Measuring CAPs in circuit? - Page 1
Forum section: electronics / beginners

author: mortos360
author_rank: Regular Contributor
timestamp: December 13, 2012, 03:28:08 pm
content: My network switch died few days ago and since I have 2 of them, I figured quick out that the PS was dead.Since CAPs seem to be 1 of the main problems with most electronicsI went and compared the values of the CAPs on the 2 PSs, broken vs. functional andfound out quick that 1 CAP had no reading at all in my Multimeter. So I replaced it with a new CAP and switch is working fine again.My question is, was I just lucky that this cap had no reading or is it OK to measure CAPs in circuit?Second question is, can you trust a multimeter or do you really need a ESR meter?I am very new to electronics and this was the first time I even tryed to measure a CAPbut since my Agilent U1272A had this capability, Ithought why not

author: PhynixViking
author_rank: Regular Contributor
timestamp: December 13, 2012, 04:22:21 pm
content: The correct way to measure caps is to remove them then test them. Which is not what you want to hear, lol. I've been repairing things for many years and yea... dead caps come in the shop every day so much that when we get a computer we check for blown caps (both on the motherboard and power supply) even if the computer turns on fine and boots fine. So many time one will be bad when not all are.Now,I knowsomeone is going to post about not doing what I'm about to tell you to do... and how its not right, or w/e, butits worked PERFECT (almost every time) for me.What I do is I take my multimeter and put it to the continuity setting and test on Pos and Neg of a capacitor.I listen for the beep and estimate how long, then I reverse my probes and listen again.If the beeps are about the same, normally that capacitor or set of capacitors are working.If the beep continues (normally it should beep-hold, then stop) then its shorted and most likely a capacitor or many are bad.If no beep at all, either way, (try a few times) then again maybe capacitor is bad.Also, if beep is longer one way and not the other, it could be bad too.IN SHORT: If beeps are same duration both polarities, then cap(s) should be good.WARNING: Discharge capacitors before doing this, and make sure your multimeter has a fuse in it, just in case!Just as a rule/safety I never test mains filter caps (above = or above 100v) only test on electrolytic low power (less then 30v) caps.Hope this works for you and helps you! ^_^I also wanted to comment that I made myself a nice decade capacitor recently that I use for testing things. I made a forum post on here explaining it and a couple pics. (PS: A lot of my stuff is low-budget so.. mind the crude design, LOL)https://www.eevblog.com/forum/projects/electrolytic-decade-capacitor-low-budget-project-__/

author: T4P
author_rank: Super Contributor
timestamp: December 13, 2012, 04:32:43 pm
content: No reading as in completely 0uF? (Shorted) Or OL? (Dielectric distance increased for some reason)I would trust a meter for general capacitance measurement but the important figure is ESR so i turn to a LCR for that but sometimes there's another figure and that's DC Leakage but hey

author: mortos360
author_rank: Regular Contributor
timestamp: December 13, 2012, 05:35:40 pm
content: Quote from: PhynixViking on December 13, 2012, 04:22:21 pmheh.. thx for the tip!definitely gonna give it a try next time I am searching for a dead capacitor!!