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Step down a stepped down voltage.


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Hi all just trying to work out if you can simply step down a stepped down voltage.

 

Scenario:

Taking 240v and stepping it down to 48v through a regularly available PSU. This is for a home automation system so I want to inject the 48v into the network cable to create a 48v POE, so far no biggy just feed power to the same pin on each of the ports, the same as the commercial POE units.

 

Obviously trying to avoid VD over long runs of network cable it makes sense to run 48v (otherwise why would the likes of Netgear do it?) Now the problem... I don't need 48v at the other end and may need to step down to 12v, 5v, or 3v (depending on what needs to be powered). I only need milliamps at the other end, is there a simple way to step the voltage down without a lot of complicated electronics since it would be located in a hard to access place. 

 

 

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This is what I am trying to achieve... A channel may use any of 48v, 12v, 5v, 3v or 0v regardless of voltage power draw will be limited to something like 20mA to 100mA each.

 

In the case of the wall switches the power required would be to light up a 12v LED in the momentary switch. You can buy some illuminated switches with 48v LEDs in which case I would just bypass.

 

Others may require voltage to power a close proximity contact switch.

 

Worse case scenario 5v may be required to power an Arduino, I don't think there would be enough juice for a Raspberry Pi 3.

 

Is this doable?

 

Voltage.jpg

Edited by Silent Screamer
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You may get a way with a simple resistor / zener method for low current, not really good regulation with that type especially if you want to run a Pi or Arduino.

 

I would look at using series connected voltage regulators 48V to 18V (see below for suggested circuit type) then 12V, 5V and 3V.

 

IC regulators will have an input to output voltage limit, ~36V comes to mind for the 78XX series, LM317 is 40V, LT1083 is 30V.

There probably are higher input / output types I am not sure what type / part number they are.

Extra components will be needed with the regulators the numbers of extra components will be dependent on the regulators used.

The only regulator really at the limit for IC regulators is the 48V to 18V ,  I suggest using 18V as this will provide enough voltage overhead for the following IC regulators like the 7812, 7805 etc.

You might use a zener controlled regulator in the 48V to 18V position, see http://www.circuitstoday.com/wp-content/uploads/2009/10/Zener-Controlled-Transistor-Series-Voltage-Regulator.jpg

A bypass transistor is then more easily found that has the required voltage / current capabilities.

 

There are many choices out there just need to look up the data on RS or Element14?

 

Have you checked to see if there is already a power supply that is off the shelf with those voltages?

 

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