Low voltage cutoff ?

[flak88]

Looking for a low voltage cutoff circuit which will cut off the load when my car battery falls below 12v the load is a lamp (10 watt).
Suggestions ?

 

[audioguru]

An LM10 has an opamp and an adjustable voltage reference in an 8-pins DIP case. It can drive a power transistor.

A problem will be when the circuit detects that the battery voltage is low and turns off the lamp then the voltage might rise high enough to turn on the lamp then it oscillates on and off. Maybe the low voltage detect circuit must set a flip-flop to turn off the lamp then you manually reset the flip-flop to turn on the lamp when you know that the battery is charged.

 

[flak88]

Any suggestions for a non ic solution ?

 

[Nigel Goodwin]

Any suggestions for a non ic solution ?

As an IC solution is simple, cheap, easy and accurate - there seems little point in not using one.

If you don't like that specific specialised IC, then a normal opamp and a zener (5.6V) is a decent alternative.

 

[ronv]

I suppose you could use a relay that you adjust to drop out at 12 volts then a button to reset it when the battery is charged. Kind of crude compared to the IC solution but.......

 

[Nigel Goodwin]

I suppose you could use a relay that you adjust to drop out at 12 volts then a button to reset it when the battery is charged. Kind of crude compared to the IC solution but.......

How would you adjust a relay to drop out at 12V

 

[MikeMl]

Looking for a low voltage cutoff circuit which will cut off the load when my car battery falls below 12v the load is a lamp (10 watt).
Suggestions ?

Thread about Low voltage alarm

Circuit for low-voltage disconnect.
Thread about low-voltage disconnect.

 

[flak88]

Are there any other circuits schematics that anyone can recommend that use a n channel fet instead and has a flipflop switch to stop oscillating when the battery voltage recovers ?
My knowledge is very new so looking for something simple but does the job.

 

[MikeMl]

The TL431 Low Voltage Cutoff circuit I posted has hysteresis. If it chatters in your setup, changing one resistor will stop that.

Isn't your load grounded? If so, you need a high-side switch, and a PChan FET is better for that than a NChan FET

 

[Diver300]

You could put a lot of hysteresis in the cut off. If you wait for the voltage to go back up to 13.5 V or so, then it will reset when the battery is next charged.

Voltage is not a very good measure of how well charged the battery is. The voltage will plummet below 12 V when the battery is just about flat. If you are trying to detect when the battery is down to 50%, that isn't really possible by just measuring the voltage.

 

[kinarfi]

Here's a method for you, the relay could be as powerful as a starter relay, but that's excessive for a 10 watt load, but if it's on a camper or vehicle that get's used often, it will keep you from killing the battery by leaving you lights on.

I haven't tried the second drawing yet, going to put it together and test it, and if it works, put on mine, I hate having to reset the radio when I leave the lights on or the dome light on over night.
Kinarfi

 

[flak88]

Thx thats just what i was looking for.

 

[MikeMl]

Except that neither circuit will work as posted

btw: A relay is usually more expensive than a FET, and the relay coil wastes power...

 

[kinarfi]

Mikes right, the 2nd one would fry the zener or transistor, but on this circuit, it worked repeatedly, dropping the relays at 12.52 volts, the zener measured 11.95 volts. I have a 50,000 µf, 40 volt capacitor and I would charge it, trip the relay on and measure the voltage when the relay opened, 12.52 volts, tried it with 2 different relays, and got the same opening voltage. One relay was 86.3 Ω and the other was 378.3 Ω. I used this exact circuit, except I didn't put the protective diode across the relay and shorted across the N. O. contact with a piece of wire.
Kinarfi

 

[MikeMl]

The second one wont fry, but it won't work, either.

Try this:

 

[kinarfi]

If your not worried about the voltage coming back on as the voltage of the battery regenerates, You could use a FET like on this drawing. The IRF4905 is a 72 amp PFET.
Kinarfi

Mike, where do I find that relay for spice. I think the 2nd would fry something, spice shows 97ma through the zener and at 14.4 volt, automotive, you would have around 2.4 volts Vbe.
Kinarfi

 

[MikeMl]

Look at "components library" here.

 

[kinarfi]

This also appears to work according to spice.
all solid state, almost. I keep playing with this so I can put it on my camper and truck.
kinarfi

 

[kinarfi]

Here's what I'm putting on my Camper and Truck. I may not need both IRF4905s, but I have them. The reset button can be the Ignition switch or a separate button or both. The two zeners should add up to the cut off voltage you want or a little less and then the cut off voltage can be tweaked with the addition of resistors or a pot.

 

[kinarfi]

Still messing around, I think I like this one the best, battery drain is about 3.25ma and 0 when off. Simpler than the hysteresis method and more adjustable for drop out voltage.
Kinarfi