LED monitor for solenoid

Question

I'm trying to determine whether it is more appropriate to place an LED in series or in parallel with a solenoid. The application is to monitor the solenoid, i.e. if the solenoid is burnt out, the LED turns off giving indication that the solenoid is burnt out. I am using 24Vdc source.

If I connect the LED in parallel with the solenoid, my concern is that the LED is of much less resistance than the solenoid, and the current will take the path of least resistance. Thus bypassing the solenoid.

If I connect the LED in series with the solenoid, I am unsure how the solenoid acts if its "burnt out." Will it be equivalent to an open circuit or a short? If it is open, then the LED would work as there is no continuity in the circuit, but if the solenoid is shorted, the LED indication would remain turned on.

Can all this be explained with Kirchoff's laws?

Answer 1

How much current does the solenoid draw? (what's it's resistance?) If it's low enough then maybe the LED in series will work. If not you'll have to monitor the current and turn the LED on when it's flowing (as tcrosley did with the 1 ohm resistor.)

^{simulate this circuit – Schematic created using CircuitLab}

This may be a bit simpler. You have to choose R1 such that it drops ~1.0 Volts for the solenoid current. R3 then limits the current to the npn and R2 sets the LED current. This assumes that the solenoid can float. If not then a pnp on the high side.

Answer 2

The following circuit should accomplish what you want:

When the solenoid is on, there will be a small voltage drop across the resistor R1, which will turn on the comparator and then turn on the LED. If the solenoid should burn out causing an open circuit, there will be no voltage drop across R1, the comparator will be off as well as the LED.

Make sure to use a resistor for R1 with the proper power rating.

The differential comparator TL331 has an open-drain output, so a FET is needed to drive the LED.

I am using the same FET for Q1 as well, but you probably already have a driving circuit for the solenoid; this was for illustrative purposes only.