This circuit determines when an engine is running, and when running, it turns on a solid-state switch to automatically power accessories. It does this simply by measuring battery voltage—when not charging, battery voltage is less than approx. 13.2V—when charging, it is greater than approx. 14.0V. Seems like a simple task, does it not? If so, why are there not already devices available to do this seemingly simple task?
While the concept is simple, there are numerous problems involved as follows:
Due to these complexities, this circuit is not recommended for the novice. However, if installed using proper circuit protection & wiring practices, the only failure mode is unintended battery discharge should it not automatically disconnect the load.
Source or native rfflow file
U1 is the voltage reference—its output is a stable, low impedance voltage source. R3, 4, & 5 form an adjustable battery voltage divider for setting threshold voltage. U2A is a voltage comparator (popular LM339 quad comparator). R6 & R7 provide positive feedback—R6 sets the hysteresis. R8, 10 & 14 are pull-up resistors for the open collector outputs. D3 is the threshold indicator LED. U2B is simply an inverter. R11 & C2 make up a 20sec delay circuit. U2D is the MOSFET driver. R13 & R12 provide positive feedback for positive switching. D5 discharges C2 when power is removed. R15 & D2 protect the MOSFET gate from over-voltage. D6 is the output ON indicator LED.
After engine is started and battery reaches float charge voltage, the circuit starts timing. After about 20sec, the load switch Q2 turns on thus powering the load—Q2 is a high-side switch. When engine is stopped, the battery voltage drops below the float charge voltage. Then the timer starts to time out again—after approx. 20sec, the load switch turns off.
MOSFET P-Channel power device
Q1 has been selected to power up to a 15A load without dissipating excessive heat. An 80A device (P80PF55) is specified and it requires a heatsink because the power dissipation = 4W @ 15A. For 10A or less, a lower current device such as the 27A FQP27P06 may be used. Or, this device may simply power the coil of a relay or contactor.
Setup on bench
Take and log voltage measurements on vehicle.
Calculate average voltage.
Set bench power supply to equal average voltage and connect to unit.
Adjust R4 to point where threshold LED (D3) just comes on.
Voltage measurements from my two old cars
Installation—first read and apply the following information
Set up on vehicle
When engine is at idle speed and battery is at float charge voltage, rotate R4 clockwise until LED D3 lights—then increase setting slightly.
Determine that load switches on properly after 20sec time delay. Then stop engine and see if threshold LED D3 extinguishes (may take a few seconds), and load is subsequently disconnected after an additional 20sec time delay. Restart engine and observe that all now functions automatically. Note that this delay is inconsistent if engine is started, stopped and restarted quickly.
For the future
AC mains load switch
Undocumented words and idioms (for our ESL friends)
perf board — perforated fiberglass epoxy sheet
one size fits all — universal size clothing (largest size)