Using PWM to drive 12 fan. What's the Pitfalls, if any?

[BobP]

Hi All,

Friend is building a kit car and has asked me to design a switched (3 or 4 position) speed controller for the interior heater fans (current between 6 to 10 amps total). I did suggest growing a beard and wearing a bobble hat like all the other kit car owners I have seen. But he wasn't to keen on that!

I plan to use a 12f683 PWM output to drive some power Fets.
Are there any do's and don'ts in driving and protecting the Fets?

Also would I need to filter the 12v supply to prevent switching spikes going back into the cars supply?
Any recommendation on what Fets to use? I imagine a low switch on volt drop is important to minimise heat generation?

Any suggestions welcome,

Thanks,
Bob

[Charles Linquis]

I have several applications where I have done this. Because I needed to read the motor tachometer (which is referenced to the negative terminal), I had to use a high-side drive. If you do it this way...

The circuit used an IRF4905 FET as a switching element. That switches power through a 15uH choke (rated for the full fan current + 25%). The junction of the two is connected to a STPS4130 Schottky diode (both sections in parallel) with the anodes of the Shottkys connected to GND. There are plenty of low ESR capacitors (about 2000uF) on both the input and output sides. Pay special attention to the INPUT side because you will need more capacitance there than on the output side. Put of bunch of .1uF caps there too, to make certain that your buddy's AM radio is usable.
To switch the '4905 gate, I used a Microchip FET driver. The inverting
type is needed, since a HIGH on the output of the PIC must turn on the FET by pulling its gate LOW. A good switching frequency is 78KHz.

[BobP]

Thanks Charles,

You have given me more information that I hoped for.

Will be building a prototype soon. Will let you know how I got on?

Thanks again,
Bob

[Archangel]

Anybody remember when cars had switches that did the work without 10 dozen little black boxes that only their designer, knew how worked? The engineers who made those cars must have been geniuses to make those cars work. I remember when heater blowers used resistors to control the blower and a relay wired directly to the power bus for the blower's high power setting.<br><br><b> Side note:</b> be advised, automotive environment is a very hostile place for electronics, especially in heater - A/C applications, I have seen functioning units smoke out a 30 amp breaker when the systems get old, likely due to dragging blower bearings, marginal (crimp) connections etc . . . I would advise using a high blower relay with your PWM controller when you want full power to operate the fan. Take this advice for what you think it's worth, it's backed up by 42 years of fixing cars.

[Charles Linquis]

One thing you have in your favor - since I'm describing a "buck" type converter, full power is achieved with the FET 100% on = almost no dissipation (in the FET). The IRF4905 is a beast - 55V, 74 Amps. I have yet to blow one. A 45V Transzorb across the device and a 15V zener across the GATE and SOURCE is a good idea, though.

[BobP]

Hi,

I remember the good old resistors used to change the blower speed. I am sure they gave off more heat than the heater matix did on my old Mini?

I have been told that on this car space is limited and my friend wants 'one of these controller things that don't heat up'.

The spec of the fan motor claims 3A current to be fused at 10A. Seems a bit low? Will check switch on and loading current before building anything.

Thanks,
Bob

[Archangel]

Hi,

I remember the good old resistors used to change the blower speed. I am sure they gave off more heat than the heater matix did on my old Mini?

I have been told that on this car space is limited and my friend wants 'one of these controller things that don't heat up'.

<b>I thought it got cold in the UK ! :D </b> What really seems to kill you in automotive is the resistance build up at every connector, cars have so much vibration, and dampness it really causes vibration induced corrosion ( I work at a testing Lab where they shake the pogies out of space and aircraft components, and it is amazing to see these super high quality components actually emit metal oxides in only an hour or two ) all induced by vibration.