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car heater blower control

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haris_216

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being into the mood of putting my old car into the "21st century":) I'm trying to modify certain mechanical, bulky or simply not cute features with the equivalent electronic ones. so the last think that hit me is the heater blower motor speed control. first I thought that I could change the 2 speed control to a 4 speed by replacing with the appropriate aftermarket resistor along with the needed switch. but then it came to mind that I'd prefer a more electronic solution. given that both blower and resistor are in the engine bay I would like either to totally control electronically the speed of the motor or, if this is far fetched due to the high currents involved, to keep the use of the resistor but switching to be done through some up/down electronic circuit that would feed the resistor, thus eliminating the heavy cables going through the firewall. any ideas would be most appreciated.
 

haris_216

Member
and to make it more clear with the replacement resistor there's gonna be 4 speed control. so the problem is to control the...control (resistor):)
 

MikeMl

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Most Helpful Member
Why not just use a Big Transistor (you have an infinite heatsink, its called the car), and control the current to the motor with infinite resolution? The power wasted will be the same.

Before someone suggests PWMing the blower, he aint going to like the hash that comes out of his stereo speakers caused by the PWM current flowing along the car's grounds...
 

haris_216

Member
thanks a lot mike. as always very fast and efficient in responding. to be honest I didn't think at all of the speaker "side effect". given the blower motor high current "needs" you really think practical/feasable controlling it through power transistor?
 

MikeMl

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Most Helpful Member
... given the blower motor high current "needs" you really think practical/feasable controlling it through power transistor?
The blower motor cant draw much more than 10A, so say 5A at half power where half the supply voltage is dropped across the transistor, so it would need to dissipate 6V*5A= 30W. That shouldn't be too difficult...
 

haris_216

Member
mike, coming back to you to ask you about you mentioning "control the current to the motor with infinite resolution". sounds like what I'd like to do but am not pretty clear (being novice in the field) as how to do it. without taking much of your time, could you please give me some guidance?
 

MikeMl

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Most Helpful Member
mike, coming back to you to ask you about you mentioning "control the current to the motor with infinite resolution". sounds like what I'd like to do but am not pretty clear (being novice in the field) as how to do it. without taking much of your time, could you please give me some guidance?

A question before I suggest something: Do you have access to both motor leads, or is one of them internally grounded to the car frame? I need to know this to decide if "high side", or "low side" control is necessary?
 

Diver300

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Most Helpful Member
I had a heater blower resistor fail. It would have been at least a whole day to get to it, so I built a PWM circuit.

It was just an RC oscillator feeding a big MOSFET. The freewheel diode is vital. The PWM frequency is fast enough that the current in the motor doesn't change much.

A big capacitor across the power supply is also needed, as the current to the PWM circuit changes fast.

There is some audible noise from the heater motor, but the stereo isn't affected.
 

haris_216

Member
A question before I suggest something: Do you have access to both motor leads, or is one of them internally grounded to the car frame? I need to know this to decide if "high side", or "low side" control is necessary?
I have access to both wires. out of curiosity, what do you mean by "high" or "low control"?
 

MikeMl

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Most Helpful Member
I have access to both wires. out of curiosity, what do you mean by "high" or "low control"?


High-side means inserting a transistor in the motor lead that would normally be connected to the battery positive pole. Low-side means inserting the transistor in the motor lead that normally connects to ground. The motor likely runs the wrong direction if the two wires are reversed, so keep track of which is which.

High-side control usually is done with a PNP bipolar power transistor (maybe a Darlington) or a PMOS FET, while low-side switching is done with an NPN or an NMOS FET. The FETs tend to be more available, cheaper and they are easier to (voltage) drive than transistors.

Using a FET as a "brute-force" (linear) speed control is analogous to putting a variable resistor in-series with the motor. A resistor can be placed in the high-side or low-side, too ;) At any given set speed, the FET will dissipate the same amount of power (W) as the resistor would... The FET can easily be bolted to a large steel heat sink (the car itself) so that the heat is conducted out the FET. You will have to use an "insulated mounting kit" to electrically isolate the FET tab from the grounded car body, though.
 

haris_216

Member
nice to see you around again mike. I hope I wasn't "invading" your life by messaging you. thanks a lot for all the info. and am glad to see you around and read your posts. take care
 
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