Re: a/c switches

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Adam,

At 04:40 PM 8/3/2002 -0400, you wrote: 
........................

>
>what exactly does a power relay do?
>Adam

In a simple toggle switch, the contacts are very small and the forces and
distances that open and close them are small.  Fine for less than 5 amp
stuff that does not require too much current.

However, suppose we want to switch 100 amps at 480 three phase with a small
panel switch.  No "switch" made will have the size and power to switch
that.  A "fault" could blow a one foot diameter hole in the panel and
vaporize any switch.  And, ruin the guy that flipped it's day...  If a big
power substation contactor "goes bad", it can knock down all the chain link
fences when it explodes...

So we get a power relay.  Basically it will have very large precious metal
contacts like 1/2 inch in diameter and 1/4 inch thick.  The contacts may
have a travel of 1/2 inch and they will slam together very hard.  A big
electromagnetic coil moves the contacts.  This coil is controlled by our
little panel switch :-)

So we switch a few amps into the electromagnetic coil with the tiny switch
which moves these giant contacts into each other in the large contactor
(preferably "far away").  If the big contactor explodes, we are perfectly
safe since the tiny switch and coil are very well isolated from the mess.

Power contactors (BIG relays are also referred to as "contactors") come in
all shapes and sizes from very small to giant ones used in power
substations.  They switch like 380kV at 2000 amps there, no panel switch
will do that!

Here is a link that has more than anyone really wants to know about them:

	http://relays.tycoelectronics-dot-com/lexicon/index.stm

But don't worry about all that stuff.  They are really very simple.  Best
way to learn about them is just get one.  You will figure it out right
away.  I could not find any really good links about them but here is a guy
that made a high voltage one that basically tells all:

	http://members.tm-dot-net/lapointe/HVRelay.html

Of course, our little 30 amp 120 VAC does not require very extreme
relays...  But the little $12 one from DigiKey will switch about 10X the
power of any "toggle switch" in perfect safety.  Radio Shack also has a
few, but I am not sure the current.  The electromagnetic coils on them run
from 5, 12, 24 volts DC and 24 and 120 volts AC.  You probably want the
120VAC coils in this case so you can just power the relay's coil from the
AC line voltage.  The like 5 volt DC ones are for transistor control
circuits and such.

BTW - Ken had a great idea about checking out "home" circuit breakers and
such at the hardware store!

Currents above 15 amps start to get really dangerous just from an explosive
standpoint.  Thus, all the rules change and it starts requiring some
serious hardware to control it.  It's not too bad really once you get into
it.  With power relays, you can control just as much power as you want with
just a "little" switch. 

Cheers,

	Terry