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12 volt light swith

D

daestrom

Jan 1, 1970
0
Ray Drouillard said:
The idea is to make and break the connection quickly, and to provide a
large enough gap to break the arc if it does happen. Any mercury switch
I have ever seen has a large gap -- if for no other reason than to keep
the inevitable splashes from closing the switch.

Well then not all mercury switches are made the same. I've seen quite a
few, both light switches and thermostats and the contacts are quite close.
One model has two contacts in one end of the glass 'envelope' spaced only
about 1/8 inch apart. The mercury either fills the space between them
('on') or runs to the opposite end of the glass 'envelope' (3/4 to 1 inch
long).

But maybe they fill them with a gas to 'quench' the arc? Something over 1
atm pressure too?

daestrom
 
D

Don Kelly

Jan 1, 1970
0
Ray Drouillard said:
The idea is to make and break the connection quickly, and to provide a
large enough gap to break the arc if it does happen. Any mercury switch
I have ever seen has a large gap -- if for no other reason than to keep
the inevitable splashes from closing the switch.

Hey, I wonder if the lights flicker in an earthquake when you use those
switches.

Anyhow, I have never had a problem switching DC, and I switch some
pretty big currents when I'm working with some of the accessories in a
car. I understand the theory of why it's harder to switch DC, but I
have never actually seen it cause trouble.


Ray
The mercury switches that I have seen (and designed for 120V 15A, may or may
not be pretty dubious at 12V, 5A DC. Small gap, contacts close together-just
roll mercury to the other end of a tube (as used in many thermostats).
Possibly gas filled as Daestrom indicates but no "color as Duane said. The
switches used in automotive work are intended for low voltage DC use. At
one time (still so?) 120V 15A AC switches were derated to 5A at 15V and
these did have a good snap action and gap.
Older household switches (heavy spring and snap action as well as a wide gap
and allowing arc extension) were capable of handling DC but newer ones are
not- in the interest of quietness and ease of operation.
However, I never tried a mercury switch on DC at 12V, any current.
I know from experience that an arc more than sufficient to light a cigarette
is possible at 120V, 5A DC.
 
B

Bruce in Alaska

Jan 1, 1970
0
The mercury switches that I have seen (and designed for 120V 15A, may or may
not be pretty dubious at 12V, 5A DC. Small gap, contacts close together-just
roll mercury to the other end of a tube (as used in many thermostats).
Possibly gas filled as Daestrom indicates but no "color as Duane said. The
switches used in automotive work are intended for low voltage DC use. At
one time (still so?) 120V 15A AC switches were derated to 5A at 15V and
these did have a good snap action and gap.
Older household switches (heavy spring and snap action as well as a wide gap
and allowing arc extension) were capable of handling DC but newer ones are
not- in the interest of quietness and ease of operation.
However, I never tried a mercury switch on DC at 12V, any current.
I know from experience that an arc more than sufficient to light a cigarette
is possible at 120V, 5A DC.

Back in the OLD days, there was a Boat Autopilot built by Metal Marine
Pilot of Tacoma, WA that used Mercury Switches to switch 12/24/32 Vdc
at better than 20 amps. They were controlling a large DC motor that
was driving a geartrain, that then drove a chain loop, that turned the
Helm Wheel. The system used two switches, setup in a mutually exclusive
state that reversed the motor rotation and keep the boat going in
a straight line. There were a number of models and versions, of this
system and they started out with Glass Switches with flexible leads
mounted on a Teeter/Tooter Board that was moved by a selenoid. (to
provide the mutual exclusivity) the last version had sealed Mercury
Switches that used electromagnetic coils to change the switch state.
Very reliable stuff. I still see some of the very old systems on
fishing boats. Some of these are 50 years old now and still going.

Bruce in alaska
 
D

Don Kelly

Jan 1, 1970
0
Thank you- I do imagine that these were better than the typical mercury
switch used for domestic 120V AC service.
 
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