# Measuring resistance of switched load

I am in the process of designing a custom vaporizer.

So the main coil with a resistance ranging from 0.2 Ω to 2 Ω is switched using a MOSFET shown in the circuit diagram below. The system is powered by 2 cells of Li ion.

My main problem arises when I try to include a feature to measure coil resistance (this is quite helpful while building coils and for overall safety of device). Below is the circuit diagram that I thought of. In black is primary switching circuit. In green, proposed circuit to measure resistance.

According to my research the way to go about impedance measurement is to use a constant current source to find the voltage drop. This would be okay to include in my circuit if I was using a p-channel MOSFET. But due to low available options, high current consideration I cannot go with a p-channel.

I am currently thinking of using a constant current source to draw current through the coil into a dummy load (drawn in green). Then measuring the voltage drop across the coil to estimate the resistance.

My questions are,

1. Is this an acceptable/safe way to use the current source?
2. If yes, do I even need a dummy load or can I just short to ground?
3. Is there any better way to measure it?
• For what it is worth, this was my attempt, GhettoVaper – an extended GhettoVape III, and my schematic – Greenonline Oct 21 '18 at 12:17
• Thanks for your help, although I am looking for something where I can estimate the resistance without firing the the vape. (a very low resistance could destroy the batteries) – Ashlesh Bhat Oct 21 '18 at 13:00
• Ah yes, I was trying to do (or going to do) something like that for temperature control, but I don't think that I ever got to implement that, unfortunately. – Greenonline Oct 21 '18 at 14:14
• I don't think the way you have that "test" wired will work, your Vgs voltage is now dependent on you current sink value. I would put a sense resistor (or two in series on the drain of the test FET (equivalent to your Isink current value). Your test FET source connected to ground, you have good Vgs, and can measure the differential voltage across your resistor. Measuring 0 indicates open coil. – isdi Oct 21 '18 at 16:29
• yes I agree, the green MOSFET is wrong to be used as a n channel, it should have been a p-channel. I think using sense resistor would be simpler but with a small margin of error because the current is not really constant. Which is not really a big issue for as long as i can estimate the error. I think I will probably do this for now. Thanks for your help. – Ashlesh Bhat Oct 21 '18 at 17:18