An interesting practical point relating to (particularly) high voltage electroylitics.
If you take a cap, say a 220uf 400volt used as a reservoir cap that has had many hours of use at near the rated voltage, then disconnect it and make sure it's fully discharged, and then put it on the shelf... and come back to it days later... you will find that residual charge in the dielectric causes the cap to develop quite a resisdual charge.... perhaps many 10's of volts if you measure with a DVM.
If you take a cap, say a 220uf 400volt used as a reservoir cap that has had many hours of use at near the rated voltage, then disconnect it and make sure it's fully discharged, and then put it on the shelf... and come back to it days later... you will find that residual charge in the dielectric causes the cap to develop quite a resisdual charge.... perhaps many 10's of volts if you measure with a DVM.
Read the manufacturer's datasheets. Electrolytic capacitors are not stable in storage. The dielectric film degrades.
I don't see why the same thing couldn't happen if run for extended periods at very much lower than rated voltage.
Hi,
the manufacturers tell us that the oxide layer degrades with time whether in storage or used at a lower than rated voltage.
The chemistry changes with time and that implies some physical change in the plates and in the oxide layer.
That will lead to variations in the various parameters of the capacitor.
The manufacturers tell us that they reform the capacitor to a set procedure before measuring any of it's parameters.
Do not expect a capacitor to return to specification by burning in. Do it properly. Reform the capacitor before installing it into it's circuit.
the manufacturers tell us that the oxide layer degrades with time whether in storage or used at a lower than rated voltage.
The chemistry changes with time and that implies some physical change in the plates and in the oxide layer.
That will lead to variations in the various parameters of the capacitor.
The manufacturers tell us that they reform the capacitor to a set procedure before measuring any of it's parameters.
Do not expect a capacitor to return to specification by burning in. Do it properly. Reform the capacitor before installing it into it's circuit.
This is something I have been doing for a long time... bring the voltage up slowly to 100% of maximum and after a few seconds slowly reduce back to zero... and no shorting them out
I still leaves the problem of caps that see no DC voltage across them at all in use... I wonder what the long term implications are of circuits that use caps this way.
So you'd recommend burning in caps for an extended period at the rated voltage, then?
Not burning in, but forming. It's even detailed in my 1967 Radio Amateurs' handbook.
Yes, it should always be done with new electrolytics or electrolytics that have been stored for a while. That's one reason that a variac is recommended when first powering up old gear.
The photo flash guys talk about reforming caps all the time. According to them; they sit - they lose capacity.
I've never been able to verify that. But I've had my hands in some studio strobes and they use lots of big, high voltage caps - lots of energy storage. Much more than we typically use in audio. So if the effect shows up, it should show up there.
I've never been able to verify that. But I've had my hands in some studio strobes and they use lots of big, high voltage caps - lots of energy storage. Much more than we typically use in audio. So if the effect shows up, it should show up there.
There is an element of truth in the original poster's quote. I noticed many years ago that if you use a 250v cap in a monostable timing circuit, the pulse width is more inaccurate than expected. From that I deduce that the value depends to some extent on the voltage applied. I did not notice whether it was up or down.
Since then I have avoided the problem by avoiding electrolytics... (and 1-shots)
Since then I have avoided the problem by avoiding electrolytics... (and 1-shots)
The subject incited me to make a quick test: I have a large number NOS capacitors that haven't seen any voltage since they were manufactured.
I tested a 10µ/25V standard Al electrolytic from Philips, manufactured 31 years ago.
Result: 12.62µF (No bias applied). Not too bad for a -20/+80% part.
In my opinion, the rest period has little influence on the capacity itself: the formation process involves voltages much higher than the natural electrochemical potentials.
What happens when no voltage is applied is the increase of the porosity channels. The reformation then "plugs" these channels, which decreases the leakage current but influences little the capacitance, because of the small area of these defects.
The deterioration of photoflash capacitors is an entirely different story, it has to do with the parasitic 'phantom" capacitor inherent to electrolytic capacitors.
I tested a 10µ/25V standard Al electrolytic from Philips, manufactured 31 years ago.
Result: 12.62µF (No bias applied). Not too bad for a -20/+80% part.
In my opinion, the rest period has little influence on the capacity itself: the formation process involves voltages much higher than the natural electrochemical potentials.
What happens when no voltage is applied is the increase of the porosity channels. The reformation then "plugs" these channels, which decreases the leakage current but influences little the capacitance, because of the small area of these defects.
The deterioration of photoflash capacitors is an entirely different story, it has to do with the parasitic 'phantom" capacitor inherent to electrolytic capacitors.
I have to wonder if electrolytic chemistry, or maybe foil processing, has changed (for the better) over the years. I've definitely had old caps that would break down at lower voltages until they were slowly reformed. The leakage current would drop lower and lower. If you stopped at some voltage below the rating, the cap would work fine, but it would also exhibit excessive leakage current again if the voltage was raised up over that value. A power surge could easily wipe it out. It only makes sense to form caps at their full rating. I haven't seen any significant value change that goes with this phenomena. I haven't seen this phenomena with caps in the last decade at all.
CH
CH
63 volt caps on 35v will be stable and just age normally.
Caps that have been stored 12 years. Well I wouldn't use any in a new build... for what they cost it's not worth it... just use them for prototyping and breadboarding etc.
Now what about caps that never see more than a few mv across them during the life of the equipment, such as interstage coupling caps etc... that's the real question
Put it another way... in 20 years of 'doing electronics' I have never, ever come across ANY circuit or description thereof that includes some system that periodically (how can I put it..) applies the working voltage any give electrolytic capacitor to each and every electrolytic capacitor in the circuit to in order to maintian it's long term use.
Yes we all know about the old fasioned paper in oil type capacitors typcially used in valve based equipment failing over time. Also the recent spate of 'bad' capcitors using duff electrolyte from China causing premature ageing..... But capacitance INCREASING if not enough voltage used.... nope! Never heard of that before.
Andy
Yes we all know about the old fasioned paper in oil type capacitors typcially used in valve based equipment failing over time. Also the recent spate of 'bad' capcitors using duff electrolyte from China causing premature ageing..... But capacitance INCREASING if not enough voltage used.... nope! Never heard of that before.
Andy
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