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I am looking for a way to detect a short in a piece of wire. I want for an LED to light up when the wire is shorted. i started off by using an op-amp comparator but this didnt work.

the specs of the task "

*The requirement is to design a circuit, to drive an electromagnet solenoid, over a length of connecting cable. The cable could be damaged (short-circuit or open-circuit), so the circuit must detect these conditions.

If the solenoid is open-circuit, a red LED must be illuminated indicating open-circuit. If it is short-circuit, then another red LED must be illuminated. If neither, then it’s assumed OK, and a third (green) LED must be illuminated.

The circuit needs a push-button to turn the solenoid ‘on’ (but only if it is OK) and another button to turn it ‘off’ again. When the solenoid is energised, a fourth (power on) LED must be lit.

If the solenoid goes open or short circuit while powered (such as by someone wiggling the cable), the drive to the solenoid must be cut off and the appropriate LED illuminated. And if it then comes good again (such as an intermittent cable), the ‘on’ button must be pressed again to re-apply power.

The solenoid characteristics are: 28V DC operating voltage (operating range 18 – 32V), coil resistance is 76-84Ω. The solenoid will not operate below 5V, definitely operates above 18V, ie somewhere between 5 and 18V it pulls in. When once pulled in, it drops out between 12V and 2V (so if the drive circuit supplies less than 2V the electromagnet will definitely let go).

Assume that a power supply 28V or thereabouts is available to operate the circuit, and that it’s quite permissible to put low-level power to the solenoid to detect if it is OK or not. The task is to sketch out a circuit which will do this (with approximate component values, and type numbers of any diodes, transistors, IC’s etc used), with a brief explanation of how it works."*

I have attached my first attempt as a sketch. enter image description here

Any help is appreciated.

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  • \$\begingroup\$ Identify EMC, ESD requirements and do you want low V detect too? or fault in the driver? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 12 '18 at 15:29
  • \$\begingroup\$ It is not so clear which wire you're actually monitoring, I'm assuming it is the red one. When open (not shorted, also: shorted to what?) the opamp's inputs are undefined: that's never a good idea. \$\endgroup\$ – Bimpelrekkie Mar 12 '18 at 15:32
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    \$\begingroup\$ Hi, Tony this is going to sound bad but i am not sure. this is part of an application. I am an electronics student and i didn't think id get this far. I'm just going on what the spec sheet say. as for the driver, only the wire needs to be detected because its not supposed to turn on unless the wire is ok \$\endgroup\$ – Steve Trezise Mar 12 '18 at 15:33
  • \$\begingroup\$ "i started off by using an op-amp comparator but this didnt work" - what about this didn't work? Also, I recommend you 1) redraw your hand-sketched circuit in the built-in schematic tool, and 2) label the solenoid and cable. It's not clear in this diagram how the red cable (which Bimpelrekkie thinks is the one you're monitoring) connects to the solenoid (which I assume is the part in the middle right of the circuit and in no way connected to the cable) \$\endgroup\$ – Selvek Mar 12 '18 at 15:39
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    \$\begingroup\$ Selvek, thanks i will clarify. sorry for the ambiguity. my intention was to have the output of the NOR gate be responsible for turning on the solenoid by ANDing it with the power on switch \$\endgroup\$ – Steve Trezise Mar 12 '18 at 15:43
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enter image description here

This is a short circuit protection circuit simulation I had laying around. The mosfet has a max input spec of 30V. You will have to change V1 to 28V, and maybe add a zener to protect the mosfet gate voltage. May also have to change the comparator supply voltage. V2 is the reference voltage for short circuit detection. Voltage at load below V2 indicates short circuit. V5 is a push button switch that supplies a pulse to start the mosfet. Output of the comparator can be used to drive LED. This circuit checks short circuit at full load, and is preferred over checking short circuit only at startup (what if a short circuit develops while the solenoid is operating?)

To detect open circuit check for ground through the solenoid. This will work as long as 28V supply is off, which is what you want. With 28V supply, op amp input is no longer at ground, so it will show open circuit. The only way to monitor open circuit while 28V is on will be to monitor current drop on the supply line (series resistor and differential amplifier).

enter image description here

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