The supply was originally a fixed 28V 8 amp regulated DC supply that I built in graduate school in 1970. Since I didn’t have the proper transformer originally – only one with a 24V output – I had added a second smaller transformer in series.
For the G-gauge application I wanted 0-24V adjustable. The original setup could be made adjustable, but the output transistors couldn’t handle the power drop required for low voltage outputs. The output transistors must “burn off” the excess power that is being dumped when the output voltage is reduced. In this case, I might have up to 6 amps with a drop across the regulator transistors of 30+ volts – close to 200 watts!
My first step was to cut down the output voltage by removing that extra transformer from the circuit. This meant I had a lower starting voltage, and hence less drop.
My regulator (a uA723) driving the output transistors wasn’t very sophisticated – a marginal overcurrent protection without any
temperature sensing. I decided to take an easy way out by adding a couple of high-power regulator chips to form a cascade of two regulators. The original regulator would provide a constant voltage source to the second regulator so that I didn’t need to worry about things like voltage ripple. The new variable regulator would control the output voltage and burn off the power drop from 26V down to the final variable voltage. The power dissipation would be shared between the two stages of the cascade.
The regulator chips are a pair of very nice Maxim LT1083 adjustable regulators with internal overcurrent and temperature sensing. I tied two of these in parallel, mounting each on its own finned heatsink. These chips will shut down if they overheat, as they might if the output is shorted due to a derailment.
For the variable control device I chose one of the linear faders that I sell as the exclusive distributor for Penny & Giles. This gives me a smooth control with a stroke of three inches.
I wanted to include a current meter on the output, but I didn’t have anything with a 10 amp range. I did have a nice 100 mA meter in an enclosure, and I was able to add a shunt to the terminals to recalibrate it to 10 amps. The shunt is just a short piece of bus wire. I determined the appropriate value by using my volt/ohm meter to read the output current to a fixed load while I adjusted the shunt to give the same reading on my power supply’s meter.
I added a double pole, double throw center off switch for reversing polarity and killing the track voltage in the center position. I also included an incandescent pilot lamp across the output as a simple output voltage indicator.
When I got done, I had a kludge, but it did what I needed. I can run my four locomotives without the supply ever cracking a sweat. Sure, I could have bought something smaller and fancier, but what’s the fun in that? Is it overkill? You bet! But it’s mine, and I love it dearly….