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 (Adobe PDF file) 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 R1 - R3 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.