12V Load Switch

Engine Running Detector, Load Switch

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:

  1. Voltage must be sensed as close to the battery as possible, and voltage sense wiring must be separate from power wiring—hence the requirement for the Kelvin connection. This is why I wrote the recent article: What is a Kelvin Connection?
  2. Since this circuit is connected directly to the battery, circuit protection is absolutely essential. This is why I wrote the recent article: Low Voltage & Automotive Circuit Protection.
  3. Voltage threshold must be accurate—requires stable voltage reference.
  4. Circuit must have hysteresis for positive switching.
  5. Load switch must have substantial delay to prevent unintended or nuisance switching.
  6. Quiescent current must be low to prevent battery discharge.
  7. Charging system must function correctly—alternator must float charge the battery as well as power all connected accessories when engine is at idle speed.
  8. Due to variations in battery charging voltage in different systems and a relatively narrow voltage threshold window, there is no one size fits all voltage setting. As a result, the threshold must be adjustable.

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.


12V Load Switch

Source or native rfflow file

Gathering components prior to assembly

Perf board prototype

Circuit overview

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.

Circuit operation

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

Engine Stopped Running Average
Chrysler 13.09V 14.49V 13.79V
Toyota 12.28V 14.58V 13.43V

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)

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