Cricuit Design help - blower motor powered by car battery

Thenior · Post by **Thenior** » Thu Dec 09, 2010 7:51 pm

Need some help on this one.

I am horribly with electricity and electronics - the only reason I have gotten as far as I have is staring out several diagrams and ask some other people questions.

Let me explain...

I have a buick lesabre that has automatic temperature control. Needless to say, it sucks. The fan won't always kick on, and since it's now 10-20 degrees here, I really need it to work. I don't like automatic temperature stuff anyway, so I am going to directly wire it to the battery, and setup a variable switch to increase the speed of the blower motor.

Here is a sketch diagram of what I think I need. Does this look right?

Spidey · Post by **Spidey** » Thu Dec 09, 2010 8:24 pm

Why use a switch to control a relay, can’t you just use the switch to turn on and off the motor?

I assume the switch won’t handle 30 amps?

Looks good tho.

EDIT: ok never mind the dumb switch question, I assume you want to mount the switch on the dash, and put the relay at the PWM.

Thenior · Post by **Thenior** » Thu Dec 09, 2010 8:39 pm

The switch is not entirely necessary. The only reason I am using it is because this is directly wired to the battery - I don't want the blower motor even possibly getting any juice when the car isn't on (or rather, when the switch isn't on). The PWM has a built in potentiometer switch to control the motor speed.

But I just found this diagram from someone else, so now I am confused : / Should I put the switch after the coil on the relay, or before (in between the battery and the relay)? The whole point of the relay is because the switch isn't made to handle heavier amps.

Spidey · Post by **Spidey** » Thu Dec 09, 2010 9:03 pm

I assume terms 1 & 2 are the coil on that relay, and 3 & 5 are NO contacts.

The switch goes in series with the coil, and can be placed on either side. (hot side or ground side)

Your drawing is better because the fuse protects both the coil and the device.

Spidey · Post by **Spidey** » Fri Dec 10, 2010 7:48 pm

The basic difference between the placement of the switch is this…

What will happen if the switch wiring shorts to ground…

If the switch is on the hot (positive) side the fuse will blow.

If the switch is on the ground side…the motor will run.

It’s a rule of thumb to place any switch in a circuit on the hot (in this case, positive) side.

In home wiring it’s a personal safety issue.

snoopy · Post by **snoopy** » Mon Dec 13, 2010 7:09 pm

I think you're doing it right.