

robc is a wealth of excellent info & as I make lots of external low noise PSU’s I  thought would my experience too.

With a linear series regulator (like  the Studer) it  is a balancing act in terms of voltage in, voltage out & heat.

 With my 12 V 4A Kraftwerk power supplies I  run them from an external 12 V transformer.  This works out really well and even at a couple of amps they barely get warm.  The linear regulator I use has a very low voltage drop so I don’t need much head room.  However running them from a 15 V transformer  they definitely get hot even at 1 amp.  As robc explained  the regulator or transistor in your case  has to dissipate the in/out voltage difference x the current.

 Anyway bottom line and my advice is definitely try some 12 V transformers.  As long as your mains voltage doesn’t drop really low I think you’ll have enough headroom especially as you’re running current it’s only around 1 amp.

 Interesting that the Studer900 sounds so good - might have to try one!

 

Edited April 6, 2019 by Gieseler Audio

[Bilbo]

@robc

"Yep, the ripple is the residual of the rectified sinusoidal AC from the transformer."

I'm probably not getting your meaning here but I was talking about the battery supply so what transformer?  Sorry for my confusion.

"Is your friend using the chassis as a heat sink or is this just the supplied heat sink?

We are both using the supplied heat sink at this stage.  It allows us to fiddle with the built units without having to reattach them to the chassis for heat sinking each time.

I'm looking forward to next week to see what each version of these supplies sounds like!  The final version will use the chassis for a heat sink.

 

@Gieseler Audio

 Interesting that the Studer900 sounds so good - might have to try one!

At around $18 a pop for the built board it's a very cheap experiment and sounds extremely good.  Pop in some quality caps and it sounds even better!

https://www.aliexpress.com/item/Lusya-5-24V-STUDER900-Regulator-Power-supply-board-super-LM317-LT1083-LT1085-DIY-kit-finished-board/32948135430.html?spm=a2g0s.9042311.0.0.61f94c4dvKx44e

 

If you consider the origin of the circuit - a condenser mic phantom power supply to provide up to +/-48V @ 500mA in a professional Studer recording console then ultra low noise and low heat would be the design requirements.  I wonder what the phantom power supplies for a Neve, EMI, API, Trident, etc...  would be like?

[robc]

Hi Bilbo and All,

15 hours ago, Gieseler Audio said:

With a linear series regulator (like  the Studer) it  is a balancing act in terms of voltage in, voltage out & heat.

<snip>

the regulator or transistor in your case  has to dissipate the in/out voltage difference x the current.

Yes, that's an excellent summary

15 hours ago, Gieseler Audio said:

Anyway bottom line and my advice is definitely try some 12 V transformers. As long as your mains voltage doesn’t drop really low I think you’ll have enough headroom

I agree!

That's the difficulty, if the mains voltage across the country was a steady 240VAC then designing these things would be a lot easier. It's when you start considering mains variations from 215VAC to 265VAC that the "balancing act" comes into play. Those values are extremes but they do happen, and in some locations, regularly.

 

From the maths (Power = Voltage x Current), the other factor that causes large variations in heat dissipation is if you have to handle large variations in continuous current. Catering for 0.5A all the way up to 3.0A means catering for a 6:1 change in heat dissipation. Easy if your using a fan, but fans are noisy. Another option is having multiple windings an the transformer and switching between them with relays. Usually a compromise is made.

 

Cheers!

Rob

[robc]

@Bilbo

21 hours ago, Bilbo said:

"Yep, the ripple is the residual of the rectified sinusoidal AC from the transformer."

I'm probably not getting your meaning here but I was talking about the battery supply so what transformer?  Sorry for my confusion.

"Is your friend using the chassis as a heat sink or is this just the supplied heat sink?

We are both using the supplied heat sink at this stage.  It allows us to fiddle with the built units without having to reattach them to the chassis for heat sinking each time.

I'm looking forward to next week to see what each version of these supplies sounds like!  The final version will use the chassis for a heat sink.

Sorry Bilbo, I was just agreeing with you that there wasn't any ripple from the battery. This agreement was a prelude to my explaining that the purpose of putting capacitors after the battery leads, i.e. right their *on* the load itself. It wasn't to smooth leftover AC ripple, but was to provide an energy reservoir for sudden transient demands for power from the load. My apologies for not being clear on that one.

 

Good luck with the 12VAC transformers, I hope you guys don't experience a drop in sound quality compared to the 15VAC transformers. You most probably won't, but you never know for sure until you try.

 

Cheers!

Rob