# Voltage-controlled-switch (or similar) to discharge battery when voltage exceeds 14V?

#### LMF5000

Joined Oct 25, 2017
108
I would like to create a simple circuit that will discharge a battery through a 50W load (resistor or bulb) if its terminal voltage goes over about 14V.

The circuit should stop the discharge when the voltage goes below 14V and switch on the bulbs again any time the voltage is over 14V.

How could I implement this with the lowest possible part count and cost? Can I do it with a single electronic component for example?

#### ericgibbs

Joined Jan 29, 2010
14,183
hi LMF
One option would be to use a dual comparator to create a window comparison, Vmin Vmax.
A LM393 dual comp, drive transistor and a relay would be minimal.
E

#### LMF5000

Joined Oct 25, 2017
108
Hi Eric, thanks for the suggestion. I thought of something like that, but I'd need a reference voltage source, the actual switch (transistor?) and probably resistors and capacitors to make it all work right? I'm a mechanical engineer so I'm a bit lost when it comes to making a complete circuit with a comparator.

Is there a way to do it with one component? Or a ready-made circuit board for $5-$10?

#### LMF5000

Joined Oct 25, 2017
108
Thank you NSA - that's almost exactly what I need. Pity about the 2A limit, I was hoping for something that can switch at least 5A at 12V (circa 50W) without needing an external relay.

Any other ideas with different approaches?

#### ericgibbs

Joined Jan 29, 2010
14,183
hi,
Use the option 'A' with the module , post #4

#### LesJones

Joined Jan 8, 2017
3,571
I think if you have exactly the same switch on and switch off voltage it will be switching on and of quite frequently. This due to the fact that the battery voltage will increase when the load is removed. (And decrease when the load is applied.) I would think you would need to switch in the load at say 14.1 volts and switch it off at 13.9 volts. You could use a zener diode and scale the voltage with resistors to a suitable reference voltage to the comparators. You could also use an LM431 adjustable precision shunt regulator fo the reference voltage.

Les.

#### crutschow

Joined Mar 14, 2008
28,201
without needing an external relay.
Just add an external low on-resistance P-MOSFET to power the load instead of a relay.