Hi i have a little circuit that must monitor a very small voltage on a shunt resistor (about 1 mV) if the voltage goes over it i must swith the output comparator. (open collector if possible)
the problem is that my supply for the comparator is 12V and the shunt is around 1 to 20V (it is a high side shunt resistor on a variable voltage,) is there any comparator that can handle voltage at input pins higher than the vcc pin?

 

Please post the schematic.

Where are you getting your 1mV refernce from?

It sounds like you need a differiential amplifier across the shunt and a comparator monitoring the output of the amplifier.

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Here is the schematic the 1mw comes from the shunt resistor

Attached Thumbnails

 

 

A high side current monitor such as INA139 will allow you to amplify the voltage and reference it to GND so that you can use something like an LM393, but the INA139 has up to ±1mV of input offset, so you would need to make your sense resistor about 10 times larger.
There are several manufacturers of high side current monitors, and various part numbers. You might find one that works better than INA139.

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Ron

 

You could an op-amp as a differiantial amplifier, just make sure use an instrumentation op-amp with a low offset error.

http://en.wikipedia.org/wiki/Operati...tial_amplifier

You could also go for a hall effect current sensor.

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is there a problem with op amp to have inputs at higher voltage than vcc?

 

In a differential amplifier both of the op-amp's inputs are held near 0V so it won't make any difference.

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If he wants gain, and the common mode signal level is 20V... How is that gonna work?

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Ron

 

Yes, you're right, he needs to power the op-amp from a higher voltage than 20V.

EDIT:
Another option is to make a differiential amp with a unity gain, that way the op-amp inputs will be only be biased at a maximum of 10V.

I like your new avatar by the way, I think I've used that image in one of my posts, did you borrow it from me or find it for yourself using Google image search?

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

There are over-the-top op amps (Google) such as LT1490A which have common mode ranges that exceed vcc. I still think a high-side current monitor such as INA139, and a cheap comparator, is the way to go.

I might have gotten it from you, although I added the red circle with MS Paint. I found the original the other day in one of my picture folders. The file is dated 4/17/07. I'm not sure why I saved it.

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Ron

 

Carmusic, at what current level do you need your comparator to switch?

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Ron

 

about 20-30 ma but in the circuit i measure up to 20A so the shunt wont be over 0.01ohm
so want to detect signal over 1 mV, i tried similar chip as the ina139 and below 5 mV they dont work very well. I must keep the circuit as simple as possible.
what can occur with the comparator (shown in schematics) if input pin are higher than vcc? i will try it.

 

LM393 won't work. The common mode range only goes up to Vcc-1.5V. It also has input offset voltage of ±5mV max.

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Ron

 

INA213 should work in front of your LM393. It has input offset of ±100uV max, and gain=50. With 1mV in, you should get 50mV out, referenced to GND so the LM393 can handle it. With 20 amps through .01Ω, you will get 10V out (20A*.01Ω*50), which will not harm your LM393.

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Ron

 

i think i will try lt1716, it seems to be perfect!