One day i went to my refrigerator and my compressor was working like it was being turned on-off really fast then internal klixon cuts the power to compressor so i tought compressor died, i searched the web to see how i can wire it directly to mains and so i did and it was working fine. then i checked the electronics first thing that came to my mind is that power supply of the electronics has gone bad and it was one of those capacitor power supplies so i said great ill measure the capacitor and it will probably be half its value but it was 1uF spot on. and this board is as simple as it gets 5v supply for the micro controller, triac for compressor, voltage divider for NTC probe with potentiometer to set temp. i have used those cheap light dimmer circuits to check triac it was ok, 5v supply with mutimeter ok, so i said oh great micro controller has gone bad but then in desperation i hooked the 5v rail to oscilloscope and it had some weird shape 50hz i don't even know how to call it wave so i went to measure electrolytic it checked ok on capacitance and since i don't have esr meter i just checked it with multimeter set to ohms and it checked ok. i checked 5v1 zener out of circuit it was ok. now it was really getting weird i put everything back on the board and supplied 5v from my bench PSU and board was drawing steady 11mA and as soon as i hook up 220v to it the power supply is not working properly so i bought 1uf 275VAC changed it, not working, changed electrolytic and what do u know board started working.
now my question is what went wrong with electrolytic cap so that when i applied 5v directly across it, it was working and when its in circuit it was failing and what instrument is capable of measuring this fault esr meter ? capacitor was looking brand new no bulges or anything
here is the board in question
http://www.supremeplumb.com/imgs/fridge_freezer_spares/BSH265109.jpg
The electrolytic was probably for smoothing the DC supply - i.e. between mains cycles. Over time, electrolytic can dry-out and lose most of their capacitance - so they no longer smooth the DC over an entire cycle . So whatever is on the DC is supplied with pulses of of rectified mains - not good for something that expecs smooth DC.
capacitor is marked 35v 680uF and it measured 692uF so in terms of capacitance it wasn't bad imo, maybe the esr meter would show some interesting reading but like i said it wasn't showing any signs of leak/bulging and i knew it was for smoothing but something tells me that esr will be in normal limits. ill just have to find a way to measure it without investing in esr meter
i guess you could use 1vpp 200khz square wave which is not that hard to make and use oscilloscope since i own one to see if esr is bad or not like in this video ( i read wiki post that u linked it suggest this method also)
capacitor is marked 35v 680uF and it measured 692uF so in terms of capacitance it wasn't bad imo,
I may have mis-interpreted your original post. The capacitor does appear to measure good (could still be ESR, but I'm not convinced). Is it possible that there was a bad/poor solder connection to the original capacitor?
@Andy Watson
i desoldered it to make measurement so its safe to say its not bad solder joint i am really interested what has gone wrong with it maybe there is something interesting to it
I just repaired a backup power supply. The caps measured good on a DMM but its symptoms made me suspect they were in fact bad. I replaced them and it was as good as new. I think the DMM can tell you if they are bad but not if they are good, if that makes sense.
I tried the above method after the fact and identified which ones were bad.
Welds can always be an issue with capacitors creating a high ESR. I seem to remember electrolytic capacitors are only a crimp to the foil. ESR is a primary indicator for capacitor failure. Capacitance can even increase before failure.
I went through a similar issue years ago with my refrigerator: the filter capacitors of a switching power supply required replacement.
As you noticed (and I did at that time as well), the capacitance alone is not enough to evaluate the remaining lifetime of a capacitor, especially because its value also varies with DC bias and frequency. The DMM capacitance measurement usually uses a very similar mains frequency (100 or 120Hz) but it does not apply a DC bias nor measures the Dissipation factor (D).
The reason I mention D instead of ESR is because capacitor's manufacturers specify it on their datasheets, therefore a LCR that gives you this value will allow you to compare the existing capacitor with a brand new one.
Physically I suspect the dielectric itself has changed or microscopically vented to the point that either the Dissipation factor (D) or the capacitance reached unacceptable levels for the circuit. In other words, it was not able to remove enough of the AC component of the input signal.
Good reads:
http://low-esr.com/QT_LowESR.pdfhttps://www.eevblog.com/forum/beginners/esr-meter/msg108683/#msg108683https://www.eevblog.com/forum/beginners/beginner-questions-about-bypass-capacitors-and-power-supplies/msg136070/#msg136070