This week I received an old photographic studio flash unit made in the early 1960'ies. Inside is a huge 40µF/2500V capacitor. I don't know the usage history of the unit; it may not have been used for 20 years or perhaps more, or it may have been used weeks ago.

What is a safe way to bring the unit back into use again? I am thinking of interrupting the mains supply (240VAC) with a relays, operated by a microcontroller, starting with 0.1s pulses and a few minutes waiting time, slowly building up. Would that be a good idea, or should I follow another approach?

  • \$\begingroup\$ Does the capacitor work? It's likely to be a film capacitor with that voltage rating. Film caps can't be reformed as far as I'm aware; I think that's something you do with old electrolytics, isn't it? \$\endgroup\$
    – Hearth
    Jun 7 '19 at 16:04
  • \$\begingroup\$ What should I do to test that without powering it on? \$\endgroup\$
    – user508402
    Jun 7 '19 at 16:35
  • \$\begingroup\$ Take it out of the circuit and stick a capacitance meter across it. \$\endgroup\$
    – Hearth
    Jun 7 '19 at 16:47
  • \$\begingroup\$ It's very likely a paper-film cap (designation: MP). Thanks \$\endgroup\$
    – user508402
    Jun 7 '19 at 17:50
  • \$\begingroup\$ I haven't messed with reforming electrolytics (if that's what it is), but everything I've read says you give them a strictly limited current and see if they'll charge up without leaking (leaking either current or goo is a bad sign). But, as mentioned, this only works with 'lytics, not other sorts. \$\endgroup\$
    – TimWescott
    Jun 7 '19 at 19:46

There are a few things I do when powering on circuits if the condition of circuit is unknown:

  • Visually inspect the circuit, and do a continuity check on the voltage rails to make sure they are not shorted to ground.
  • I find a way to monitor the current to the supply and estimate the current usage, if I can I find some way to limit the current. If it's an AC device, I use a kill-a-watt meter.

  • The best thing you can do is get a hold of a thermal imaging camera and watch hotspots. I turn the supply on momentarily and see where power is being dissipated. Parts usually die thermal deaths. It really depends on the design, but it would be unusual for anything to get above 90C, if anything gets hotter than 120C there is most likely an issue with that part it it should be inspected further. Then increase the amount of time the supply is being turned on for and watch the temps of components.

If your worried about the old caps, unsolder them from the board and do an ESR test and leakage test


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