Help with simple smart switch that allows current flow at 14.5v but restricts at 13v

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Joined Apr 4, 2012
I am trying to run a load in an automobile but what I really want to activate the load automatically only when the alternator is up and running at its nominal 13.5-14.5 volts dc.

Before the car is turned on the normal voltage of the battery is usually less than 13 volts, when the alternator is on it can jump to 14.5 volts.

I need a smart circuit which will not activate or send power until it recognizes an increase in voltage or it detects the 14.5 volts. It will then shut down when the ignition is turned off and the alternator goes offline. Only needs to be enough power to trigger a dc relay.

What I really want to do is allow the circuit to pass a voltage to a relay which will run the load but will release power from the relay when it sees a voltage drop, in other words turn itself off when the alternator is turned off and the voltage starts dropping down to the batteries normal voltage reading of less than 13 volts dc. It would be more or less an automatic electronic switch that only turns on when the alternator is running.

I could tie into an ignition wire or another relay in the vehicle but I am trying not to do any modifications to the factory wiring. It would be a small enclosure that is directly connected to the battery and will remain dormant until the car is turned on and the alternator is supplying excess voltage at or around 14.5 volts, then it will pass voltage through to the relay which will actually power the load.

Any ideas on how to go about designing a simple circuit to do this? Any help would be greatly appreciated...


Joined Jul 17, 2007
Just a simple comparator won't work.

We no longer support discussions about automotive modification topics per the TOS paragraph 6, so this thread will be closed when a Moderator sees it. The TOS is not open to debate; it was established by the owners of the site.

I sincerely recommend that you not proceed with your idea; it's far more complex than one would think.
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Joined Mar 24, 2008
I am closing this thread as it violates AAC policy and/or safety issues.

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