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I'm having a bit of a hard time diagnosing a problem with a circuit I have just soldered onto stripboard, the circuit works perfect in simulation.

The scenario is that I have a circuit with a few opamps and transistors and am using a lab power supply in series to provide +- voltage rails.

If I connect positive and ground to my circuit everything seems fine. In terms of the power supply. However if I connect the negative rail to the board then the supply that's providing the negative rail trips and goes into CC mode, the voltage on the display drops from 12v to around 0.1v (I can't remember the exact but its low), if I remove the negative rail connection the supply restores and comes out of CC mode.

This happens with both just ground and negative and positive and negative connected. So the problem is with the negative rail / connection.

However, when I test with my multimeter on the continuity setting, there is no beep when I touch the ground and negative or positive and negative. So this to me means there's no short?

So I'm not sure what I'm looking at or what the problem is...I assumed the power supply going into CC mode meant there was a short somewhere but I can't find anything and I'm not sure why its happening for both ground and positive rails.

This still happens when I remove all the opamps from the chip holders so theres no shorting happening through a potentially dead opamp.

EDIT: added schematic photo. Also another thing I discovered is that when I do a continuity test from the black dot and the ground rail I get a beep.

Testing is so done with pin 3 of the first opamp disconnected. schematic of problem circuit

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  • \$\begingroup\$ Schematic? (Ctrl-M brings up a schematic editor BTW) \$\endgroup\$
    – Adam Lawrence
    Nov 10, 2013 at 20:42
  • \$\begingroup\$ Well, the circuit in simulation works 100% so I'm assuming its a soldering or dead component problem, and looking for advice on how to go about debugging. But you think it could be the circuit itself even if simulation works? \$\endgroup\$
    – binarysmacker
    Nov 10, 2013 at 20:46
  • \$\begingroup\$ Being able to visualize what you've come up with can lead to targeted debugging advice instead of general advice. Sometimes impossible circuits can simulate too (i.e. thousands of amps in a trace somewhere) \$\endgroup\$
    – Adam Lawrence
    Nov 10, 2013 at 20:53
  • \$\begingroup\$ You also need to specify how much current your lab supply is capable of delivering / what it is adjusted to. Tripping into CC mode with a 1mA current limit is very different than with a 1A current limit. \$\endgroup\$
    – Adam Lawrence
    Nov 10, 2013 at 20:55
  • \$\begingroup\$ I have added a photo of the schematic that I have. That's a good point, I wouldn't be able to say what it was set to but I do believe I tried upping it a bit. I believe the limit is 5A \$\endgroup\$
    – binarysmacker
    Nov 10, 2013 at 21:02

2 Answers 2

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Just because the continuity test function of your meter does not detect a short does not mean there is an unintended path for excessive current between the supply rails. There are many potential faults that could exist but won't be detected with a meter like that, but here's one: a forward-biased diode between the rails.

Most meters, except the very cheapest of them, test continuity with less than 0.6V precisely so that they can't forward-bias a silicon P-N junction. Thus, they will not detect a diode as "continuity" regardless of which way you place the diode with respect to the meter. However, to any power supply significantly more than 0.6V, the diode might as well be a dead short, if it's forward-biased.

If your circuit is simple, the easiest way to troubleshoot is usually to inspect it very carefully, looking for errors. If that fails, you can connect the power supply and leave it in CC mode, with the current set low enough that you aren't burning anything. Then, with a good, precise voltmeter, start measuring the voltage drop between points on your board. Where there is a voltage drop, there is high current (remember, \$E=IR\$), and thus you know the fault is somewhere between those two points. Be methodical about it and you should be able to find the fault by process of elimination.

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  • \$\begingroup\$ There's no diodes, but I guess the same principle can be applied to the transistors? Could you advise how I would go about debugging this? \$\endgroup\$
    – binarysmacker
    Nov 10, 2013 at 20:14
  • \$\begingroup\$ Thanks for the tip on testing! Any idea on the black dot? I'm on my way to the lab now so will be testing soon! \$\endgroup\$
    – binarysmacker
    Nov 11, 2013 at 8:11
  • \$\begingroup\$ @binarysmacker really hard to say. There could be a solder bridge there, or your meter could be causing something (transistors, op-amp) to conduct. If your current limit is at 5A, it's also possible that some of the components have been fried. Generally you want that current limit as low as possible while still being sufficient. For the couple of op-amps and transistors in the schematic, 5A is a few orders of magnitude more than I'd estimate is necessary. \$\endgroup\$
    – Phil Frost
    Nov 11, 2013 at 12:39
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Are you certain the two power supplies are isolated? Without the circuit connected, do you get the same behavior when connecting the supplies in series?

(If the supplies aren't isolated, and/or are both referenced to earth ground, you're likely building a ground loop)

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  • \$\begingroup\$ No, it only happens as soon as I plug negative supply into the circuit. The set up works great with another circuit. \$\endgroup\$
    – binarysmacker
    Nov 11, 2013 at 8:11

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