Re: a/c switches

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

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

Hi Adam,

Don't use 12 volt DC automotive switches.  They are high current but very low
voltage.  AC switches are made to open and break 120VAC (277VAC) safely without
catching fire or exploding.  That requires special contacts, materials, and a
certain level of quality that many car switches don't have.  You need a "real"
switch for 20 to 30 amps!

I can get 20 amp switches from a local shop but a quick look in a few catalogs
did not have them.  The problem is there are many safety standards now that
don't like high power AC hand switches.  Too many people have had them explode
in their hand...  Probably a good thing since they are pretty dangerous
really.  Running a light or something is not too bad but if they switch into a
short or big inductive load they can arc and dramatically burn up.

Exactly what is hooked to you switch?  If it is inductive or above 20 amps, you
almost have to use a power relay instead.  Check these out from DigiKey

http://info.digikey-dot-com/T022/V5/0753.pdf 

The # Z818-ND  is a DPDT 25 amp 220VAC unit for $11.70.

This baby will switch 240 VAC into a dead short and come back for more :-))

The coil is 120VAC so you just wire it up with any low current AC switch like
from Radio Shack to the 120 volts.  You can also pull the relay out of the case
to inspect it or make an emergency repair if needed.  That is very useful if
you zap the relay hard and wonder if it was damaged.

I would avoid the SPST relays since they cost almost as much as DPDT ones and
the DPDT ones are far more versatile.  You can also wire the contacts in
parallel for 40 amps (it really does not work that way, but...) 

Big contactors are commonly found in old high power things like air
conditioners and such that you may be able to find for free.  Problem is, they
may be in pretty poor shape.

You can sometimes use circuit breakers as switches, but good ones are pretty
expensive.  Maybe not to bad since they act as current trips too.

DigiKey as some nice ones for about $25. like the #PB420-ND

http://info.digikey-dot-com/T022/V5/0868.pdf

(upper right W23 & W31 series)

This may be a great way to go.  Note that these are "thermal" breakers that do
not like dead shorts.  They are made for "mild" overloads, not flaming dead
shorts.  For that you need the "Magnetic Hydraulic" types like at the bottom of
the page.  If you need higher current, you may be able to get the dual or three
phase ones and separate the breakers for individual use.  That requires a
drill, some small screws and voids the warranty... ;-)



I assume you mean a panel meter.  DigiKey has some for about $60 each.

http://info.digikey-dot-com/T022/V5/0950.pdf

A lot of money, but they are pretty nice.  

Check out Ebay Item # 1754396200 

New meters run in the 50-80 dollar range but used or salvage is 0 - 10
dollars.  Not that many ammeters for high current AC run with a transformer. 
You may want to study the data at:

http://www.simpsonelectric-dot-com/anaind.htm

http://www.simpsonelectric-dot-com/pdf/webpdfe/Wide-Vue%20AAC.pdf

To see how they are used.  I think the current transformers are "extra cost".

Cheers,

        Terry



At 11:53 AM 8/3/2002 -0400, you wrote: 
>
> Where can i get some switches that can take 20-30amps? i went to an auto
> store and they had 50amp switches, i am not sure of the voltage though. Would
> these work for 120 volts? Also, where can i get an amp meter?
> Thanks
> Adam