Electronics > Projects, Designs, and Technical Stuff
Lab Power Supply - The Lost Current
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radoczi94:
Hi.

I built the power supply shown on the schematic.It looks like working as a treat, but not really.The CV mode is ok, the CC is what causing the nightmare. I started the testing with a massive short circuit on the output, slowly turning the current pot up. The comparator doesn't wanted to switch over. I probed around for a while, found out, there isn't enough voltage drop across the shunt (R35-R25) resistors.I started to question the quality of those, modified the voltage divider on the comparator input, then came the idea: measuring the current thru the shunts.Okay, now, here comes the funny part.I measured 1A on the shunt, while on the output was 3A. I lost track of 2amps!!! I measured over and over again with 4 different DMMs and analog meters, the results were the same. It turned out, the lost 2A is not a fixed value, it depends on the load on the output, it's about 2/3 of the output current. I removed the shunts and measured resistance to see, if there is anything weird, but only measured something in the 200MOhms range, which is basically nothing. Anyone has any idea, what should I try, or measure? Because I'm out of ideas.

Thanks.
C:
just a quick look

If you have a power supply that has a CV mode & a CC mode, you have a OR.

Output = the lesser of (set voltage or set current)
Kilo Tango:
Hi,

most power circuits I have come across measure current in the  +ve rail, otherwise it means your -ve output is different to your circuit common terminal, the thick blue line, so which one is earth ?.

You can't loose current  (easily !) , it has to flow somewhere. if it is flowing out of X6-2 and back into X6-1, it has to make its way back to the input along the thick blue line, which means either thro your shunt resistors or by another path. By the way you don't need to remove the shunt resistors to check them, there isn't anything significant in the circuit to disturb their measurement easily, also if they now measure 200M then they are blown.

I'm still trying to get my head round the Q4 emitter follower pulling up the bases of the 2 2N3055's and both op amps are running without any feedback ?. is this a published circuit ?

Cheers Ken
radoczi94:
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.

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