Barry Schwartz wrote:
> Just a little clarification here - ... The switch can be either
> before, or after the relay (or light or whatever). It doesn't
> really matter to the device being switched. Just because the
> switch is connected to the frame or ground after the switched
> device doesn't mean your switching the negative lead. Actually,
> I guess it depends on where one determines the switchover
> point I consider any "lead' coming from the positive side of
> the battery to be a positive lead until it contacts the frame
> or earth ground - however as I stated earlier, the switch
> doesn't care :-)
Barry,
You're right that it doesn't matter when the circuit is on
where the switch is. However, when the switch is off, it does
matter, for reasons of safety and avoiding fires, etc.
I'll use the example of household wiring, because this is
where it is really obvious. Consider the standard light
switch for an overhead light in a room. As you point out,
the light switch will work perfectly well whether the
neutral wire (white in household circuits), or the "hot"
wire (black in household circuits) is switched.
(Yes, alternating current (AC) household circuits have
a "neutral" wire, and a "hot" wire.)
Regardless of where the switch is, if the switch is
closed (on) then the current flows, and the light bulb
lights. Similarly, if the switch is open (off) then
no current flows, and the light bulb is off.
However, if the neutral wire is where the switch is,
then the wiring at the light socket is "hot", even when
the switch is off. This means that there is 120VAC there
at the light socket, just waiting for a path to ground.
If somebody goes to change this light bulb, and accidentally
touches the socket, they're in for quite a surprise, even
though the switch was off! If the switch were wired
properly (in the hot wire, before the light), then when
the switch is turned off, somebody could stick their finger
in the light socket all they want, and be perfectly safe.
Needless to say, for this reason, household electrical
codes require the hot wire to be switched. However,
this kind of thing isn't understood by all "DIY"
electricians, and can thus get them seriously hurt.
Now, of course, a car is different than a house, but
the same principles apply. Basically, one generally
wants the "hot" portion of the wiring to be as short
as possible when the switch is off. In this way, there
is less wiring that is prone to shorting out when the
switch is off. Also, in this way, when the device is
switched "off", there isn't a "hot" wire at the device,
in case somebody goes to replace that device, and is
suddenly surprised to find out that there's power there,
even when the device is turned off! (This is the reason
that the shop manuals almost always say to first disconnect
the battery before touching anything in the wiring system.)
For standard operation, you are 100% correct. However,
in order to maximize safety, and minimize the likelihood
of undesired shorts, it is always best to switch the
hot lead, and keep the section of unswitched hot wire
as short as possible.
> (donning flame retardant suit, fire extinguisher at the ready!)
Make sure you have that fire extinguisher real close
by if you've done any of your household wiring with
switching the neutral wire! :-) :-)
--ken
'70/'74 TR6 Daily Drivers and Digital Multimeter in the trunk, always!
--
Kenneth B. Streeter | EMAIL: streeter@sanders.com
Sanders, PTP2-A001 |
PO Box 868 | Voice: (603) 885-9604
Nashua, NH 03061 | Fax: (603) 885-0631
|