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Signal relays

S

Spehro Pefhany

Jan 1, 1970
0
Didn't you know that mercury has been and is illegal?

They outlawed a naturally-occurring element? Sounds like something a
politician might do.


Best regards,
Spehro Pefhany
 
W

whit3rd

Jan 1, 1970
0
Yes this is a one off. I'm building a custom thermostat to replace my wall thermostat. I know these furnace control circuits are 24VAC...
I did look at SSRs, but I'm leery about those because when they fail, they often fail closed.

The failures are unlikely, but you can work around them by using a fuse and having
a second switch (needn't be an SSR) which is normally OPEN when the SSR is closed.
The second switch, when the SSR is commanded OPEN, gets commanded CLOSED
and blows the fuse. This kind of thing is common in interlocks (microwave oven door
switches, for example).
The 3V limitation is actually a preference because the microcontroller used is 3.3V. I could have another regulator to provide relay coil power, but thought it might be nice to have one regulator.

Relays don't require regulated power. You can use a single transistor to switch the coil
current, and just drive the transistor's base from the logic (with suitable current-limiting).
 
S

Spehro Pefhany

Jan 1, 1970
0
The failures are unlikely,
^^^^^^^^
You misspelled "inevitable".
but you can work around them by using a fuse and having
a second switch (needn't be an SSR) which is normally OPEN when the SSR is closed.
The second switch, when the SSR is commanded OPEN, gets commanded CLOSED
and blows the fuse. This kind of thing is common in interlocks (microwave oven door
switches, for example).


Relays don't require regulated power. You can use a single transistor to switch the coil
current, and just drive the transistor's base from the logic (with suitable current-limiting).


Best regards,
Spehro Pefhany
 
R

Robert Baer

Jan 1, 1970
0
Spehro said:
They outlawed a naturally-occurring element? Sounds like something a
politician might do.


Best regards,
Spehro Pefhany
Well, what do you expect from the EC?
They outlawed lead, forcing us to make PCs with "cheaper" silver..
 
F

Frank Miles

Jan 1, 1970
0
Didn't you know that mercury has been and is illegal?

That might be great flame-bait but it ain't so, at least not as stated. Example: there is
a family of connectors using mercury raceways that work in rotated systems that
are still produced and sold.
 
Don't over-complicate this. Just use a mechanical relay! Dirt simple

and stone cold reliable.



Every electronic thermostat whose circuit I've analyzed uses bistable

relays.

I just thought it would be interesting to note that I took apart the original wall thermostat, a Honeywell, to see what it used. It made no noise so I figured it was solid state. It was a......6 amp TRIAC! With what appeared to be a varistor across it's main terminals.

I still went with latching relays. I'll post the web page with the design later when it's done.
 
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