Last Change
25 May 2000

Power Supply


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Every electronics hobbyist needs a good power supply. Generally to start off, you'll want a 5 VDC and an adjustable power supply. I used an old 24VAC center tap 'wall wart' transformer, a 7805, 7812 and LM317T for my supply. So I could use my multimeter for something other than monitoring supply voltage on the variable supply, I purchased a Radio Shack ammeter.

The chips are each only capable of delivering one amp of power, and the transformer can deliver 1.6 amps. The Radio Shack ammeter measures up to 500mA. This means that if I wanted it to go full scale at 25 VDC, it would be using half of my supply current! Deciding this was unacceptable, I did the only thing an engineer could do - I took it apart. Here is the inside:

You should note that the ammeter actually contains a shunt across the resister and meter movement (see arrow). I don't have a schematic program yet, but here is an ascii diagram for laughs:
mtrintarrow.jpg (10636 bytes)
Input+  ---+-------+
           |     .-^-.
  Shunt->  :    |meter|
           |     '-.-'
Output- ---+-/\/\/-+

I clipped one end of the shunt and bent it backwards (in case future modifications warrented a shunt) And played with it. I found that clipping the shunt made the meter into a 5mA ammeter, much more acceptable a current loss than .5A. I put the appropiate resisters in series and added a switch so I could measure 0-25VDC, and 0-5VDC for low power applications

The end result, after a box from RS, is a power supply with +5, +12, and 1.25-24VDC outputs. Eventually I may put another switch or two in so I can use the meter to measure current as well as voltage.

I plan on building a switching power supply into this box as well, since supplying the 5 volt regulator with 24V unregulated creates quite a voltage drop and resulting power loss.

Everything on/in this site is copyright Adam Davis, 1999, except where otherwise noted
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