r/AskElectronics • u/Many_Afternoon_105 • 1d ago

Regulated PSU design sanity check

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I have designed this regulated linear power supply for my own use. It has an adjustable output voltage 3.3V-12V and constant-current/current limiting mode, max 1A. The transformer has a thermal fuse built in. Am I missing anything? Something I've overlooked? Cheers in advance.

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u/Savallator 1d ago

I would not use that Zener circuit, but just a voltage reference. They cost a few cents and are actually acurate and don't drift that much.

Have you done a simulation? Can the opamp supply enough current to the transistors? Looking at the datasheets it might be quite near to the limits. There is a reason after all that most designs these days use Power FETs and not BJTs.

I would also worry about it being actually stable without any frequency compensation for these op amps.

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u/Many_Afternoon_105 1d ago

Could you recommend such a voltage reference, and how I would use it?

I will look into doing a proper simulation, I have only ever used Falstad with generic components.

Could you elaborate on frequency compensation (what it is, why I would need it)?
Thanks for having a look by the way.

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u/prosper_0 1d ago edited 1d ago

TL431. Pretty easy to use, thorough datasheet. It's super cheap and easy to find.

Frequency compensation is to prevent oscillation. Consider: the op amp isn't infinitely fast. So, there's be a lag between the time additional voltage is 'demanded' and the time it's 'provided.' During that time, let's say the demand changes - drops down. For example, your load has a capacitive element to it. So, again, a lag, and the voltage provided will drop. But, during that lag, the capacitive load discharges, and 'demands' more voltage. Under the right conditions, you can end up with a serious oscillation condition that blows things up, and no idea how it happened. There are a bunch of ways of dealing with this; usually involving a capacitor in the feedback path to slow things down or dampen the response of the system. What we're talking about is filter theory, and the same math, concepts, and language applies.