Two 12v inputs switching

[john2k]

i have an LED light that's powered by two +12v inputs, one for full brightness and another for dim mode. The two power sources are not on at the same time because i've got an auto relay switching kit that switches off the main one if the secondary power source activates. However, just wondering what I can do as a failsafe in the event that the auto relay switch fails and does not stop the main power and I will end up having two +12v lines active at the same time.

Thanks

 

[duke37]

Putting a diode in each supply line will stop any backfeed if the supplies are connected together. This will drop about a volt.

You can connect your relays (with an extra contact) so that when the auxilliary relay is pulled in, the main relay is isolated.

 

[BobK]

If they are both +12V I don't see how one is bright and the other dim.

Bob

 

[KrisBlueNZ]

What is the problem if both inputs are on at the same time?
Show us a diagram of how the LED is driven from the two inputs.
If you're just using two different current limiting resistors, you can modify the circuit slightly (addition of two diodes) to get the behaviour you want, and throw away the relay.
Show us the circuit diagram first.