# At a resistance cut power

Discussion in 'General Electronics Discussion' started by Antleo, Sep 6, 2016.

1. ### Antleo

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Sep 6, 2016
I am trying to create a circuit that uses a specific resistance (actually it'd be great to have it work off of 2 specific resistances but I can create 2 circuits if need be) to apply power to a normally closed relay causing it to cut power to a system. I can read circuit diagrams and solder and understand electronics well, but I don't know how to go about doing this. Id imagine I will need a micro controller, but I'm not sure which. Any help is much appreciated.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
What you want is a window comparator and tho either poised a known current through the resistor or to place it as one leg of a voltage divider.

A window comparator detects a voltage between 2 limits, which can be set as close as you desire.

3. ### Antleo

12
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Sep 6, 2016
I cant apply power to the circuit. as it is for a gauge. as far as i can tell it is a set of reed switches. and as each trips it sends a different resistance value. the first and last switches give the 2 resistances that i need. However i can not tap off of these as they are completely inaccessible. hence needing it to read the resistance. I wish it was as simple as that.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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2,809
Jan 21, 2010
You can't measure resistance without passing a current through it.

5. ### Antleo

12
1
Sep 6, 2016
I need to find a way to take this reading without messing with the gauge reading. What i am thinking is what if i added a voltage divider upstream of the circuit that takes away the power that is applied by this circuit?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Ok, if its already in circuit then it already has current flowing through it and you just need to detect a voltage range on one end of it.

7. ### Antleo

12
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Sep 6, 2016
Clearly Im not using my brain here. I should have thought of that haha and i should have known about the voltage thing..I went to school for this 2 years ago. jeez *facepam* ( ive also been out of it for 2 years so)

(*steve*) likes this.
8. ### Antleo

12
1
Sep 6, 2016
So I went out and took measurements today, I found the applied voltage to be 8.4v and the resistance is actually the same when it hits either limit switch, being 167Ω meaning current would be 50.3 mA. So what type of circuit would be best to cut power at or below this amperage? (actually I'd use something a little higher for some leeway considering the next reading below it is 133Ω) so say 50.7mA and lower it would send signal to a relay to cut power. How would I go about doing this?