Switch on load only when alternator is running?

You need an accurate voltage reference and a comparator. The easiest way is probably to use an LM3914 chip which has a reference and will drive a led display in either bar mode (like a thermometer) or in a dot display.

By setting to a bar display and setting the sensitivity to make the bar display say the 8th output at 13V then use the 9th output to drive a transistor which switches a relay.

So the answer to your question is yes.

 

Here's a circuit that should do what you want.



Here's a circuit description.

The circuit connects to an automotive power supply of +12V (nominal) and 0V using CN1 and CN3. When this voltage is higher than a set level, for example 13.0V, relay K1 is activated, and feeds the +12V supply out on CN2. The relays specified are rated for 2A switching and carry current; if more current is needed, an extra interposing relay should be used.

U1 (TL431AC) is a shunt voltage regulator that is used here as a voltage comparator with a fixed detection voltage of 2.5V. When the ADJ pin goes over 2.5V, U1 will conduct current through its K and A pins, activating the relay. D1 protects U1 against back EMF from the collapsing magnetic field when the relay is turned OFF.

An adjustable fraction of the battery voltage (which comes from CN1) is provided into U1's ADJ input. This voltage is smoothed by C1 to prevent U1 from responding to noise and rapid variations. Hysteresis is provided by R2 and R3, and provides a voltage deadband between switching ON and switching OFF; this prevents chatter and indecision when the voltage is near the threshold voltage.

Initially assume that K1 is open and the voltage at the top end of R3 is negligible. If VR1 has been adjusted for a 13.0V voltage threshold, VR1's resistance can be calculated as 2.5 / ((13.0 - 2.5) / R1) which is about 6430 ohms.

Once the supply voltage reaches this threshold and the relay turns ON, current through R2 raises the voltage on R3 to about (13 R3 / R2) which is about 48 mV. The supply voltage required for the voltage divider to yield 2.5V on U1's ADJ pin has now dropped to (((2.5 - 0.048) / VR1) * R1) + 2.5 which is (2.452 / 6430 * 27000) + 2.5 which is about 12.8V. So a hysteresis deadband of about 0.2V is created.

R4 protects the circuit against surges at load dump; only minimal protection is needed as there are no ICs connected across the supply rails.

The relay types given are all high-sensitivity relays; coil current is 12.5 mA or less. The IMB06 parts are TE Connectivity brand; the G5V is an Omron part. They are all rated for a maximum switching and carrying current of 2A; if more is needed, an external relay must be used.

 

Yes you can use a P-channel MOSFET instead of a relay.

Connect the source to the +12V rail (from CN1).
Connect the gate to the K pin of U1.
Connect the drain to the switched output voltage (to CN2).
Replace D1 with a 1K resistor.
No other changes are needed.

 

And you still don't since there is no voltage comparator in the circuit. Kris is using a trick based on the fact that a shunt regulator will draw a current only when the voltage is above the set voltage. A comparator is a bit different and would require a separate reference, which is why this circuit is clever.

Bob

 

Yep, as you can see there is actaully a comparator in the TL431 as well as the reference and a pulldown transistor, so basically it is used here as a comparator with a builtin reference.

Bob

 

Hello, verry nice cirquit..
How would you adapt the values for a truck with a 24V battery system wich should be switched at 26V (2x13V)

Thanks,
Nicolas

 

Changes for 24V operation:

1. D1 change to 1N4004
2. K1 change to a relay with a 24V coil such as Omron G5LE-1A(4)EDC24 (10A contacts), see http://www.digikey.com/product-detail/en/G5LE-1A4 DC24/Z2254-ND/369017
3. R1 change to 56k
4. R2 no change (hysteresis will now be about 0.4V)
5. R3 no change
6. R4 (varistor) delete
7. U1 no change
8. VR1 no change
9, Add a 33V 1/2W zener diode across U1 with its cathode to the K pin and its anode to the A pin. A suitable zener is BZX79C33 see http://www.digikey.com/product-detail/en/BZX79C33/BZX79C33-ND/977909

These changes relate to the relay version. If you want to use a MOSFET instead of a relay, a change is needed to the gate drive so the maximum gate-source voltage is not exceeded. Let me know if you want details.