07-11-2012, 06:43 PM
Yes, that might work well, given some tweaks.
You need to consider the operating range of the inputs - if your PSU gets close to 0V or 24V, it will stop working reliably. All op-amps specify their common-mode operating voltage - some are rail to rail, most aren't.
I'd use a couple of resistors in series with each input - perhaps 10K or thereabouts. That will protect the op-amp to a decent extent should the voltage be allowed to exceed the safe range of the op-amp.
You might need to introduce some hysteresis for this to be reliable. A high value resistor between op-amp output and the non-inverting input - such as 1M (or more) should do the trick.
Also, the output of the op-amp won't be rail to rail. You'll find that the LED will be stuck on because the op-amp might only get down to 1-2 volts when saturated against the negative supply. Needless to say, you'll need a resistor for the base of the transistor - perhaps 22K or more. Depending on the current capability of the chosen op-amp, it might be able to drive the LED directly...
But as a concept, a good place to start.
Incidentally, for most power supplies, the "sense" resistor can be included in the negative feedback loop, so the regulation of the supply is maintained.
The reed switch is a reasonable idea, BTW, but it does depend on the minimum current you wish to detect. Some years ago I investigated a module made by/for Audi that detected failed brake lights, and that used reed switches - the coil was only 5 or so turns of heavy-gauge wire - of course, brake lights are 21W, so nearly 2 amps to play with...
Get your breadboard out
You need to consider the operating range of the inputs - if your PSU gets close to 0V or 24V, it will stop working reliably. All op-amps specify their common-mode operating voltage - some are rail to rail, most aren't.
I'd use a couple of resistors in series with each input - perhaps 10K or thereabouts. That will protect the op-amp to a decent extent should the voltage be allowed to exceed the safe range of the op-amp.
You might need to introduce some hysteresis for this to be reliable. A high value resistor between op-amp output and the non-inverting input - such as 1M (or more) should do the trick.
Also, the output of the op-amp won't be rail to rail. You'll find that the LED will be stuck on because the op-amp might only get down to 1-2 volts when saturated against the negative supply. Needless to say, you'll need a resistor for the base of the transistor - perhaps 22K or more. Depending on the current capability of the chosen op-amp, it might be able to drive the LED directly...
But as a concept, a good place to start.
Incidentally, for most power supplies, the "sense" resistor can be included in the negative feedback loop, so the regulation of the supply is maintained.
The reed switch is a reasonable idea, BTW, but it does depend on the minimum current you wish to detect. Some years ago I investigated a module made by/for Audi that detected failed brake lights, and that used reed switches - the coil was only 5 or so turns of heavy-gauge wire - of course, brake lights are 21W, so nearly 2 amps to play with...
Get your breadboard out
