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Polarity Sense Circuit

jw0752

In one of my recent builds it was necessary to come up with a circuit that would sense if the inputs were hooked up reverse polarity. I came up with this circuit:

 

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I have tested this circuit to 100 volts DC on its Test Input terminals. If the Test Inputs are under normal polarity the output is LOW and if the Test Inputs are under reverse polarity the Output is HIGH. Unfortunately the Test Inputs will not sense a reversed polarity below 3.5 Volts. While these limitations were not a problem for my build I am curious if there are other better circuits to test polarity than what I cooked up here. Also if you see problems with the circuit above any critique would be appreciated. R1 limits the forward current through LED1 which acts like a zener and keeps the voltage Gate to Drain from exceeding breakdown limits. D1 is reverse biased under proper polarity. R2 keeps the P CH MOSFET off until there is a Negative Voltage applied at the gate by a reversed Test Input polarity.

 

Thanks John

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  • 0 in reply to johnbeetem

    Hi John

    I like the thinking. I clicked on the link you provided and I was looking at my own behind. I actually tried to make the circuit originally with a comparator but I did not do a good job with the input circuit so I move on to a different approach. Since the inputs I was testing and the comparator had a common ground with the comparator my reverse polarity pulled the inputs of the comparator well below Vss. Now I am thinking that I can get around that problem with a split supply or a better design of the circuit that feeds the inputs of the comparator.

     

    My continued quest on this subject is motivated primarily by curiosity. The circuit in the DC Load is working well within parameters for my application. Frequently, however, an application catches my interest and I like to push it to see what the limits are and what other approaches there are to the problem.

     

    Thanks for your input and for reminding me of the unused LM339 sections.

    John