[SOLVED] High ESR Capacitors can cause "no power" ???

Hello all...I am working in industry and recently I found one of my controller assembly from the machine cannot power up. So i removed this certain controller and did a basic troubleshooting. This controller can be power-up by 110Vac and it consist of 1 back-plane pcb with 5 pcb modules plug-in on it. What i did was I removed all 5 pcb modules at a time and power-up. But still cannot power-up even I already removed all the 5 pcb modules. What's left is the backplane pcb. I noticed that the backplane have 8 capacitors soldered on it. So i decided to removed all caps and power-up...so it CAN POWER-UP already. Then right away i measured all caps and found all 8 have high ESR up to 100ohm. I compared with the good one which only have <1ohm...(caps specs are 100uF, 25V)... I am happy in that moment I confirmed it was the capacitors were faulty. However, when I soldered back the original high ESR caps it can also power up. Then this makes me confuse....Hope some experts can explain to me how the high ESR caps can cause no power, but when you removed and soldered it back it can power up again. Thanks in advance.

Looks mysterious.

It is possible that somehow there was some "cold-solder" and in course of time it has come loose.

It is possible that desoldering and resoldering corrected some soldering "error".

This is perhaps the simplest explanation...

yes, an age / soldering issue most likely, not around the caps but around other connections to the backplane ...

A capacitor with high ESR will also normally have a very high leakage current, at the voltages your multimeter gives out the leakage looks alright but if you power the capacitor at its rated voltage then the leakage current can be very high, it is this effect that stops most circuits from working not the high ESR.

the point that the ckt worked with all the caps out shows it to be a soldering / connection problem elsewhere, flexing etc the backplane made it work again ....

Hi,

There is a lack of information.
* is it a conventional 50/60Hz supply or an SMPS
* at which node(s) are the capacitors --> schematic
* are they all connected in parallel?
* how did you detect and what did prevent from power power up? Did it oscillate? Is there an overvoltage protection circuit that gets activated?

Desoldering capacitors ... and then power up a circuit may cause damage. Immediate or long term. Thus I recommend not to do this.
Capacitors wit ESR of 100Ohms are dead.
--> Throw them away.

With ageing the ESR of a capacitor may rise (especially of wet electrolytics). This is a long term process.
The ESR rises until the circuit fails to operate correcty. This is condition you see. The circuit is at the edge to operate or not.
Any tiny improvement may cause the circuit to operate.
The improvement may be caused by: temperature (soldering), movement/ vibration...

Klaus

All 8 are DC capacitors...I think 2 or 3 of them are the only connected in parallel...some are independent...really appreciate if some can give tips on how calculate the current consumed by high capacitors...or simple circuit i can experiment to measure current..see attach image for the photo of back plane pcb (1 cap not accessible to view).