As I understand your description, the accepted method would be to run
hot to the common relay terminal (the jumpered connection between one
leg of the coil and one side of the relay contacts). Then the other
leg of the coil runs to your turn-on switch -- the switch is then
grounded on its other side. The other side of the relay contacts goes
to your fan, and the other fan terminal is grounded. This setup is
used because if a fault develops in your turn-on switch (the most
likely location for trouble) then the fan will just turn on -- a
shorted switch in this setup just closes the coil circuit and doesn't
provide a full current short to ground. Horn relays are typically set
up like this, and when the horn switch fails the horn blares
continuously (annoying, but you're alerted to the problem and there's
not the fire risk of a hidden short to ground).
Another possibility is to ground the relay's common terminal. Then
the turn-on switch is wired to the relay coil on one side, and hot on
it's other side. Likewise, the fan is wired to the relay contact
terminal on one side, and hot on its other side. This setup is
inferior, since it relies on a properly fused hot supply to avoid fire
risk in the event of switch failure. It works, but is not recommended.
A third possibility seems to be what you were suggesting -- using the
turn-on switch to switch ground to the common relay terminal. Here
the coil terminal would be wired to hot, and the fan would be
connected to the relay contactor on one side and to hot on its other
side. The common relay terminal would be wired to the turn-on switch,
then on to ground. This is definitely not recommended, since it
defeats the purpose of using the relay. The relay's purpose is to
reduce current flow through the turn-on switch for reliability, or to
allow lo-current dash-type switches to activate the device. You don't
want the full device current running through the turn-on switch. This
setup is actually worse than using no relay at all, since full device
current flows through the switch, plus now you've added coil current.
It would operate (once, anyway!), but eat up your switch even faster
than just wiring the fan and switch directly without a relay.
M Dietsche
---Dave Vrba <Dave_Vrba@mail.sel.sony.com> wrote:
>
> ATTENTION electrical gurus on the list.
>
> Note: I have a NEGATIVE ground system on my '62 sprite with a
1275
> engine.
>
> I am going to add a "push fan" to my radiator for the "stop or
slow in
> traffic times" to attempt to keep cooler. Traveling from 0 to
15 MPH
> for extended periods of time is a potential overheat condition
for my
> sprite. I wish to use a relay. Should the relay coil and the
fan
> motor GROUNDS be the correct legs to switch? Another way of
putting
> it would be to attach the HOT + to the relay common. (One side
of the
> pull in coil and common terminal of relay, which are separate
> terminals but jumpered at the relay base) Then when the NO
contact
> closes, when the coil activation switch (manual now , thermo
maybe
> later) is closed, the fan would be connected through the fan
motor to
> ground to complete the circuit. Thus running.....
>
> I propose this way to minimize the run on the HOT wire through
the car
> thus reducing the potential of shorting somewhere.
>
> Does anyone know of a good source for a (adjustable ?) thermo
switch
> that could be inserted into the spare temp gauge probe outlet
in the
> radiator? (I have this plugged now) Currently, I have the temp
gauge
> probe connected to the block outlet for better gauge feedback
of real
> engine temp conditions.
>
> I hope I haven't rambled to much to confuse the issue(s) at
hand.
> Once again, thanks to the list for help.
>
> David L. Vrba
> '62 A/H Sprite MKII
> HAN6L14083
> Dublin, CA
>
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