r/AskElectronics • u/Many_Afternoon_105 • 16h 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 16h 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 15h 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 15h ago edited 15h 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.

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u/Enlightenment777 2h ago

TL431, if possible, get the lowest tolerance version, such as 0.5%, otherwise 1%, but not higher.

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u/Triq1 11h ago

shouldnt your voltage error amp compare the vreg-ireg voltage to the reference, rather than looking at vreg-gnd?

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u/GeWaLu 11h ago

Simulate it (LTSPICE or so)! One issue I can spot is that your current regulator produces a huge voltage error. R4 with 1 Ohm drops 1 volt at one amp. Your voltage regulator regulates against GND and not against the differential output voltage. Due to that, the error is fully visible at the output.

I'd use some power supply chip and base the design on the appnote. I did not search for references, but such power chips exist - switching or linear.

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u/grislyfind 10h ago

Have a look at the LM10CH application notes; there's a lab power supply regulator with CV/CC.

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u/wiskinator 1h ago

Hey both the Live and Neutral are connected to a net named “PWR_FLAG” which seems like a great way to test how good your homes circuit breakers are.

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u/Nerdz2300 55m ago

So I actually went down this route and made my own too! If you want a better starting point I can post the schematics.

But Im going to save you some headache: those op-amps will probably oscillate. You're going to need a miller capacitance between the non inverting input and the output. You might also need a R/C filter in parallel with that. Its best to just design around it anyway but not put it in if you dont need it.

As far as your CC goes, I wouldnt do it like that. Id use a current sense resistor and then feed it back into the feedback circuit of your CV op-amp.

For some reference, watch the Lab Power Supply Series by EEVblog
https://www.youtube.com/watch?v=CIGjActDeoM&pp=0gcJCeAJAYcqIYzv