I'd like to built a circuit for sensing voltage of a battery. For example if it's a 12v battery, I'd like the circuit to continuously monitor the voltage, and if it drops below say 11v, to give a signal that will go somewhere else. This will need to have as low current draw as possible.

What I was thinking about is using a simple low current op-amp, then have a voltage divider bring down 12v to say 5v, feed that to one side of the op-amp, then have a 5v reference feed the other side of the op-amp.

Is this an elegant solution? If so, which 5v reference should I use? I don't think zeners will work with under 1ma draw?

 

What sort of output do you need?

Personally I'd drop in a micro controller that spends most of it's time in sleep mode drawing little to no current. Same with the divider: it would switch off during sleep.

But for an analog approach... Micro semi makes some zeners with a test current of .25mA I found after a 30 second search for "low current zener." Do not forget the divider is also drawing current so use large value resistors. A two zener approach seems good to me, say 2.5V as reference and 5.0V to run the op amp. Put two equal value zeners in series to reuse the zener current in both devices.

Of course, if you are allowed to define your own signal for somewhere else... just define that signal as >11V good and <=11V as bad.

That way your whole circuit can be made out of two wires.

 

Depends on what you call low current draw? Depends on what you want to do with the signal. Personally I would use a simple comparator circuit, you may want to give this a read. There are also as mentioned micro controller options. Each method has good and bad points. As to an operational amplifier the comparator rather than Op-Amp is more along the lines of what you want. I would use a low power 5 volt regulator and as you mention divide down the 12 volts for the input to the comparator.

Ron

 

Here is a forum posting about micropower voltage reference driving a micropower comparator specifically for sensing a 12V SLA battery.

Here is a forum posting about using a TL431 as a voltage sensor, which is still pretty low power.