With TJ Byers
I have a power transformer that can deliver 125 volts at 500 mA, and I want to use it to construct a variable 0-150 volt regulated power supply for some tube applications. Do you have a good circuit for this?
Trying to get 150 volts DC from a 125 VAC transformer is not easy because you don’t have a lot of headroom. For good regulation, you need some kind of voltage difference between the input and output voltages. Even without a load, the best you can hope for from this transformer is 175 volts — and that decreases as the load increases. Fortunately, you can get more current as the voltage is lowered; you should get maximum current somewhere around 120 volts and under.
With that said, here’s the best design I could come up with.
The LR8 voltage regulator is a three-terminal device — like a 78L05 — that has an adjustable output voltage range of 1.2 to 450 volts. Unfortunately, the LR8 can only supply 10 mA of current. Which is why the external pass transistor is needed. This transistor is configured as an emitter follower; it has very low voltage drop and will pass as much current as your transformer can provide. That is, it’s not short-circuit protected as is the LR8 chip by itself — so be careful. Mouser Electronics (800-346-6873; www.mouser.com) has the LR8 in stock.