Hello,i have disconnect 2 x electrolytic capacitors 47uF/50V from one pcb.
With two measurement instruments i read 62uF capacitance. They are defect?

 

Electrolytic capacitors have a wide manufacturing tolerance, typically up to ±25%, so those caps are probably fine.

 

Hello,i have disconnect 2 x electrolytic capacitors 47uF/50V from one pcb.
With two measurement instruments i read 62uF capacitance. They are defect?

As Alec_t states they have wide tolerance, AFAIK: some types can be as much as +100%.

Aluminium electrolytics are "formed" - the forming current grows an oxide layer on one of the plates which is actually the dielectric. Forming thicker oxide gives you a higher breakdown voltage rating, but decreases capacitance. If the capacitor is left a long time with no voltage applied, the caustic electrolyte etches the oxide layer - capacitance increases, but you lose voltage rating. Its actually the normal leakage current that maintains the oxide layer.

It is entirely possible that a capacitor that has increased capacitance can no longer withstand the voltage it says on the can. dumping the full rated voltage on it will likely cause a leakage current hot spot. The capacitor could build up a head of steam and chuck its innards at you.

Forming the capacitor requires bringing it up to voltage with a safe current limited supply, it takes around 24h - within reason, you can trade off capacitance for a little more voltage rating.

 

Thank you very much for your information.

 

If the capacitor is left a long time with no voltage applied, the caustic electrolyte etches the oxide layer - capacitance increases, but you lose voltage rating. Its actually the normal leakage current that maintains the oxide layer.

Interesting. So all those capacitors in my junk pile should be attached to a power supply?

 

Interesting. So all those capacitors in my junk pile should be attached to a power supply?

None of mine are - but I don't just assume I can bang full rated voltage on them after they've been stored a while.

A company I worked for that made capacitor discharge stud welders routinely "formed" freshly built capacitor banks. They probably could've got away with not bothering - but an employee getting a face full of capacitor foil wouldn't look good on the profit/loss ledger.

 

What about supercaps? I have some 2.7V 500F -20/+80% ones that have been lying around for about a year. I do not yet have a way to charge them in a reasonable amount of time. So they have not been used once. Is that an issue?

 

What about supercaps? I have some 2.7V 500F -20/+80% ones that have been lying around for about a year. I do not yet have a way to charge them in a reasonable amount of time. So they have not been used once. Is that an issue?

Really? One Amp will charge then in 22 minutes. Of course you do need something to ensure they will not overcharge.

Bob

 

What about supercaps? I have some 2.7V 500F -20/+80% ones that have been lying around for about a year. I do not yet have a way to charge them in a reasonable amount of time. So they have not been used once. Is that an issue?

We're talking about formed aluminium oxide dielectric - I don't think supercaps have any of that.

Doesn't mean you shouldn't read the data sheet and see if there's any safety critical handling procedures though.

 

Really? One Amp will charge then in 22 minutes. Of course you do need something to ensure they will not overcharge.

Bob

That is still a very long time if you are going to be discharging them within a few seconds. And is it actually possible to overcharge them if you use a voltage less than the rated voltage?

We're talking about formed aluminium oxide dielectric - I don't think supercaps have any of that.

Doesn't mean you shouldn't read the data sheet and see if there's any safety critical handling procedures though.

Umm....

I got them on amazon a while back and do not have the link anymore. I think they might say the part number on them, but I don't think they do. Is there anything other than overvoltage and reverse polarity to look out for, though?

 

All it takes to deliver enough charge to re-form a capacitor is a supply that will provide enough current to bring the voltage up to the required value. That is usually simple and sometimes easy. The important part is to limit the voltage so that it does not get above the posted value.

 

Hello,i have disconnect 2 x electrolytic capacitors 47uF/50V from one pcb.
With two measurement instruments i read 62uF capacitance. They are defect?

What are the two instruments you used to measure the capacitors? Are they the type of instrument which allows you to choose the measurement frequency? If so, at what frequency did you make the measurement?

 

Beware cheap capacitors from China. When I buy PCBs/modules/PSUs for projects I always replace the standard "wingwongcatpoo" capacitors with something more reliable. I've seen brand new 1000uF capacitors that measured as low as 600uF on a calibrated capacitance/ESR meter.

 

... I always replace the standard "wingwongcatpoo" capacitors with something more reliable.

Best laugh of the day so far!