Hobbies And Interests
Power Supply Circuit
Electronic devices run on direct current, or DC, in which electricity flows in one direction and at a fixed voltage. Electricity in a household AC outlet flows rapidly back and forth at 60 hz. A power supply solves this problem by converting AC to DC. In the power supply circuit, capacitors remove electrical noise, providing cleaner power to the device it powers. Capacitors work by storing electric charges; they can retain a voltage long after disconnection from the AC outlet. If you touch the capacitor's terminals, the stored voltage will pass through your skin, giving you a shock.
Linear and Switchmode
Linear power supply designs have capacitors only for the DC part of the circuit. Converting AC to DC leaves a 60 hz. "hum" in the signal; if left alone, it causes problems with the DC-powered device. It also produces an objectionable low-pitched noise in audio equipment. The capacitors smooth the 60 hz noise from the DC. Circuits such as these use capacitors mostly for low-voltage DC, which is safe. Other power supplies, called switchmode, use a capacitor on the 110-volt AC side of the circuit. The 110-volt value is an average used by electricians and engineers; its actual voltage varies between -150 and +150 volts along the 60 hz wave. When unplugged, the AC capacitor retains some of this voltage and may give a painful shock.
Hazardous Voltages
The voltages produced by many power supplies runs between three to 24 volts and are not harmful. Older power supplies, especially those used for vacuum-tube equipment, have voltages ranging from 300 to 500 volts. A capacitor carrying this voltage is hazardous. In older television sets and computer monitors with a cathode-ray tube, one of the power supply's terminals carries thousands of volts. A trained technician knows which parts he can safely touch and which he avoids.
Discharging Capacitors
The voltage in the capacitors of a disconnected power supply are not permanent; they can deliver a strong jolt, but after you discharge them, they contain no electricity. A discharging device made of an insulated wire and resistor allows a technician to safely remove the electricity from the capacitor, rendering it safe.