Current sense comparison

Thread Starter

Nano001

Joined Jan 12, 2010
101
Hi, I am trying to compare the signal from a current sense circuit to that of a reference voltage to detect no load, then switch the output to a high state through a pull-up. I want to detect when the (-) goes below (+). When the load is removed, there is no current flowing so the voltage drop is zero. The issue is I am creating a feedback loop because I am now trying to pull up the point that I am comparing with initially. This is causing v+ = v-. How can I decouple this?
 

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Thread Starter

Nano001

Joined Jan 12, 2010
101
Output is just before R24. High state is the voltage at R48 (20V in this case). R48 is the pull up. V+ and V- are LM324 inputs.

I want the output of LM324 to go high when R24 is large ("open"), due to no current flowing through the sense. When LM324 goes high, I want the voltage just before R24 to be pulled up. Basically, I want to indicate an open by pulling the output high when the load is removed. I have other things that I want to switch on as well through the analog switch, but for now they are grounded.
 

djsfantasi

Joined Apr 11, 2010
6,439
Hi, I am trying to compare the signal from a current sense circuit to that of a reference voltage to detect no load, then switch the output to a high state through a pull-up. I want to detect when the (-) goes below (+). When the load is removed, there is no current flowing so the voltage drop is zero. The issue is I am creating a feedback loop because I am now trying to pull up the point that I am comparing with initially. This is causing v+ = v-. How can I decouple this?
This hands down is the most confusing description I have read on AAC. First, there is the vagueness that @crutschow has already alluded to. Then, I tried to make sense of the various scenarios you describe. Wasn’t successful. I may be losing my mind.

In particular, what #^** do you mean by (-) goes below (+)? At first glance, that doesn’t seem possible...

Please explicitly define the terms you want to use. A’la crutschow’s post.

Then, how about defining the various scenarios, one-by-one... Personally due to my OCD, I’d create a table first of all possible combinations in a scenario. Then, I’d use that to write my post (I usually compose a post offline before submitting it. But that’s me)

If you can organize your thoughts and post an update, that would probably help you AND us. Thanks!
 

crutschow

Joined Mar 14, 2008
24,997
The issue is I am creating a feedback loop because I am now trying to pull up the point that I am comparing with initially. This is causing v+ = v-.
Still a little confusing, but I think your problem is the current sense amp is basically running open loop as a comparator.
Edit: This means the op amp offset could cause the output to be high with no load current.
Add a resistor between the left side R28 and U4's minus input.
Value of resistor determines the current gain sensitivity.
What sensitivity do you need? What is the minimum current you want to detect (don't say zero)?
 
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