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Created July 19, 2013
Last modified July 20, 2013
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The sim starts with both the 12V supply at 0V (i.e. off) and the battery disconnected by opening SW1.

Dled1 is off.

Dled2 is off.

At T=10ms SW1 closes connecting the battery. D1 conducts and V(load) goes to about 8.3V.

D3 is reverse biased.

Q2 turns on and feeds about 1mA to Dled2.

Dled2 is on.

The 12V supply then climbs up to > V(batt). D1 conducts and D2 is reverse biased. V(load) climbs to about 11.3V.

D4 also conducts and turns on Q1. V(V12_ON) goes to almost 12V and feeds current to Dled1.

Dled1 is on.

Via D3, which is now forward biased, V(V12_ON) also pulls V(base) up to above V(V9) which turns Q2 and therefore Dled2, off.

Dled2 is off.

Dled1 LED is on only when 12V supplies the load.

Dled2 LED is on only when the 9V battery supplies the load.

Low current LEDs are recommended. Reduce R4 and R6 to increase the LED brightness but it may be found necessary to reduce R3 and R7 by the same ratio as R4 and R6 (i.e. R3/R4 must be a constant and R7/R6 must be a constant).

Reducing the resistors increases the amount of power wasted just driving the LEDs.

If indicator LEDs are not needed then just use the basic Diode-ORing circuit of D1 and D2.


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