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“Lab” PSU design help

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CountChocula:
Hello—

Absolute beginner here; I am trying to learn a bit about analog electronics by designing and wiring up a simple CC/CV power supply. My goal is for it to take a regulated 12VDC input and provide a variable 0-12V / 0-1A output.

Using several reference circuits available online as a guide, I ended up designing the schematic below. I breadboarded it, and, somewhat to my surprise, it mostly works—I can set the voltage from 0 to ~11.95V, and the constant current limit works correctly between 0 and 1A as seen by R10.

However, the load regulation is poor; for example, at 8V 1A, I see a drop of around 250mV, which seems excessive. I thought that perhaps there could be a problem with the input, but it stays at a solid 12V, so there is clearly something wrong with my circuit; just to be absolutely sure, I replaced R2 with a 5.1V zener reference, because I thought that perhaps a drop in VIN could skew the voltage set point, but that did nothing. I would be grateful for any suggestions!

While I'm at it, I would also love some suggestions on how this circuit can be improved in general (or whether it even can be improved at all… most of the other circuits I've seen seem much more complex and go well over my head, so I suspect that I'm drastically underestimating the difficulty of the task at hand :-) ).

For example, am I correct in assuming that the Q1/Q2 and Q4/Q5 darlingtons are superfluous? I wanted to be careful about not exceeding U1's 50mA output current (not the least because these TLC2272s cost $8 a pop!), but, if my calculations are correct, there should be no risk of that—even if either Q2 or Q5 were to drop all 12V across R7 or R8, they would be sinking ~60mA, and the worst-case beta of the 2N2222 is 30, which would mean that the circuit should never draw more than 4mA from the op amp).

In any case, thanks in advance for any help. Cheers!

ledtester:
Do you have a scope? The poor load regulation you're seeing might be an indication that your control loop and thus your output is oscillating. Such oscillations could be in the kilohertz and above range -- something a multimeter wouldn't pick up.


--- Quote ---... not the least because these TLC2272s cost $8 a pop! ...

--- End quote ---

octopart.com is a convenient site for searching several major electronics distributors at one go. It looks like the SMD versions of this chip are significantly cheaper than the through-hole package. And with adapter boards you can get from ebay/aliexpress they can be made just as easy to use.

SMD adapter boards: https://www.aliexpress.com/item/2251832772375001.html

magic:
General remark: you don't need two power transistors in series, the two opamps could control one power transistor.

General remark: I never built such a PSU, but others did and you should be able to find schematics somewhere on the forum or maybe somebody else could post links.

It seems that you want rail to rail output. This is somewhat unique and it precludes a simple emitter follower output. It's very likely to be unstable without compensation because the output stage has voltage gain. Stability will depend on the value and ESR of the output capacitor - low resistive impedance provided by the capacitor reduces voltage gain of the common emitter output stage.

Note that your current sense resistor reduced maximum output by 1V at full load.

The datasheet says that your opamp can sustain output short circuit to either supply rail indefinitely.

strawberry:
C5 small as possible (can destroy sensitive devices )
temperature stable resistors in voltage/current sense and reference circuits
TL084 should be good enough down to couple mV levels (rail-rail OP amp wont go down to some  mV anyway)
differential amplifiers to rule out wire voltage drop (0.01ohm * 5A = 0.05V)
zener stability is aprox -1..+10mV/K (depends on zener voltage and circuit design) compensated zener circuit less than ~0.1mV/K and LM399 1ppm/K
TL431 with current source would get more stable (LM336 bit better bandgap reference ~20ppm)

magic:
Output capacitance may destroy devices connected to the PSU by temporarily exceeding the preset current limit, but it is also essential to stabilizing LDOs, which OP apparently tries to build.

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