PSU capacitors ESR - how to test economically?

[RickNel]

I'm needing to check the state of capacitors in a Sony switch-mode PSU from my Mac IIsi.

Putting the continuity tester from my DMM across them, I read resistance rising as the caps charge, then falling back to a much lower value - mostly around 100R but one or two up at 400R.

Does this tell me anything useful about ESR?

I've been looking at this DIY ESR adapter for use with an oscilloscope, since I'm not sure I want to shell out for a professional ESR meter.

Any suggestions?

Rick

 

[Chuck(G)]

An ESR meter is basically an AC ohmmeter. To test ESR, you need to impress an AC voltage across a capacitor and measure the series resistance. There are plenty of DIY plans and kits as well commercial products around.

You don't need an exact-reading meter; most PSU caps with problems have ESRs that are an order of magnitude or more from factory spec. If you own a 'scope, a quick and dirty ESR tester is a matter of a few minutes' work.

 

[Unknown_K]

The problem with measuring caps is you want to do it in circuit, and there tends to be a bunch of caps in parallel on the board. I have been tempted to get an ESR meter but just havn't bothered to do so yet.

 

[Chuck(G)]

That's generally my take. If I'm going to unsolder a mess of capacitors, I might as well replace them and be done with it. Museum-grade restorers may want to preserve the look. But any PSU using wet electrolyte capacitors for filtering will have those capacitors fail eventually.

 

[Unknown_K]

The ESR meters are said to work in circuit which is why people get them. Most of the bad caps I deal with are on old Macs (I just replace them ALL) and a few old PC motherboards (tend to be the ones by the CPU that get cooked by CPU heat) and I replace them all as well. The only things that would need a meter to find individual bad caps would be in monitors and power supplies, both of which I havn't realy looked into repairing yet.

 

[DOS lives on!!]

This is somewhat to the topic, but must capacitors be removed from the board to be tested? I've seen caps being tested, but they were always removed. I'm asking because a motherboard I have shuts off after a few seconds, but has no signs of bulging caps. When I tried testing the caps onboard, the capacimeter showed no reading.

 

[Chuck(G)]

Well, consider that most motherboards have many caps in parallel. How the heck are you going to figure out which is the bad one? They're generally connected by wide traces (if they're not connected to supply and ground planes in a PCB, so the inter-capacitor resistance is probably in the low milliohms range.

 

[modem7]

This is somewhat to the topic, but must capacitors be removed from the board to be tested? I've seen caps being tested, but they were always removed. I'm asking because a motherboard I have shuts off after a few seconds, but has no signs of bulging caps. When I tried testing the caps onboard, the capacimeter showed no reading.

Capacimeter

A capacimeter - a meter that measures the capacitance property of a capacitor. Yes, in most cases, you won't get a reliable capacitance reading unless the capacitor is taken out-of-circuit.

ESR Meter

An ESR meter - a meter that measures the ESR (equivalent series resistance) property of a capacitor.

Why use one instead of a capacimeter? Quote: "Why not use a readily-available capacitance meter? Because when electro go faulty, they normally don't lose their capacitance significantly (as many technicians assume they do). Rather their equivalent series resistance (ESR) 'goes though the roof'. Capacitance meters don't tell you this; about the best they can do is give a low reading if the electro is nearly open circuit."

In a lot of cases, the ESR of a capacitor can be measured with the capacitor in-circuit. That is due to the way that the ESR is measured, and the fact that a capacitor's ESR is a low value (comparatively speaking).

More information is in the document [here].

In the document, note the, "... it can't identify leaky or short-circuited capacitors."

 

[Chuck(G)]

However, to add to what I said, if you know the sum total of the capacitance connected on a motherboard (do a head count of the capacitors sharing a line and add the capacitance up), you can at least verify that the total capacitance is or is not where it should be. No meaningfrul ESR readings in circuit, nor the identity of the single capacitor(s) that are at fault.

Large capacitors in power supplies, I treat like headlight bulbs on a car. If one headlight lamp goes out, you replace them both because sure as shootin'', the good one will go bad shortly after you replace the bad one. Might as well save some effort.