Page 91 - Testing Electronic Components

CAUTIONS – Never use a capacitor into a circuit with higher
voltages than the capacitor is rated for otherwise it becomes hot and may
explode. It’s all right to replace a 0.22µF of 200WV capacitor with one
rated 0.22µF, 250WVDC.

Capacitor Safety
A capacitor can store a charge for some time after equipment is power
off. High voltage electrolytic caps, and even large value, low voltage
electrolytic, can pose a safety hazard. Usually these caps are in power
supplies, and almost have a resistor in parallel with the cap(s), called a
bleeder resistor, to discharge the cap after power is switched off. If a
bleeder resistor is open, or there is none, then the cap can retain a voltage
charge after the unit is unplugged.

A large 75V capacitor in a high power audio amplifier can easily melt the
tip of a screwdriver, accompanied by flying metal sparks if the tool shorts
across the capacitor terminals. This could cause eye injury, burns, or
secondary injury. When in doubt, take time to discharge power supply
capacitors before working on a piece of equipment.
Be sure to connect electrolytic capacitors in the right polarity i.e. negative
to negative and positive to positive. If the electrolytic is connected in
opposite polarity, the reversed electrolysis forms gas in the capacitor. It
becomes hot and may explode. This is a possibility only with electrolytic
capacitors.

How to read capacitor numeric code

The non-polarized capacitor of nominal value of less than 1000pF is
usually plain marked. For instant, for a 220pF capacitor, it will be
marked 220 only. For capacitance values of 1000pF or more, a three digit
code is used. The first two digits represent the two significant digits and
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the third digit represents the decimal multiplier. For instance, 102

represents a capacitance of 10x10² = 1000pF and 104 represents a
capacitance of 10x10 = 100000pF = 0.1µF. Basically it has the same
calculation method as resistor.

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