Does an electrolytic capacitor degrade each time it receives reverse voltage?

Question

I know that I mustn't connect an electrolytic capacitor reversely. It will explode if I apply the reverse voltage long enough.

But, what happens if the reverse voltage is applied for a short time? For example, a fault occurs in the circuit and the capacitor becomes reverse biased or exposed to AC voltage for a short time, but it still looks OK from outside. Does the internal physical and chemical structure of the capacitor change permanently? Would it still have the rated capacitance, voltage and life time? Is it okay to keep it using if it doesn't explode? I feel that the answer is "no", but I'm looking for an explanation for it.

Answer 1

There are many types and subtypes of electrolytics, I will concentrate on those most common for hobbyists: the liquid electrolyte aluminum one.

Most details can be read in the corresponding wikipedia article but in short:

• The cathode oxide layer can withstand up to 1.5V of reverse voltage
• it can receive permanent damage earlier, but 0.5V is deemed safe

When applying too high reverse voltages for too long, one of two things can happen:

• gassing makes the cap explode
• dissolving of the oxide layer shorts the cap

These numbers and probability of failure mode can greatly differ when going far outside of what is considered "room temperature".

Even short application of too much reverse voltage can permanently damage the capacitor. Sometimes the self healing abilities can reverse this a bit over time, but there will be permanent damage.

Some ways to see the damage is:

• increased leakage current (due to partly dissolved oxide layer)
• decreased capacity (due to self healing of oxide layer holes)

Other types of electrolytics have different kinds of behaviour, but most people deem short exposure of up to 0.5V reverse voltage ok. This is why a lot of LCR meters measure capacitance with 0.5V AC regardless of any polarity.

Answer 2

Electrolytic capacitors can withstand for short instants a reverse voltage for a limited number of cycles. In detail, aluminum electrolytic capacitors with non-solid electrolyte can withstand a reverse voltage of about 1 V to 1.5 V.

Solid tantalum capacitors can also withstand reverse voltages for short periods. The most common guidelines for tantalum reverse voltage are:

• 10 % of rated voltage to a maximum of 1 V at 25 °C,
• 3 % of rated voltage to a maximum of 0.5 V at 85 °C,
• 1 % of rated voltage to a maximum of 0.1 V at 125 °C.

These guidelines apply for short excursion and should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently.

More on Wikipedia

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What you're talking about mostly happens in ac where for a short period of time a reverse voltage is applied and then a positive voltage immediately after that to reverse the small damage.

Reverse polarization does not occur so fast enough to damage the capacitor permanently. Time for it to get damaged depends on the reverse voltage applied, size of the capacitor and the material used for the dielectric and the electrodes.

Generally when used in ac, most common application being the filter, I have not see capacitors sustaining damage sufficient enough to interfere with the operation. Capacitors are used in filter in dc chargers and we use them every day, and they work for years. When used in such applications, there could be slow damage and oxide layer could form, but no hindrance in the normal operation.

Although, frequent transient voltages can damage the capacitors quite fast. That's why, always when turning off any device, turn down the volume, switch off the device and then remove the ac power plug.

Answer 3

If you feel that you circuit may have a failure mode like this (Holy cow - at the places where I rely on the polarity of electrolyte capacitors reversing the polarity would usually fry the whole circuit!), it would be the best to

• consult the data sheet of the specific part, not wikipedia on reverse voltages of the specific cap you are using
• use a bipolar electrolyte capacitor
• take protection measures (diodes or diodes + fuses) against external polarity changes/battery flips
• use voltage watchdog circuits to only power up the electronics if all supplies are ready - and power down fast if one fails (i observes a really weird failure mode where the loss of the negative rail lead to a input clamp diode of an opamp sending the positive rail to the supposedly high impedance input....)

Never make that a normla oprating mode