Microswitch question

[stuhaynes]

Following the excellent advice received from forumites I believe I've sorted my water pressure switch problem. However something happened an hour ago that came as a complete surprise, but before that...

I've bypassed the inbuilt pressure switch that caused so much grief and fitted an external inline switch, I also fitted an external 24 volt / 30 amp relay. It's wired in such a way that the only current carried through the (new) microswitch is the small amount necessary to energise the relay. Everything working perfectly, up to an hour ago....

The problem had to be the microswitch in the inline pressure adjuster. Any way, took off the complete inline switch and removed the micro switch, tested it and sure enough it wasn't working. What surprised me was that when I pried the lid off the switch the insides had collapsed. Fortunately I'd bought a spare and it's up and working again.

My question is.. Do these switches come in different qualities? The ones I bought off Ebay were only 99P each and I wonder if these are just poor quality imports. I'm pretty sure that the vibration in the water line has literally shaken the switch apart. Are there grades that resist vibration?

 

[Evadne]

I've never seen that before, without the aid of salt water, and I use the larger sizes (V3 for instance) at work. But then I've never paid so little for a microswitch. Generically sized components go from the sublime (US made mil-spec) to the ridiculous (made in China, sold on Ebay) and the prices generally reflect that.

I missed your original thread, I'm afraid, but if you paid 3 or more times that amount for one from Farnell or RS then it will probably be a lot better, you can get them in IP68 rated housings as well.

 

[VicMallows]

Did the microswitch fail open circuit or shorted?

Vic

 

[lw395]

Yes, the cheap ones can be rubbish.
I just bought some from rswww.com, there are many variants so it's worth having access to the manufacturer's drawings.

 

[nimbusgb]

The problem is that the coil of the relay needs a flyback diode fitted to it.

When the microswitch goes open circuit the back-emf caused by the collapsing field of the relay coil causes all sorts of trouble, probably arcing the light contacts in the microswitch. Not long before it falls apart. get a 1N 4007 and fit it across the relay coil. ( the line on the diode goes to the positive side of the circuit )

pictures and explanation - http://en.wikipedia.org/wiki/Flyback_diode

 

[stuhaynes]

The problem is that the coil of the relay needs a flyback diode fitted to it.

When the microswitch goes open circuit the back-emf caused by the collapsing field of the relay coil causes all sorts of trouble, probably arcing the light contacts in the microswitch. Not long before it falls apart. get a 1N 4007 and fit it across the relay coil. ( the line on the diode goes to the positive side of the circuit )

pictures and explanation - http://en.wikipedia.org/wiki/Flyback_diode

Did the microswitch fail open circuit or shorted?

Vic

The switch failed leaving power to the relay and forcing the PRV to exhaust on the calorifier. I checked the contacts inside the switch and they looked pristine. Apologies for multiposting, but the multiquote button won't work for some reason.

 

[stuhaynes]

The problem is that the coil of the relay needs a flyback diode fitted to it.

When the microswitch goes open circuit the back-emf caused by the collapsing field of the relay coil causes all sorts of trouble, probably arcing the light contacts in the microswitch. Not long before it falls apart. get a 1N 4007 and fit it across the relay coil. ( the line on the diode goes to the positive side of the circuit )

pictures and explanation - http://en.wikipedia.org/wiki/Flyback_diode

I wondered about this. I'll get one. However, the contacts were fine inside, it just physically fell to bits inside. The contact link is held on a very small pivot pin, it was that that fell out collapsing the gubbins inside.

 

[Billjratt]

You get what you pay for don't you... Good ones used to be made by Honeywell.

 

[stuhaynes]

IN 4007 flyback diode question

The problem is that the coil of the relay needs a flyback diode fitted to it.

When the microswitch goes open circuit the back-emf caused by the collapsing field of the relay coil causes all sorts of trouble, probably arcing the light contacts in the microswitch. Not long before it falls apart. get a 1N 4007 and fit it across the relay coil. ( the line on the diode goes to the positive side of the circuit )

pictures and explanation - http://en.wikipedia.org/wiki/Flyback_diode

Thanks nimbusgb

Been in touch with RS and they've given me a page with 7 diodes. I'm not sure which would be most suitable, bearing in mind that we're 24 volts

IN4007 from RS

Any advice?

Many thanks

 

[VicMallows]

Thanks nimbusgb

Been in touch with RS and they've given me a page with 7 diodes. I'm not sure which would be most suitable, bearing in mind that we're 24 volts


Many thanks

1N4007 will be absolutely fine. It's the 'default' silicon diode that everyone uses for non-fussy applications like this. It's been around for over 40 years now!

Trivia: there was originally a whole family 1N4001 up to 1N4007. 1N4007 had the highest maximum voltage rating and accordingly was the most expensive - around 25p each individually even back then. They are now so cheap that AFAIK only the 1N4007 is produced.

 

[elton]

I really don't think you need a diode to protect the microswitch. The contacts will easily handle the currents and back-emf voltage of the relay. The whole point of using the relay was so the microswitch contacts would't be switching high currents as I recall.

 

[stuhaynes]

1N4007 will be absolutely fine. It's the 'default' silicon diode that everyone uses for non-fussy applications like this. It's been around for over 40 years now!

Trivia: there was originally a whole family 1N4001 up to 1N4007. 1N4007 had the highest maximum voltage rating and accordingly was the most expensive - around 25p each individually even back then. They are now so cheap that AFAIK only the 1N4007 is produced.

Thanks for that Vic, I understand it now. I've just got to find a stockist who'll sell me one, rather than a shovel full!

Thanks again.

 

[stuhaynes]

I really don't think you need a diode to protect the microswitch. The contacts will easily handle the currents and back-emf voltage of the relay. The whole point of using the relay was so the microswitch contacts would't be switching high currents as I recall.

Hi Elton

Using the relay you recommended some weeks ago is helping to sort the problem. The microswitch is fine. However the contacts in the relay are getting hammered, this is why I want to dampen the back current, definite evidence of arcing.

Thanks again for the help, Stu

 

[elton]

Hi Elton

Using the relay you recommended some weeks ago is helping to sort the problem. The microswitch is fine. However the contacts in the relay are getting hammered, this is why I want to dampen the back current, definite evidence of arcing.

Thanks again for the help, Stu

Sorry, I must have misundersood this:

I wondered about this. I'll get one. However, the contacts were fine inside, it just physically fell to bits inside. The contact link is held on a very small pivot pin, it was that that fell out collapsing the gubbins inside.

 

[elton]

Hi Elton

Using the relay you recommended some weeks ago is helping to sort the problem. The microswitch is fine. However the contacts in the relay are getting hammered, this is why I want to dampen the back current, definite evidence of arcing.

Thanks again for the help, Stu

Ah, Ok, I was a little slow on the uptake. Must remember not to post so soon after waking from a nap

 

[stuhaynes]

Ah, Ok, I was a little slow on the uptake. Must remember not to post so soon after waking from a nap

Thanks Elton. When I ordered the 2 new micro switches (one being a spare), I also ordered the relay. I used Ebay and 'saved' £15 against RS comps. The first switch simply collapsed inside, I had a look. The relay also pulled a wobbly and stopped working in a very strange way..... Anyway, back to RS yesterday and had a new (expensive) relay delivered today. Fitted and working, switch okay. Checked inside the relay and real evidence of arcing across the contacts.

It's working okay now, but I've ordered the IN4007 diode (there's actually 50 of 'em!) because I'm convinced that if I don't lose the back EMF this is going to continue to happen.

This pressure switch problem has been a total pain in the *ss, but I think I'm getting there. G*d I hope so. I think there is a little saying, you know the one, something about 'something' being a dangerous thing.

Thanks for all the advice... I hope we've heard the last of my adventures into water pump land!