You may sometimes need to turn on a secondary device, such as a lamp or an alarm, when a device that is normally on loses power. You can build a simple circuit using just a transformer and a relay for this purpose. In the circuit, a primary load is in series with an ac-mains transformer (Figure 1). The transformer connects in an unusual way. Its usual secondary low-voltage winding is Winding 1, and its primary ac-mains winding is Winding 2. Under these conditions, the main lamp's voltage is slightly less than during its ordinary operation—the ac-mains voltage minus the voltage drop over Winding 1. That situation is acceptable in most cases because the lower voltage doesn't greatly affect the operation of the load—that is, the luminosity of the main lamp. Select Winding 1 to match the main load's current needs. In this circuit, a 220V, 50-Hz ac voltage appears at Winding 2.

Connect a relay to Winding 2 so that the secondary loss connects to the relay's NC (normally closed) terminal. Use a relay with a winding that can operate at 220V, 50 Hz for your ac-mains voltage. For example, you can use a TR91-220VAC-SC-C relay from Tai-Shing Electronics Components Corp. This relay's coil operates at a 220V, 50-Hz, SPDT (single-pole/double-throw) commutation of 240V ac under a 40A load.

Using an SPDT relay adds flexibility in controlling the spare load. It lets you switch a load on or off with no need for additional electronic components. In the figure, a spare lamp turns on when the main lamp burns out because the secondary load connects to the relay's NC contact.

Select a transformer whose secondary winding (Winding 1 in the figure) has a low-rated voltage that provides sufficient current for the main load—the lamp. Match the relay's rated coil voltage to the ac-mains voltage and frequency specifications.