# Measure current change in circuit

Discussion in 'The Projects Forum' started by Blackaxe, Sep 21, 2009.

1. ### Blackaxe Thread Starter New Member

Sep 21, 2009
3
0
I have a circuit with 4 resistors (heaters) in parallel. They are running 12v dc and 18w each. The heaters are encapsulated so you cant see them and i need an indication if one dies (blows).

I cant seperate the power supply to each heater (so there goes the easy LED/resistor off the neg leg) The only way i can see is to measure the current but am more than happy with any other suggestions.

The whole thing is very small and won't support an amp meter or anything large. I am hopping for some way of turning on a LED as a warning device but like i say am open to any advice by your esteemed personages.

Thanks

2. ### JDT Well-Known Member

Feb 12, 2009
658
87
Measure the corrent using a series resistor and send voltage to a comparator.

See the attached circuit diagram.

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3. ### Blackaxe Thread Starter New Member

Sep 21, 2009
3
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See i new the problem would be easy if i spoke to the correct people of above average intelligance

Anyone else have a better idea? hard to think it would get simpler tho.

Many thanks JDT and nice diagram

4. ### SgtWookie Expert

Jul 17, 2007
22,198
1,800
Alberto, your idea would result in four LEDs glowing whether the heater resistors are working or not.

JDT's idea is a good one.

Your total power dissipation is 18W x 4=72W with a 12v supply, which means your current is 6 Amperes. Depending on the gauge and length of your supply wires, you might be able to simply monitor the voltage drop across the wiring to determine the current draw.

For example, 5 feet of AWG 16 wire has a resistance of about 0.0204 Ohms; a 6A current will give a drop of 122.4mV. If one of the heater resistors failed, current would drop to 4.5A, and the voltage drop would be 91.8mV.

Here's a wire resistance calculator: http://www.stealth316.com/2-wire-resistance.htm

5. ### Blackaxe Thread Starter New Member

Sep 21, 2009
3
0

Alberto, the problem i have is i cant seperate the pos and neg sides of the heaters from each other. Imagine two solid metal bars with the elements wedged between them.

Sarg, My next question i think you have answered but im not getting it. i was going to ask where the hell i get a .005 ohm resistor. are you sugesting the resistance in the neg wire from the elements to the battery may be enough on its own?