Question

Safety Considerations for Electrolytic Capacitors

Investigate safety considerations when dealing with electrolytic capacitors on the internet and in your textbook. Include a discussion regarding how to avoid safety concerns when dealing with charged electrolytic capacitors as well as design considerations to avoid damage to circuits utilizing these type of capacitors.

Answer 1

What are electrolytic capacitors?

An electrolytic capacitor is a polarized capacitor which uses an electrolyte to achieve a larger capacitance than other capacitor types.

An electrolytic capacitor is a type of capacitor that uses an electrolyte to achieve a larger capacitance than other capacitor types. An electrolyte is a liquid or gel containing a high concentration of ions. Almost all electrolytic capacitors are polarized, which means that the voltage on the positive terminal must always be greater than the voltage on the negative terminal. The benefit of large capacitance in electrolytic capacitors comes with several drawbacks as well. Among these drawbacks are large leakage currents, value tolerances, equivalent series resistance and a limited lifetime. Electrolytic capacitors can be either wet-electrolyte or solid polymer. They are commonly made of tantalum or aluminum, although other materials may be used. Supercapacitors are a special subtype of electrolytic capacitors, also called double-layer electrolytic capacitors, with capacitances of hundreds and thousands of farads. Aluminum electrolytic capacitors have a typical capacitance between 1µF to 47mF and an operating voltage of up to a few hundred volts DC. Aluminum electrolytic capacitors are found in many applications such as power supplies, computer motherboards and many domestic appliances. Since they are polarized, they may be used only in DC circuits.

Polarity and safety

Due to the construction of electrolytic capacitors and the characteristics of the electrolyte used, electrolytic capacitors must be forward biased. This means that the positive terminal must always be at a higher voltage than the negative terminal. If the capacitor becomes reverse-biased (if the voltage polarity on the terminals is reversed), the insulating aluminum oxide, which acts as a dielectric, might get damaged and start acting as a short circuit between the two capacitor terminals. This can cause the capacitor to overheat due to the large current running through it. As the capacitor overheats, the electrolyte heats up and leaks or even vaporizes, causing the enclosure to burst. This process happens at reverse voltages of about 1 volt and above. To maintain safety and prevent the enclosure from exploding due to high pressures generated under overheat conditions, a safety valve is installed in the enclosure. It is typically made by making a score in the upper face of the capacitor, which pops open in a controlled manner when the capacitor overheats. Since electrolytes may be toxic or corrosive, additional safety measures may need to be taken when cleaning after and replacing an overheated electrolytic capacitor.

There is a special type of electrolytic capacitors for AC use, which is designed to withstand reverse polarisation. This type is called the non-polarized or NP type.

Applications for electrolytic capacitors

There are many applications which do not need tight tolerances and AC polarization, but require large capacitance values. They are commonly used as filtering devices in various power supplies to reduce the voltage ripple. When used in switching power supplies, they are often the critical component limiting the usable life of the power supply, so high quality capacitors are used in this application.

They may also be used in input and output smoothing as a low pass filter if the signal is a DC signal with a weak AC component. However, electrolytic capacitors do not work well with large amplitude and high frequency signals due to the power dissipated at the parasitic internal resistance called equivalent series resistance (ESR). In such applications, low-ESR capacitors must be used to reduce losses and avoid overheating.

A practical example is the use of electrolytic capacitors as filters in audio amplifiers whose main goal is to reduce mains hum. Mains hum is a 50Hz or 60Hz electrical noise induced from the mains supply which would be audible if amplified.

Safety Considerations

Do not charge by higher current or higher voltage than specified.
Doing so may generate gas inside the capacitor, resulting in swelling, fire, heat generation or bursting.

Do not reverse placement of (+) and (-).
Capacitors have polarity. If the (+) and (-) side of the capacitor is reverse inserted, it may cause short-circuit or over discharge of the capacitor on some equipment and it may induce overheating, explosion or fire.

Do not solder directly to the capacitor.
If soldering is performed directly to the capacitor, the capacitor will over heat and, consequently cause leakage, explosion or fire due to overheating from internal short-circuit.

Do not heat, disassemble, nor dispose of in fire.
Doing so damages the insulation materials and may cause fire, heat generation, leakage or bursting.

Do not discharge by force.
If the capacitor is discharged by direct connection to an external power supply etc., voltage of the capacitor will decline lower than 0 volt (electrical reversal) and will cause the capacitor case to expand, overheat, leak, explode or burn.

Charge-discharge proof

Frequent charging/discharging cycles may lead to a decrease in capacitance. Due to their special design, electrolytic capacitors are charge-discharge proof. This means that 106 switching cycles will cause a capacitance reduction of less than 10% (Charge-discharge test to IEC 60384-4).

Design Considerations:

Polarity

Aluminum electrolytic capacitors are polarized. Never apply a reverse voltage or AC voltage. Connecting with wrong polarity will short-circuit or damage the capacitor with the pressure relief vent opening early on. For circuits where the polarity is occasionally reversed, use a bipolar type of aluminum electrolytic capacitor. However, note that even bi-polar type capacitors must not be used for AC circuits.

Operating voltage

Do not apply an over-voltage that exceeds a rated voltage specified for the capacitors. The total peak value of the ripple voltage plus the DC voltage must not exceed the rated voltage of the capacitors. Although capacitors specify a surge voltage that exceeds the full rated voltage, it does not assure long-term use but limited use under specific conditions.

Ripple current

Do not apply an overcurrent that exceeds the rated ripple current specified for the capacitors. Excessive ripple current will increase heat production within the capacitors, causing the capacitors to be damaged as follows: • Shorten lifetime • Open pressure relief vent • Short circuit The rated ripple current is specified along with a specific ripple frequency.

Operating temperature

Do not apply high temperatures that exceed the upper limit of the category temperature range specified for the capacitors. Using the capacitor at temperatures higher than the upper limit will considerably shorten the lifetime of the capacitor and make the pressure relief vent open. In other words, lowering ambient temperatures will extend the expected lifetime of the capacitors.

Charging and discharging

Do not use capacitors in circuits intended for rapid charge and discharge cycle operations. If capacitors are used in the circuits that repeat a charge and discharge with a large voltage drop or a rapid charge and discharge at a short interval cycle, capacitance will decrease and/ or the capacitors will be damaged by internal heat generation.

Failure mode of capacitors

Non-solid aluminum electrolytic capacitors have a limited lifetime which ends in an open circuit failure mode, in general. Depending on the product type and operating conditions, the failure mode may involve in opening of the pressure relief vent.

Capacitor insulation

Electrically isolate the following sections of a capacitor from the negative terminal, the positive terminal and the circuit patterns. • The outer can case of a non-solid aluminum capacitor. • The dummy terminal of a snap-in type non-solid aluminum capacitor, which is designed for mounting stability

Outer sleeve

The outer sleeve of a capacitor does not assure electrical insulation (except for screw-terminal type capacitors). It should not be used where electrical insulation is required.

Operating conditions

Do not use/expose capacitors to the following conditions: 1) Direct contact with water, salt water or oil, or high condensation environment. 2) Direct sunlight. 3)Toxic gases such as hydrogen sulfide, sulfurous acid, nitrous acid, chlorine and its compounds, bromine and its compounds and ammonium. 4) Ozone, ultraviolet rays or radiation. 5) Extreme vibration or mechanical shock that exceeds limits in the catalogs or product specifications.