Automatic low voltage shut off circuit.

[delciotto]

Anyone got a schematic for a simple low voltage shutoff? I have a project I'll be building soon that uses 4 18650 unprotected batteries and I want a small circuit to stop the battery pack from going below 12V so they don't get damaged. The circuit it will be powering will be drawing around 3.5A too if it matters.

 

[Cyparagon]

What are you powering, and how good with electronics are you?

 

[delciotto]

What are you powering, and how good with electronics are you?

I'm a trained wireless communications/electronics technician, but I can't design for shit. If I have a schematic I can make a PCB from it and build it or fix it. Although it's been a couple years since I've had to use those skills.

Its going to power a high wattage LED lamp driver.

 

[The Lightning Stalker]

Try this.

 

[Cyparagon]

There are many ways to do this. Here's one I picked from google images that would work, and is one of the simplest:

Have the "12V" come from the battery, and have the power to the driver go through the relay. The parasitic draw would be quite high, >100mA which would drain your pack in a couple days, so your power switch should probably go in series with the relay coil. Add a flyback diode (google it) to the coil. This would lower your parasitic draw to ~3mA which isn't too bad.

Add a volt or so to the zener value to figure your cutoff voltage. 12V zener would cut the voltage off a bit over 13V. 10V zener would cut if off a bit over 11V etc.

Before anyone jumps on me with "USE A FET YOU MORON " I'll point out that I'm going for simplicity. Gate drive is a lot more picky than coil drive, and adding the needed hysteresis to the system complicates things.

 

[delciotto]

There are many ways to do this. Here's one I picked from google images that would work, and is one of the simplest:

Have the "12V" come from the battery, and have the power to the driver go through the relay. The parasitic draw would be quite high, >100mA which would drain your pack in a couple days, so your power switch should probably go in series with the relay coil. Add a flyback diode (google it) to the coil. This would lower your parasitic draw to ~3mA which isn't too bad.

Add a volt or so to the zener value to figure your cutoff voltage. 12V zener would cut the voltage off a bit over 13V. 10V zener would cut if off a bit over 11V etc.

Before anyone jumps on me with "USE A FET YOU MORON " I'll point out that I'm going for simplicity. Gate drive is a lot more picky than coil drive, and adding the needed hysteresis to the system complicates things.

Well time to dig through my parts and see if i got that transistor. I know i have some darlington transistors somewhere in my mess of parts, just don't know what ones.

 

[delciotto]

Well in my search through my parts i didn't have the transistor i needed and the local electronics shop doesn't have them, but i did find some IRF510s and a bunch of 2N2906s and a 3.7V zener which I was able to make this circuit with

Which worked great, I should be able to replace that zener with a 10-12V and it should work fine still.

I also found a baggy full of LM317Ts, so if I want to make a bunch of laser drivers i guess I'm good to go lol

 

[delciotto]

Oh man i just realised i'm going to need a crazy dummy load for this when i calibrate the driver since its going to be powering a 100W LED lamp at 30-36V. I only have experience with making RF dummy loads and from a quick glance at google one for something like this is a lot different. Looks like a whole bunch of diodes to simulate the voltage drop and a MASSIVE 1 ohm load resistor.

 

[The Lightning Stalker]

I would say just forget about the dummy
load. LEDs are far more tolerant of current
spikes and ESD than laser diodes. In this
case a dummy load would cost more than
the LED, so just use the LED for testing.

For the resistor, you can use one of these or
a shunt ammeter.

------> WNDR10FET <-----

 

[delciotto]

Ha ha! The protection circuit works great! Time for some pics.

The soldered circuit on a solderable breadboard

Testing, it works!

I also discovered I REALLY need a bigger work area.

 

[The Lightning Stalker]

Very nice. You have constructed your own
op-amp from discretes. What happened to
the zener diode?

 

[delciotto]

Very nice. You have constructed your own
op-amp from discretes. What happened to
the zener diode?

Yeah I looked up ones with discrete components because I didn't feel like waiting for shipping again and i knew i had those parts around. and the zener is under the blue resistor.