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I'd like to design a simple device that uses a microcontroller (in my case a TI Tiva C Series Launchpad) and a relay to drive a load, then monitor whether the load is actually drawing any current. The load will draw about 5-10 A at 12 VDC. What is a good method to determining if the load is actually drawing any current so I can use that as feedback into my microcontroller for other functions? What electronic components would be used?

I'm essentially trying to duplicate how my car knows if I have a failed headlight. If I flip the switch to the headlights, a signal to an ECU that then commands a relay to close. If the relay closes, but the lights don't come on, I get an indication on my dashboard info panel (not a dummy light, but a text readout; in other words, the indication comes via the ECU). The only wires going to my headlights are two power wires (one each for high and low beams) and a common return/ground.

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  • \$\begingroup\$ I happened to ask a question about how to sense current in a circuit of mine a few months ago, and got a few recommendations for parts. But you're probably looking for something much simpler, less precise and cheaper that would tell you whether there's current drawn or not. Well, here's the link, anyway. \$\endgroup\$ – Ricardo Jul 9 '14 at 1:31
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I don't know any part numbers off the top of my head but they do make Current Sense IC's which basically measure the voltage drop across a very low value (0.5 ohms or less) resistor. You set the threshhold at one of the IC pins with a voltage divider and another pin gives you a TTL high or low output.

We used these to monitor current levels in an RF amplifier system, but if the threshold value is set close to zero I suppose you can use it to monitor open/load conditions.

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Four options for measuring a DC current of this sort of size would be:

  • Place a low value resistor in series with the load and measure the voltage drop across it. You need to choose the resistance so that the voltage drop across it (V = IR) is small enough to make negligible difference to the load, but large enough to measure, and you need to make sure the power rating of the resistor (P = I2R) is adequate.

  • Pass the load supply through a coil of wire wrapped around a reed switch. When enough current flows the magnetic field generated will close the reed. You'd need to experiment with the number of turns and gauge of wire needed and again you'll need to consider the voltage drop and the heating due to the coil's resistance, but this would be the simplest method in terms of circuitry.

  • Use a Hall effect sensor to detect the magnetic field due to the current flowing in the load supply wire, possibly again using a coil but it would need many fewer turns than the reed relay as the sensitivity is greater. You'd need to be aware of any other magnetic fields that might be present where the sensor is going to be installed, that might interfere with your measurement.

  • Use a current sense IC which gives you a linearised voltage output proportional to the current - I think these usually use Hall effect rather than a resistance, but either way this is probably the easiest option if you don't mind spending a few dollars/euros/etc. You can get these ICs ready mounted on breakout boards from suppliers like Pololu.

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