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I’ve produced a simple unregulated power supply PCB, consisting of:

  1. AC input terminals
  2. Fuse
  3. 4 rectifier diodes
  4. Filter/smoothing capacitors
  5. DC output terminals

Only when the PCBs arrived did I get the sinking feeling that the traces connecting the outputs of the rectifier diodes to the rest of the board (which consists of nice chunky pours) may be too thin for the current I need to drive. Doh!

What is the best/safest way to stress test one of these boards?

I’m happy to kill one of the PCBs in the process of stress testing.

If possible, I’d also like to spare the capacitors - during testing, would it be a good idea to leave the caps off? And of course I don’t want to jeopardize the transformer (a large toroid).

My idea is to build one of the PCBs (minus the big expensive caps) connect it to some beefy dummy resistors (pulling the desired current) and then to mains. Then wait and see if anything spontaneously combusts, or if the thin traces start melting.

Update: I’m not looking for assistance in determining the current handling ability of the trace, or for strategies to transfer heat/increase the trace width. For the sake of science, let’s assume it’s too thin. I’m looking for a general stress-test method to find the real world upper limit of what a PCB like this can safely handle.

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    \$\begingroup\$ What are the trace sizes? oz copper, width, length? What is maximum amperage and voltage through the rectifier? Output power? \$\endgroup\$ – Misunderstood Jul 2 '18 at 22:29
  • \$\begingroup\$ Connect the single trace you want to measure to your lab power supply with a low current, then turn up the current until it melts? BTW the PCBs are very likely salvageable (even after this destructive test), you'll just need to solder extra wires in parallel with the thin traces. \$\endgroup\$ – immibis Jul 3 '18 at 0:48
  • \$\begingroup\$ @Misunderstood - I suppose I’m asking for a general “stress test method”, rather than for help in calculating whether the trace is capable of handling the current. But the trace is 5mm wide, 50mm long, 1oz. 28v rectified, worst case 7A throughput. \$\endgroup\$ – abza Jul 3 '18 at 4:28
  • \$\begingroup\$ I understand what you want. It is necessary to know what needs to be tested. Since you posted the requested info you now have a couple of valid answers. \$\endgroup\$ – Misunderstood Jul 3 '18 at 17:09
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You can use one of the online Trace Width Calculator to determine how much is your situation close to the required handling current. it seems that you intend to use your circuit to very high power application. BE CAREFUL please as excessive current can lead to a catastrophic explosion which may result in injury.

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  • \$\begingroup\$ Thanks Ahmed (and for the warning ;) But I’m looking for a method to stress test the board I have - I’ve already determined that the trace size is borderline close to the limit of acceptable. \$\endgroup\$ – abza Jul 3 '18 at 4:22
  • \$\begingroup\$ I've never tried it, but it would be interesting to thermally bond a thermocouple to a thin PCB track and actually test some of those numbers about temperature that are often quoted for PCB tracks. Perhaps that might be more informative than just whacking current through a board until we get a Muppets-Swedish-chef moment? (OK, that's fun as well ...) \$\endgroup\$ – dmb Jul 3 '18 at 4:38
  • \$\begingroup\$ @abza I'm still confused as to why you can't just pass the expected current through the trace, and see how hot it gets. \$\endgroup\$ – immibis Jul 3 '18 at 5:50
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I would recommend buying yourself a handful of power resistors and simply testing increasing currents until the trace temperature becomes higher than you're comfortable with.

There are programmable loads you can buy, but that's probably overkill for this.

If you have a variac on hand you can use that in combination with your load resistors to really dial in the max current.

A FLIR camera or non-contact thermometer is useful for this sort of testing; Although you should be aware that these tools can be very inaccurate depending on the surface you're looking at.

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If the THIN traces are short in length, then heat can be dumped to thicker regions that serve to heat-sink the thin pieces of copper.

The thermal resistance of default copper foil (1 ounce/foot^2, 1.4 mils thick, or 35 microns) is 70 degrees Centrigrade per square of foil.

A piece of foil of length 0.2 inches and width 0.02 inches has 10 squares, thus has Rthermal of 700 degree Centrigrade per watt.

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I’m looking for a general stress-test method to find the real world upper limit of what a PCB like this can safely handle.

You can use a variac and a current transformer.

The current transformer you can wind yourself, using any traditional transformer and adding only a few secondary windings to it.
(notice: the transformer is for short term usage only, it will get hot)

The variac can be used at the primary to adjust the current.

You can get hundreds of amps like this. But the maximum impedance of the circuit under test depends on the transformer.

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