Circuit to cut off voltage at 1.2 volt

[electrocub]

I want to hook a 1.2 volt lamp up to a single cell nicad battery and when the battery voltage drops down to 1.2 volts, the circuit cuts off. Any thoughts?

 

[DerStrom8]

I would start with a basic comparator circuit (unless you have experience with PICs). Simply set the reference voltage of the comparator to ~1.2 volts so that it shuts off the output if the input is less than the reference.
Der Strom

 

[BrownOut]

I see a "gotcha" in that solution. Unless you have another stable voltage to run the comparator on, that won't really work. What you really need is a breakdown device, like a zener that's rated for 1.2V. That needs to feed a switch which in turn supplies current to your lamp. Or maybe just a couple forward biased diodes connected to the base of a transistor.

 

[DerStrom8]

I see a "gotcha" in that solution. Unless you have another stable voltage to run the comparator on, that won't really work. What you really need is a breakdown device, like a zener that's rated for 1.2V. That needs to feed a switch which in turn supplies current to your lamp. Or maybe just a couple forward biased diodes connected to the base of a transistor.

That's a good point, BrownOut... I didn't think of that In that case, I agree. The zener diode would work much better.
Sorry about that...
Der Strom

 

[ronv]

NiCads have a nominal voltage of 1.2 volts and there voltage is not a good indicator of their state of charge.

Nickel-cadmium battery - Wikipedia, the free encyclopedia

 

[RCinFLA]

Besides the nominal discharge voltage being 1.1-1.25vdc, NiCads can be run down with little detrimental effect. Although when taken off charger at full charge they will be between 1.45v-1.55v they quickly drop to 1.25 vdc with initial discharge current flow.

NiMH should not be totally depleted and LiIon is severally damaged if run below about 2.1v to 2.8v depending on discharge rate.

If you want a good 1.2v reference IC you can use one like LM4041 in an SOT23 package. An ordinary Zener diode does not have a good sharp reference point on zener voltages below about 2.7 vdc. Your are better off using a band-gap type reference voltage IC that is well temp compensated and have a very stable, sharp, reference voltage output.