Dangers of Computer Power Supply Capacitors
The dangers of computer power supply capacitors fall into two broad categories: stored energy and electrolytic breakdown. The capacitors used in computer power supplies include larger-value capacitors used in the filtering steps after rectification, where the 110 VAC input voltage is stepped down and filtered for smooth DC use. These larger-value capacitors will most often include electrolytic capacitors that use an electrochemical reaction as part of normal operation, but they are sometimes given to leakage or even bursting.
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Capacitor Construction
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The simplest capacitor is two metal plates separated by an air gap. Air is the dielectric, and charges build on the plates.
Most capacitors are two strips of metal foil (the plates) separated by a paper or mica layer (the dielectric).
For higher capacitance versus capacitor size, electrolytic capacitors use a liquid or paste that forms an oxide or gas layer for a higher dielectric effect. This electrochemical reaction is based on the capacitor seeing only the correct polarity of voltage (positive voltage on the + side and negative voltage on the - side).
Capacitors in Power Supplies
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After input AC voltage is rectified, the half-wave ripple voltage is run through a filter stage, where the half-sine waveform is converted into a DC level with a ripple, which is further smoothed by later filtering devices.
The capacitors doing most of this smoothing function absorb the high-voltage swings and return energy to the system, averaging out the voltage.
The main filter capacitors must have high capacitance (storage) value as well as a voltage rating above any likely spikes or transients the supply would see. In most commercial power supplies, those large capacitors are electrolytic types.
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Identifying Electrolytic Capacitors
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The power supply electrolytic capacitors will most often be metal cylinders wrapped in blue or yellow plastic with the positive (+) lead marked. They may be as small as pencil erasers or up to D-cell battery size or larger.
In most power supplies, they will be near the power supply transformer and rectifying diodes.
Stored Energy
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The most serious danger a capacitor offers is part of its function. A capacitor stores electrical charge. Even if power to the computer has been turned off, the power supply capacitors may still retain enough charge to give an unwary technician a jolt.
Unless a capacitor has been isolated from the rest of the supply (for instance, by a cut wire or burned circuit card trace), the charge will bleed off into the filter in a minute or less. Applying a metal screwdriver across both leads will short out any charge safely.
Small disk-type capacitors carry too little charge to be a threat. The primary concern must be the power supply electrolytic capacitors. Test for charge, or ground before touching or soldering/desoldering.
Electrolytic Leakage
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Since electrolytic capacitors are foil and fluid or paste-filled cans, pressure may build up inside the can and cause the capacitor to leak or vent. Electrolyte leaking from the capacitor is corrosive, causing skin irritation, damage to eyes or mucus tissues and damage to circuit cards. This should be wiped away with a cloth or wet-wipe, taking care to avoid contact with the eyes. Wash your hands before eating.
An old, reverse-polarized capacitor or a capacitor exposed to excessive heat may also burst or vent the gases formed by the electrochemical process. This gas is an irritant to eyes, nose and throat. The quantity will be small, and a few minutes of ventilation should suffice before cleaning the power supply and computer cards.
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