Hi Greg.

30-Mar-04 09:32:00, Greg Mayman wrote to Jasen Betts



 JB>> the really old ones were bimetalic.  those capacitor ones don't
 JB>> work to well when they get hot... (I've not compared a bimetalic
 JB>> one's performance)

 GM> I did see one bimetal relay used as a timer. The bimetal unit was
 GM> extremely tiny, and was heated by a coil around it. But it was so
 GM> tiny it could only operate a tiny microswitch contact, and the
 GM> shortest time delay it would work down to was about 20-30 seconds.

bimetalic timers are common in pop-up toasters :)

 GM> I have seen thermal flashers from US, British, Euro and Japanese
 GM> cars. They all used a VERY thin wire that stretched when heated.

hmm, that would be faster... but I'm sure I saw a bimetalic strip, but I
didn't see the device running (it was non-functional before I opened it).
maybe  the strip was for temperature compensation...

 GM> The thermal inertia was thousands of times lower than the smallest
 GM> bimetal strip I ever saw, and even then they couldn't flash faster
 GM> than about 2-3 times a second.

I've got a spare one here that I have no need of, made in mexico, looking
inside I see a thin wire with a heating coil wrapped arouund it welded at
both ends to a spring-steel diaphragm which holds the moving switch
contact.  (seems you were right)

I'm guessing that when the wire gets hot it expands and the expansion
causes the diaphragm to click into the closed contacts configuration
turning off the heater...

OTOH I've seen bimetalic strips in christas-tree lamps, and other
self-blinking lamps :) but they only do about 1 per seconds or less.

the switches to control cooking rings also use bimetallic strips
IIRC they switch about once per second too.


 -=> Bye <=-

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