# Op amp circuit help

#### SwatKat

Joined Nov 25, 2009
1
Hi

I found this Battery Equal Charge Indicator circuit here

http://freecircuits.wikidot.com/
(The third circuit down the page)

I was wondering why such an elaborate circuit is used. Cant I just stick the batteries as inputs to an op amp and use the output (if they are unequal) to drive some sort of indicator?

I am sure there is some sort of explanation that eludes me. I'm a beginner. Could someone explain this to me please?

#### steveb

Joined Jul 3, 2008
2,431
Hi

I found this Battery Equal Charge Indicator circuit here

http://freecircuits.wikidot.com/
(The third circuit down the page)

I was wondering why such an elaborate circuit is used. Cant I just stick the batteries as inputs to an op amp and use the output (if they are unequal) to drive some sort of indicator?

I am sure there is some sort of explanation that eludes me. I'm a beginner. Could someone explain this to me please?
First of all, that circuit is not all that elaborate. However, there is always more than one way to meet a goal. It's quite possible you can come up with a simple arrangement with an opamp. I can imagine some ways myself. However, I don't know if that approach will allow as accurate a control on the voltage tolerance.

Feel free to post your attempt and submit it here for critique.

#### BMorse

Joined Sep 26, 2009
2,675
yes, you can just use 2 opamps, actually a dual opamp such as the LM1458 would work, then you can set trip points on both and feed the ouput into a 4011 to switch/turn on an indicator.....

My .02

#### steveb

Joined Jul 3, 2008
2,431
yes, you can just use 2 opamps, actually a dual opamp such as the LM1458 would work, then you can set trip points on both and feed the ouput into a 4011 to switch/turn on an indicator.....

My .02
Two opamps and a 4011 is just as "elaborate" as three comparitors.

I took his question to be asking if one opamp could be used. This would require a creative method. It's easy to use one opamp/comparitor to indicate if battery A is greater than battery B, but testing for equality is more challenging. It can be done, but the tolerance range will be less well controlled, I think.

#### mik3

Joined Feb 4, 2008
4,846
Hi

I found this Battery Equal Charge Indicator circuit here

http://freecircuits.wikidot.com/
(The third circuit down the page)

I was wondering why such an elaborate circuit is used. Cant I just stick the batteries as inputs to an op amp and use the output (if they are unequal) to drive some sort of indicator?

I am sure there is some sort of explanation that eludes me. I'm a beginner. Could someone explain this to me please?
What do you want to achieve by comparing the batteries' voltage?

What indication do you want? Just to see which battery has greater voltage across it or to see how much is the difference between their voltages?

How accurate do you want this to be?

These are a few questions to answer first and then decide if you will use a simple or more complex circuit.

#### SgtWookie

Joined Jul 17, 2007
22,201
Well, you would not want to connect two batteries in parallel unless they had the same voltage at no load. Otherwise, you would have very heavy current flow as one battery tried to charge the other battery.

This comparison could likely be performed with just a single opamp and an LED (or two) with (a) current limiting resistor(s).

[eta]
Here's an example.

The Zeners would have to be very closely matched for it to be a valid comparison.

If the battery voltages were the same, both LEDs would be on at about equal intensity.

#### Attachments

• 12.3 KB Views: 27
Last edited: