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MINIATURE OFF-LINE SUPPLY
DELIVERS LOW POWER DC
(June 9, 2000)
designed by David A. Johnson, P.E.

Often a low power circuit needs to be powered from the 120vac line. Usually, the circuit only needs a fraction of one watt and requires good isolation from the high voltage power line. The traditional circuit usually chosen is a classic linear regulator consisting of a small iron core transformer, a bridge rectifier, a filter capacitor and a voltage regulator. Although such a scheme provides the needed power and isolation, the assembly is seldom as inexpensive and as small as desired. The circuit shown strives to keep both the size and cost of a line powered DC supply small.

The circuit transfers power from the 120vac line to a voltage regulator circuit by discharging a capacitor through a small high frequency transformer, twice each power line cycle. A bi-directional discharge circuit, consisting of two small SCRs (Q1 and Q2) and two current steering rectifiers (D1 and D2), provides the needed power switching while a transformer furnishes isolation and voltage reduction. The resistor divider network consisting of R1R3 defines a voltage trigger point of about 140 volts for the two SCRs. Each time the capacitor C1 is discharged voltage spikes are induced in the primary winding of the transformer T1. The pulses are translated to the transformer secondary, where they are rectified, filtered and regulated. An inexpensive three terminal regulator (A1) provides voltage regulation. With the components shown the circuit supplies an output voltage of 12 volts with a maximum current of 15 milliamps.

 
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