*In-Circuit* Capacitor Reform for Medium Quantity Old Stock SMPS - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: January 15, 2021, 01:39:47 am
content: Hi all,I've seen a couple of threads about electrolytic capacitor reform recently, in addition to the boatloads of other threads from other forums...going back many years.At work, we are trying to come up with a reasonable electrolytic cap reform procedure for old stocked power supplies, like 5-10 years in the warehouse. Most people talk about reforming capacitors out of circuit. That's simply enough. Other people talk about slowing increasing voltage with a variac, but this requires someone sitting there and moving the dial up every couple of...minutes(?) for a few hours. I'd like to find a solution that is basically set-it-and-forget-it with a high liklihood of working.My main idea so far is to:Apply 240VAC, but through a 1K - 10Kohm resistor. This limits the initial surge current into capacitor, and if 10K is used, will take about 5 seconds to reach full voltage.( Is that long enough?) Power supply idles through 1K - 10K resistor for approximately 1hour per year in storage.The issue here is that somewhere around 70-80V, the power supply will try to turn on. With such a high source impedance, my guess is that it will, at the very least, stutter as it tries to start up, and more likely, require multiple attempts over several seconds...Normally, I don't think that's much of a problem, but the very act of turning the switching converter on will demand some high peak currents out of the bulk cap. Does this just defeat the purpose of the "gentle" reforming process?Is there a more reasonable way to reform capacitors in circuit?Thanks,Tim

author: TimNJ
author_rank: Super Contributor
timestamp: January 15, 2021, 02:00:35 am
content: As this has been discussed ad infinitum,https://www.eevblog.com/forum/chat/old-electronic-devices-electrolytic-capacitors/https://www.eevblog.com/forum/beginners/doubts-about-electrolytic-capacitor/https://www.eevblog.com/forum/renewable-energy/electrolytic-capacitor-truth-or-myth/https://www.eevblog.com/forum/beginners/electrolytic-reform-school-how-long-a-period-of-inactivity-to-be-necessaryhttps://www.eevblog.com/forum/chat/reforming-aluminum-electrolytic-capacitors...I kind of regret making this thread, on second thought. But hey, if you have any input, specifically to my question let me know.Another option is to simply just power it on (as normal). If it blows, toss it. If it works, leave it idling, and call it good.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 15, 2021, 02:58:08 am
content: If you search for 'dim bulb tester' you'll find warnings from me and others about using any kind of ramping input voltage with switching power supplies. The info is mostly anecdotal, but the story is that switching supplies can get upset if you feed them too low a voltage. Two possible problems are that part of the circuit wakes up before another or lower voltage causes higher currents than are safe. Both can result in magic smoke. A dim bulb tester avoids this, but if the input capacitor is weak, it could blow due to the mains voltage. The dim bulb should prevent further damage. Replacement of the input capacitor would then be required.If you decide to go ahead with a reforming operation, realize that as soon as the voltage is high enough for the supply to wake up, there will be a current spike that will drain the input capacitor. This will occur at a voltage much lower than mains voltage so you won't really be doing much reforming. If the supply has, or you can create, a 'disable' function, you might be able to keep the supply off while you reform the input capacitor. This will restore the input capacitor only, but that's usually the one to worry about.As for the actual reforming operation, typically you limit the current and allow the voltage to rise slowly. I'm not aware of any rules of thumb as to the magnitude of the current, but it's typically in the milliamp range. Think charging times of minutes rather than seconds. I don't know if any capacitor manufacturers discuss this in datasheets or app notes.