Residual charge on electrolitic capacitor under operating value, deteriorate device ?

[andre_luis]

[SOLVED]Electrolitic capacitor is deteriorated w residual charge under nominal value?

Hi friends,


Due to specific topology used on the circuit, an electrolitic capacitor will retain about 1.3v over plates ( that´s the diode drop voltage ).
Does it is relevant to think about put a high value resistor to drain the charge after long time ?

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[betwixt]

When the voltage is likely to cause harm it is normal practice to connect a 'bleed' resistor across it to allow a small current to flow and discharge it. However, 1.3V is unlikely to be dangerous so unless there is a risk of it being shorted to something else, it may not be worth adding one. The capacitor will discharge through it's own internal leakage after a while anyway. The choice is yours...

Brian.

 

[jpanhalt]

Does it is relevant to think about put a high value resistor to drain the charge after long time ?

Depends on what you mean by "long time."

Maybe the small voltage that you are seeing is beneficial. Electrolytics may need to be "reformed" after storage for long periods completely discharged. That small charge may help delay that degradation. (long = few years)

John

 

[andre_luis]

The question was raised because I heard long time ago, that Electrolitic Capacitors must operate under a range of voltage ( min / max ), due to excedding both limits, can damage it.
It was pointed - for instance - that using that device bellow 15% Vnominal, could damage capacitor in log term operation.

This information was presented to me, at technical course, about 20 years ago, but I don´t know if fabrication technologies is the same.



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[betwixt]

I think the min/max voltage still applies but isn't of any significant consequence. The problem with exceeding maximum is of course gassing and failure of the dielectric but the only problem with working at minimum voltage is gradual adaptation to that voltage. Put a different way, if you used a 10V rated capacitor at 5V for a long time it might need 'reforming' to be safe at 10V again. I think this only applies to wet electrolytic capacitors, as far as I know it isn't necessary to reform solid dielectric types. Ask yourself how many times you have switched on the power to something new, the voltage would have gone from zero to full, have you ever had to raise it slowly to allow time for capacitors to reform?

If in doubt, add a resistor, the cost is almost nothing.

Brian.

 

[jpanhalt]

The point you make about solid tantalums not needing reforming is certainly often stated, and using tantalums at lower than rated voltage (derating) to extend life would seem consistent with it as well.

However, according to this patent, the same claim was made erroneously for wet tantalums.

Source:Reforming wet-tantalum capacitors in implantable defibrillators and other medical devices - Cardiac Pacemakers, Inc.

Because reform expends valuable battery life, manufacturers developed wet-tantalum capacitors, which are generally understood not to require reform. Yet, the present inventors discovered through extensive study that wet-tantalum capacitors exhibit progressively worse charging efficiency over time. Accordingly, to address this problem, the inventors devised unique reform techniques for wet-tantalum capacitors.

Are there any reports actually showing that solid tantalums do not need reforming after long periods?

John

 

[andre_luis]

...only problem with working at minimum voltage is gradual adaptation to that voltage...

Brian,

So, the raised question really seems make sense.
Despite minimum voltage to be not a significant problem, it must to be considered at residual charge, wright ?
( I mean : A discharging resistor must be planned ? )

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[betwixt]

I would say probably not, there is a difference between 'residual charge' and using a capacitor at less than rated voltage. The residual voltage is what is left when the capacitor has been used at higher voltage but the source of voltage has been removed. As it discharges, the current flows from it into the surrounding components and a little into it's own internal leakage, the 'residual' part is what's left when the external current stops flowing because the voltage is too low to make the components around it conduct. An example would be a circuit containing silicon semiconductors where they cease to conduct when the voltage across them goes below about 0.6V. Bleed resistors are used across capacitors because they conduct at all voltages so they can discharge any residual amount eventualy.

Bear i mind that the memory effect of an electrolytic capacitor may take several years to show, in general only capacitors that have been in storage for say 10 years or more may need to be reformed. I have seen capacitors dating back more than 50 years still work perfectly when full voltage is put straight across them.

If in doubt, add a resistor, it probably won't make any difference but to give you peace of mind! Electrically, the effect is almost nothing.

Brian.