Wiring for demister fans

[Fingolfin]

Hi all,
I've been undertaking some homebrew upgrades to Mog's round Smiths heater, which I'll detail in a later post. For now, I need another pair of eyes for the wiring; I'm adding some small CPU fans into the demister vents to act as boosters for the airflow.

What I want is this:
1) the demister fans run with variable power from the rheostat, alongside the blower motor;
2) the demister fans have a separate toggle switch to run them at full power without the blower motor running (for clearing the screen on hot/rainy days)

Here's the wiring I've come up with. Let me know if you think it will work, or if I need to make modifications. I'm particularly curious about what will happen if both switches are on simultaneously...

Heater wiring.png (9.12 KiB) Viewed 4142 times

[philthehill]

The way you have done the wiring diagram - as soon as you engage the toggle switch the full current will feed the blower motor.

If you only want the demister fans working you need to install a two way isolating switch instead of a one way toggle switch.

The two way isolating switch will allow the rheostat to control both the blower motor and demister fans as required and/or isolating the blowing motor so allowing only the demister fans to work.

[StillGotMy1stCar]

Hello, what you could do is add a diode (1N4002). As you've drawn it, cut the wire from the rheostat to the switch and add the diode with the band facing the small fans, that is assuming the car is negative earth, if it's positive earth turn the diode around.
Regards John

[IslipMinor]

Something like this?

Heater Wiring + Diode.jpg (32.83 KiB) Viewed 4109 times

That would give you 3 options:

'Demister Switch' 'OFF' and rheostat 'ON'- variable speed to all fans
'Demister Switch' 'ON' and rheostat 'ON'- variable speed to heater fan and full speed to demister fans
'Demister Switch' 'ON' and rheostat 'OFF'- heater fan 'OFF' and full speed to demister fans

Plus both 'OFF' of course!

[StillGotMy1stCar]

That is correct, thanks.

[Fingolfin]

Intriguing! Thank you all for identifying such a simple solution. I have a fair grasp of the principles of wiring, but my practical knowledge is limited when it comes to things like this.

Updates to come...

[Fingolfin]

Now I have another problem...but at least I'm learning!

My other major upgrade for the heater is a new blower motor from an '80s Jeep Cherokee. I selected this motor because it fits nicely into the round core, and its squirrel-cage wheel is easily modified to fit as well.

With the new blower motor wired into the rheostat as above, the circuit runs at full speed when the rheostat is just turned on (like you'd expect), quickly cuts to half when the rheostat is turned to 3 o'clock, and cuts out completely around 5 o'clock. Turning the rheostat further causes the rheostat to smoke! Reversing connections at the rheostat makes no difference (though it does reverse rotation at the motor).

Am I correct in believing that the new motor is trying to pull more current through than the rheostat can resist, and so it's being overloaded? If that's the case, an inline resistor (or even the original Jeep blower motor resistor) would solve the problem. Where should such a resistor be placed in circuit? Between rheostat and motor is my guess.

What am I missing? And, having smoked a little (obviously I shut off power as soon as I saw smoke), would the rheostat likely need replaced?

[StillGotMy1stCar]

Your best place to start is with the Jeep resistors. What you want to do is select the resistor or resistors (may need to use all the resistors in series) which gives the desired slow fan speed when connected from the battery supply to the motor, leaving the rheostat in circuit when you turn the rheostat on you should get the speed control you want. The load will be shared with the resistor/s so hopefully take enough load off of the rheostat.
You will need to add an on/off switch to the supply to turn fan off.
So the slow fan speed will be achieved by turning on the new switch then increased speed by turning the rheostat. Hope that makes sense.
Please note the resistor will get very hot, normally placed in the air flow of the fan and can/does start fires if mounted near plastic. Cars should have a thermal fuse mounted next to the resistors in case the motor stalls and resistors overheat.
The rheostat may have survived.
Regards John.

[StillGotMy1stCar]

If the above fails to be satisfactory then your best bet would be the fan switch and resistors from the Jeep or another vehicle with a rotary fan switch for the period look.
Regards John

[Fingolfin]

If I follow your diagram correctly, the resistor is wired in parallel rather than in line - which would explain the additional on/off switch. In the original Jeep wiring diagram (granted using a different switch), it's wired in line. Would wiring the resistor in line cause damage to the rheostat or motor?

I will (carefully) play with the Jeep resistor tonight and see what I find. Thanks!

[StillGotMy1stCar]

The Jeep switches the resistors in and out of circuit to get the different speeds, so it is inline.
 I have suggested the resistor in parallel with the rheostat to take some of the load off the rheostat.
If you put the resistor inline (series) with the rheostat it would still overheat and you would not get full speed from the fan.
It is going to be a bit of trial and error but I hope will work as I've suggested.
I've not got a rheostat to look at and have assumed it hasn't got a switch so uses the wiper on the winding to start current flow.
If it does have a switch it may be possible to use it.
Regards John

[Fingolfin]

What I've come to discover is that blower motor resistors like discrete steps in power, rather than continuous variety. After experimenting with various options - Arduino boards and MOSFET controllers - I'm tossing out the old rheostat.

A blower switch from a 2001 Mazda Protégé looks like it will fit...http://www.ebay.com/itm/like/3211143987 ... dispItem=1

We'll take the Mazda's relay and resistor as well. Since I've already modified the Jeep motor to fit, let's hope Jeep and Mazda circuitry play nice together.

In that case, we can follow the Mazda wiring diagram (https://ww2-secure.justanswer.com/uploa ... 49_pic.gif) for how to wire the modified Minor circuit. This is what I come up with:

Wiring.png (10.79 KiB) Viewed 3779 times

The only funky thing about this is that I placed the blower switch live feed for the demister fans on the switch's ground lead. I thought this made sense, because there will only be power coming through this lead when the switch is in one of the four "on" positions.

Also, obviously, all of this will be fused. The Mazda diagram shows a 40 amp fuse, which is a lot of juice. I'll have to consider my options there.

Am I close?

[StillGotMy1stCar]

Just the diode in the wrong location and if connected to correct location between blower motor and blower switch would make the demister fans run flat out on the blowers slow setting and vice versa. Opposite to what you want.
The funky blower switch has to go in the supply lead with the end of the diode moved from the blower motor earth to the supply side.
Not surprised the rheostat smoked with the higher current.
I haven't looked at the links yet, will look later in case that alters the way it has to be wired.
Hope that all makes sense.
Just had another look and you could switch the earth side of the demister fans and keep the blower switch on the earth side as well, gets confusing, I will see if I can workout alternative arrangements later.
Regards John

[StillGotMy1stCar]

Revised wiring diagram switching the earths.
The 40A fuse suggests 26A current draw by the motor, possibly a high start up current but could be up to 5A less as the next fuse down is 30A. I think they have correctly selected what appears to be a high value fuse to stop heat buildup in the fuse which would melt the fuse holder. Would be interesting to know the actual though, I guess it's too high to measure. Jeep motor may well be less?
Regards John

[Fingolfin]

Most interesting, John! In my fiddling about with my own wiring diagram I'd come to a pretty similar layout. When I have time, I'll wire it up as you have it, and see what happens.