Hysteresis for low-voltage cutoff circuit

[Aeternam]

Hey gang,

I want to add hysteresis to a low-voltage cutoff circuit (see attachment.)

I am designing a dew heater for telescopes. Since these things are used in the field I plan on powering it with a lead-acid battery (already present to provide power to telescope mount etc.) We all know that discharging these below a certain point is inadvisable, hence the low-voltage cutoff.

Problem with my current design is, once the cutoff kicks in the load on the battery disappears and voltage rises again. The circuit sees enough voltage and turns the heater back on. Hysteresis to the rescue: cut off the load once voltage drops below 11.5V and do not turn back on until it's above say 11.8V.

Problem is, I have NO idea on how to implement something like that. Is that even doable with the attached subcircuit?

Sorry for the sucky scan btw. The 10k thing on the opamp's positive input is a 10k pot, VIN is connected to the battery, the rest of the circuit continues to the right of the relay. The opamp compares VIN to a zener reference voltage. As long as VIN is above the zener voltage the transistor is keeping the relay open which in turn powers the rest of the circuit.

I'd be happy if someone could point me in the right direction.

Cheers!

[Alex Nikitin]

1) add a resistor (say, 1K) in series with the base of Q7 . As drawn, you are relying upon the short-circuit protection of the opamp, not a good thing.

2) connect a resistor between the output of the opamp and the non-inverting input, say 470K . The value would define the hysteresis width.

Cheers

Alex

[Aeternam]

1) oops 

2) brilliant, thank you 

[Whales]

Just be wary that this hysteresis resistor will change your trigger point.  You'll need to re-adjust your pot after you install it.

[Alex Nikitin]

Yes, and one more thing - add a diode (something like 1N4004 is usually sufficient) across the relay coil (cathode/bar to the + supply, obviously) to protect the transistor from overvoltage by back EMF from the coil.

Cheers

Alex