Why is this solenoid coil increasing in resistance? - Page 1
Forum section: electronics / beginners

author: jmh
author_rank: Contributor
timestamp: January 04, 2022, 06:14:22 pm
content: I have a solenoid from an electric shower that failed. When extracted it read 3.5M ohms on a DMM (the replacement is 3.5k). But I noticed the resistance increasing.I've cleaned it (just in case) and attached it to a bench DMM and it has been sat for a few hours increasing in resistance, to 8.5M and still climbing. I've not been at all scientific about it, no time measurement or environmental control. But, why is it increasing? The only thing that came to mind was it has got water in and is drying out...Not at all important - I mean it's duff and has been replaced but I am interested to learn what effect might be in play here.

author: Capernicus
author_rank: Frequent Contributor
timestamp: January 04, 2022, 06:19:41 pm
content: Thats happened to me before checking charcoal sticks, is it a fault in the multimetre? running out of battery? Ive got no idea.

author: bob91343
author_rank: Super Contributor
timestamp: January 04, 2022, 06:23:41 pm
content: I suspect moisture drying out.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 04, 2022, 06:26:39 pm
content: Clearly the coil has been failed as open circuit, which is why it isn't working.Additionally, there is contamination (likely water) that bridges the tiny gap, making it conduct just enough you can detect it with your multimeter. Now it's just drying out.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 04, 2022, 06:27:39 pm
content: Quote from: jmh on January 04, 2022, 06:14:22 pmMy guess: The wire is broken at some point and the resistance really should be "infinite" (open circuit) for the DMM. There might be a small amount of water trapped inside the coil that slowly evaporates, so this could be the cause for the increasing resistance. DMM should say "overrange" if you wait for long enough or heat the coil. So basically I agree to your guess.

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2022, 06:46:32 pm
content: Its a solonoid - wound wire on a core. It should measure at most a few kiloohms if its got a *LOT* of very fine wire in it.We therefore know the solonoid has gone open circuit. The odds are, moisture has got into it and corroded the wire away at some point along its length, most likely near its terminals, where it wouldn't be protected by its enamel insulation.Chances are the change in resistance is electrochemical: Copper is being eroded from inside the wire enamel and plating out on the much larger terminal, increasing the gap between them, and thus the surface leakage path length. It *could* be due to it drying out - you can test that by disconnecting it and only reconnecting it to take measurements occasionally. If the trend stays similar, its drying out. If it vastly slows or even reverses slightly, its electrochemical.

author: Capernicus
author_rank: Frequent Contributor
timestamp: January 04, 2022, 08:00:17 pm
content: there could be parastic capacitance and inductance developed in there maybe?

author: emece67
author_rank: Frequent Contributor
timestamp: January 04, 2022, 10:11:00 pm
content: .