Reed switch testing

A common failure mode of reed switches is for the reeds themselves to get magnetisedand so the leaved repel each other , regardless of an external magnet. Back in the day of audio cassette/R-R players, an audio head demagnetiser was your friend

Are you sure it's a reed switch (two wires)? If it has more than two wires it could be a hall sensor, which would need to be powered to have it working. The current from the multimeter probes may not be enough to activate it fully.

Bigger pics:

Theo

It does not sound normal - at least not for a "normal" reed switch. However you can get some with built in resistors. These tend to be bigger encapsulated packages though, rather than the bare glass tube.

Thanks Theo. Let me upload some photos of mine. Hopefully that will help

Thanks John. I have uploaded some photos here

Hope these help

OK, that's a regular reed switch and it's dead Jim. My first port of call would be to find one of the same dimensions and replace it.

I'm guessing there's some muck on the contacts which is why it's high resistance. Or a micro fracture somewhere.

Theo

Agree 100%

I am in the throes of re magnetron-ing a microwave. Apparently any resistance between the cathode heater pins and ground is a nono...

I could buy a whole new smaller oven for the price of the magnetron. It had better be good. The one I took out had seriously bent fins on it.

Failed after 5 years. How long do your ovens last?

Thanks Theo. I have had a quick look online. I can get a replacement board from eSpares for £20! All it has on it is the reed switch.

I found a 250v reed switch on RS but you have to buy 500 of them! Is it only physical size and voltage I need to worry about? When looking online besides the RS one I could only find ones from China. Is there a UK company who would sell me one or a few?

Many thanks

Lee.

From the photos you have posted are you sure that its not just a dry solder joint? Try putting your multimeter probes on the wires from the glass encapsulation rather than on the circuit board tracks or connector pins.

In matters such as this, ebay is your friend. Some bloke in a bedsit in leeds who imports stuff in 500s from china will pop one in an envelope and grab a few quid on his paypal account

If you want one guaranteed at 250V, which I doubt you need - I'd expect this to feed some low voltage electronics - there are plenty more on ebay

Of course you need to be able to successfully remove (cut the wires and remove the lead tails individually) the old part and solder the new part in ....

Otherwise £20 for the board seems reasonable to me. certainly it would take me at least 20 minutes to replace a reed switch in a board, and that is at £60/hr, £20...

Our Hitachi microwave is 42 years old and still in use every day.

Yep. Back in the days when they cost £1000 instead of £100. I knew a man one who claimed that he was employed by John Lewis back in the day and accidentally updated a price list for all their microwaves to a figure that was much lower than it had been. John Lewis sold so many that they reduced the price permanently.

Yup, more than likely....

although do a quick test with the meter on diode mode and check it in both directions - you can get some with a diode built in.

That's true - now I see the pics on a bigger screen it looks like there could be fractures where the solder joint meets the track, with the joint having torn off the track slightly. Scraping off the soldermask and reflowing the solder joint could bridge that gap.

Theo

Never had one fail. Our original was passed on to a friend, and it was still going strong when his dementia meant he could no longer use it.

Is replacement lasted 20 years, needed a replacement turntable motor due to a broken gear, but only replaced to match new oven...

Dave

Thanks both. I thought this was a great idea. I tried testing it using the wires on the reed switch as suggested. Seems like it has now stopped working all together. Even using the board connectors I used before came up with the same. So maybe the magnet I have been using for testing was too strong and killed it for good or maybe finished it off ?

In terms of replacement when I measured the voltage on it it was almost

This one seems ok but has a max switching voltage 100V DC and high voltage of 250V DC. Not sure what the difference is and assume AC Vs DC is not an issue for this??

Many thanks

Lee.

You probably could physically see the switch closing - perhaps use a torch behind it to illuminate the reed within the glass enclosure

The position of the magnet may/will make the difference to its operation.

See

My guess is that the contacts have simply arced up and no longer conduct, and with a sealed reed switch that's the end of it