Electronics > Projects, Designs, and Technical Stuff
Lab Power Supply - The Lost Current
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xavier60:
Wow! that is one serious power supply.
All I can suggest, if you have not done so already, is to remove one of the shunt resistors and solder in PCB pins so that you can experiment and take measurements while the PCB is mounted.
radoczi94:
Yeah,  next time will make some wire loops for test points. Always learning.
Damianos:

--- Quote from: radoczi94 on January 30, 2018, 11:36:25 pm ---
--- Quote from: Damianos on January 30, 2018, 10:39:46 pm ---How you measured 1A on 200MOhm resistance? Is there a 200MV source?
Also how you measured the output current?

On the circuit:
- What is the role of T1?
- What about C16?
- Missing capacitors on D4 and D5 (bases of respective transistors)...
- The reference voltage (VREF 8V) is relative to the other side of the shunt, so the current limit adjustment will vary with load current...
- The 100nF capacitors, here and there, are like the designer had a bag of them, without knowing where to use them! I'm wondering if any calculation of time-constants / filter-poles had been made.
- ...

--- End quote ---
I did not measured 1A on 200MOhm resistance, I removed the shunts and tried to measure resistance on the pcb to see, if there is some sort of short circuit or something.

T1 transistor shorts out the output of IC2, when the negative supply dies, which happens almost instantly, when the mains goes off.Whitout that, after switching off, the output would jump up to the maximum voltage.

Honestly, I have no idea, what C16 supposed to do.

I placed caps there on the PCB, just missing on the schematic.

--- End quote ---
So, the 1A is imaginary or real current?  :-// How you lost the 2A? How you know that the total was 3A?  :-//

T1(Q7) and IC2: See the notes of the maximum ratings section in the Op-Amp datasheet.

C16: see what does C14 and you will see that it, at least, cancels the compensation.

Consider the current reference, noted above...


--- Quote ---
--- Quote from: C on January 30, 2018, 10:49:58 pm ---
--- Quote from: radoczi94 on January 30, 2018, 08:51:43 pm ---I know, the schematic is a bit annoying, working on a version for better readability. You can find many versions of this schematic on the web, even an ebay kit is based on this PSU.

--- Quote ---A lot of junk on web

--- End quote ---
The IC2 gets feedback directly from the output, IC2 has no feedback, since it works as a comparator.

--- End quote ---

If IC2 is a comparator then it would be a switcher supply.
IC2 feedback includes Q4,Q5 & Q6

If you look at T1, as current drop changes across current sense resistors it's resistance changes effecting output.
As R17 voltage changes due to output voltage change, IC2's output changes.
This is not an OR, it is a voltage modulated by current.
With this happing then remaining parts of circuit has to compensate for this error or you have a voltage modulated by current output.

--- End quote ---

Sorry, I meant IC1 has no feedback.Which is actually not true, because it has a cap, so its an integrator.My bad.

I do not know if this thing oscillates or not, I do not have access to a scope to see what is happening.One thing came into my mind: if I tear off IC1 from the circuit, and put a reasonable load on the output, I would be able to disqualify a lot of theories. If the missing 2A comes out, then something crazy happens with IC1, if not, then it's an another kind of problem.

Anyway, made a new schematic, I hope, it's better.

--- End quote ---
The IC1 is neither a comparator nor an integrator, it is intended to operate linearly by reducing the voltage setting when the current tends to be greater than the set limit...
radoczi94:
No, I measured that 1A series with the shunt resistors, 3A was present AT THE SAME TIME on the output, parallel(Intentionally).
If IC1 oscillates, it may have tricked the dmm.

I do not know which data should I look in the table, but it has unlimited short circuit protection in the temperature range, if you think abot that.

Yes, it moves a little bit, but this was not meant to be a high precision thing. It can be solved by whacking in an another reference chip, tied to the fixed GND. I was aware of that, but probably it will be good enough for the girls I go out with.
xavier60:
It wouldn't be too surprising if it is oscillating in CC mode because both op-amps are in the loop making the response rather complex.
I see in other designs that either the CV op-amp or the CC op-amp can take sole control of the power transistors.  The response of both loops can be individually optimized.
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