How to make a non-variable power supply variable?

[broncoman529]

Hi,

I have a hobby of slot car racing which requires me to have a high amperage variable power supply, It needs to be able to variate from 0-13.8 volts DC. Is there any way that I can make an adapter that attaches to the output terminals on my power supply to variate the voltage? I have two different power supplies that I can use, An Astron Model SS-30 or a Pyramid PS-21KX. I need to be able to maintain no less than 5 amps at 3 volts. I do not want to butcher my power supplies since I do use them for my radio hobby and to buy one is very expensive. I need to know what value parts I need to do this. The reason I posted this question here is because there are a ton of electronic geniuses on this website Please let me know.

Thanks,

 

[nomadradio]

Hi Broncoman.

Your uh, ton of geniuses seems a few pounds short.

Commercially-made power supplies that will turn all the way down to zero Volts are designed differently than the fixed-output models you mention.

You could find the tiny screw-adjust "trimpot" that sets the output voltage. It will usually be on the circuit board with the other regulating-circuit parts. Even if you remove the trimpot, and put long wires onto an external control, you may not get the full range of adjustment you want.

One design detail that makes the full-range adjustable supply different tends to make it cost more for the same rated output. This has to do with how most of them perform the regulation. A transistor, or bank of them, serves as a variable resistor. They are in series with unregulated source inside, and constantly change their internal resistance, as controlled by the stuff on the circuit board.

The voltage drop, between that unregulated source inside and the output setting is called the "pedestal". Because it looks like one on a graph. If you pull 3 Amps at 14 Volts, and the unregulated side is 22 Volts, here's the math. The regulator transistors will have to dump 8 Volts times 3 Amps, or 24 Watts. That's how a series regulator works, turning all of the "step-down" pedestal voltage into heat.

Now, consider a setting of TWO Volts. Now that difference is 20 Volts, times the same 3 Amps, is now SIXTY Watts. That's a lot more than 24, and the normal "13.8 Volt" fixed supply will only be designed for the smaller heat load this requires.

Figure it this way. If you convert a commercial fixed-output supply, just use one rated for at least three, or better four times the 3 Amps you'll draw. An Astron or Tripp-Lite type supply rated for 12 or 15 Amps should survive this conversion okay.

But you're still left with wondering how far the output-voltage control will "turn" when it's made external. If it won't go all the way down to zero, you can't use it, right?

Don't have any handy way to predict this from experience. Doesn't come up a lot.

On the other hand, suppliers that cater to service shops will sell full-range regulated power supplies with the control already on the front. Trouble is, they cost twice to four times as much for the same Amp rating as fixed supplies cost. For a reason.

73