Fault on mains voltage electrolytic capacitors

Thread Starter

Willen

Joined Nov 13, 2015
281
I was repairing a SMPS. A 33uF400V capacitor leaked its liquid all over the supply and made the fuse burn. There was no newer one. I scrapped/borrowed few such capacitors. Some two of them shows nothing in my DMM's capacitance meter. One 33uF used to show just 8.5uF in the DMM however I used that in the SMPS and got BOOM within a minute!!!

Another one with 33uF was showing 13uF in the DMM and I used that one. It's working from an hour with 800mA (around 1A) load and getting little hot. Not more. I also had measures some high voltage used caps and they were showing very low capacitance in the DMM. Other low voltage (eg, 10uF 16V) shows pretty nice and close to its rating.

Question is: is it normal to get very low capacitance with high voltage electrolytic capacitors or they seems partially damaging capacitors?
 

AlbertHall

Joined Jun 4, 2014
8,498
It is not normal.
Does your meter read higher value, e.g. 100uF, low voltage capacitors correctly?
If not you may be at the limit of what your meter can read. What make/model is your meter?
 

Thread Starter

Willen

Joined Nov 13, 2015
281
It is not normal.
Does your meter read higher value, e.g. 100uF, low voltage capacitors correctly?
If not you may be at the limit of what your meter can read. What make/model is your meter?
Hi Albert,
My DMM have 2nF to 200uF rage of capacitance (CX) to measure. There's a pot inside of the DMM to calibrate capacitance so I calibrated the accuracy with some 1% capacitors so now it can display little accurate value. When measuring 100uF 16V electrolytic, I get around 85uF to 90uF. But I get little more accurate reading in tantalum capacitors and film capacitors. Thus capacitors measurement is OK in my DMM.

Model of my DMM is DT9205A: http://www.yourduino.com/sunshop/images/products/large_150_DT9205A-1-600.jpg A $5 DMM. :)

Hi Bertus,
The high voltage capacitors I am talking about are not old but I found in many SMPS, the same capacitors are being faulty and making switching IC and fuse burn. Thank you for the ebook. Maybe I need to make a DIY ESR meter found around internet.
 

bertus

Joined Apr 5, 2008
20,085
Hello,

When measuring capacitors with a DMM, be sure there is no voltage on it.
This is also likely given in the DMM's manual.

DT92XX_capacitor_measurement.png

Bertus
 

Thread Starter

Willen

Joined Nov 13, 2015
281
Hello,

When measuring capacitors with a DMM, be sure there is no voltage on it.
This is also likely given in the DMM's manual.

View attachment 120298

Bertus
Yes, high voltage capacitor of some faulty SMPS have stored deadly voltage if the SMPS has no bleeder resistor. So to save DMM from the deadly voltage I touch pins by fingers and check first (the foolish test mistakenly happened with me few times). :) Then I short pins with metal and then only I test with DMM.

Wayneh,
I will collect ideas from the links too.
 
Top