Power Lamp Indicator
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
This article describes a low cost circuit which detects any failure incoming from a two-secondary, low voltage power plug transformer. The failure, i.e., any wire discontinuity or short circuit between two leads of the two-secondary transformer, is detected by means of a simple circuit made up of a bipolar transistor, a light-emitting diode (LED) and a few resistors. As long as the power plug transformer is good (no open wire, no short circuit between two loads) the transistor conducts for about each half period of AC, and the LED is lit. Otherwise, the LED does not light. D1 and D2 protect the transistor against reverse voltages. Z prevents the LED from lighting when the middle point of the transformer is opened. R1 and R2 adjust current in the LED. Typical values are: R1= 390 ohms, R2= 750 ohms, R3= 51 K ohms.