# Voltage Comparator question

Discussion in 'The Projects Forum' started by SomeGuy1, May 5, 2013.

1. ### SomeGuy1 Thread Starter New Member

Jul 16, 2012
1
0
Hello,

First off, let me start by saying I am a complete noob when it comes to electronics. I understand the very basics but I struggle when trying to figure out how to do something different or exactly why a certain component was used in an example circuit.

But I am learning slowly and hope to develop a hobbyist level of knowledge in electronics in due course.

I have a idea for a project that I am stuck with. I'd like to be able to develop a simple circuit that could be used to plug into a battery with the aim of draining that battery down to a certain level. At that point, the circuit would shut itself off. The idea is to maintain a good level for (in particular) lithium batteries whilst in storage. This will hopefully prolong their life.

I've looked at op-amp voltage comparators and my idea is to build something using this method that will turn on an LED plus a resistance circuit when the voltage is higher than a reference.

My problem is two fold. Firstly, I'm not sure how to get an accurate reference voltage from a single source of power. The second problem is that if I do the voltage comparison whilst the supply is under load, then I will need different voltage targets (for the comparator) depending on the battery capacity. For example:

A 2 cell lithuim ion or lithium poly pack is 8.4v fully charged.
I'd like to drain the pack to, say 7.8v.

If the pack is 5.2AH then a reading of (perhaps) 7.4v under 1A load will yield a pack voltage of 7.8v under no load.

But if the pack is a 2.6AH pack then a reading of 7.4v under 1A load would be closer to 8.0 volts.

So my circuit design would have to try and test for 7.8v whilst the pack is under very little load. If the voltage was still to high, then it would engage the resistor/led sub-circuit for a time before testing again.

I realise this can be done much easier with some kind of processor like an Arduino, and I think I could do that myself. But I'd like to make the circuit a simple one without a processor if I could - mainly to learn.

2. ### wayneh Expert

Sep 9, 2010
12,394
3,246
That much is easy. There are various ways to get a reference voltage, for instance a zener diode or a dedicated voltage reference IC. You would then use a resistive voltage divider to divide down your sampled voltage to compare it to your reference(s).

The rest of your project is a challenge without a microprocessor.