What economical device - varistor - transistor etc can you use to supply power to a relay until the voltage drops to 10.5 volts?

Answer A zener diode, a transistor, a rectifier diode, and a few resistors. A 10.5V zener diode will conduct until the voltage across it drops to below 10.5. If the relay is small enough, a zener diode is all you need. However, more than likely the current draw of the coil would burn the zener out.

Use the zener to drive a transistor, and use the transistor to trigger the relay V 10.5V | | /___\ | 3|| Relay 3|| Coil 3 0---------- /___\ 3 3 4.7K | | / 2.2K / |____ ___| \ \ | / / | | | B | / C 2N2222 E When the voltage is above 10.5V the zener will conduct ant turn on the transistor. The transistor will power the relay. When the voltage drops below 10.5V the zener will stop conducting, shutting off the relay.

The rectifier diode absorbs transients caused by the field breakdown in the coil, protecting the transistor Just a plain resistor will do it or you may follows suggestion and use a computer to do that The last time I used those ancient devices I find out they operate as a current device like minimum pull in current source. So a passive resistor in series will insure the minimum pull in source however it could be higher then the 10.5 v you need. The above designi just plain silly.

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