Wednesday, December 2, 2015

Switched 12V circuit adjustment

The circuit I built in the last post worked well in theory, but in practice with my car it wound up cycling power to the output quite often when the car was running. I put a voltmeter in the car to watch when driving.  It appears that when the car decides the battery is absolutely topped off, it completely removes alternator power from the line.

At these times, the bus voltage drops as low as 12.3 volts.  At other times it's as high as 15 volts. Typically it's in the 13.2 to 13.8 volt range.  The trigger point for my circuit is about 12.6 volts.

As built with a 2.2μF capacitor, it would shut off almost immediately, no more than a couple of seconds delay.

I modified the circuit by replacing that with a 680μF capacitor.  It does now take about 12 seconds to switch on while the capacitor charges, but it takes 3 minutes to shut off. I never observed a low voltage situation on the line that lasted more than about 20 seconds so this should be great. In fact I don't mind that the camera continues to run for a few more minutes after shutting the car off.  Bonus feature!

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