How to drive an electric fan?

[oden]

Just mounted an electric fan in front of the radiator to stop the car overheating in the Australian summer, currently its controlled by a switch under the dash.

Looking at the Ravage low side driver schematic, the PWM outputs from VND14NV04 would only drive 12A and I have a 20A fan which I assume is common consumption. So do people intend to use this?

Thinking about the BTN7930B, a 20A motor fan controller but seems hard to get in small quantities. http://search.digikey.com/us/en/product ... ND/1989748

ideas?

[Fred]

Go with a HEAVY FET that can push double your load and put it on a heatsink in the engine bay and control it via PWM (or switch for now) remotely. If you get a nice OMNIFET again, then it'll be easy :-)

http://octopart.com/vnp35n07-e-stmicroe ... ics-846361

35 will probably be enough, get one of those and try it. If it dies, no big deal, try again :-)

There is an ozzy company that sells a box which is just a heatsink with something like that inside too.

Fred.

[DeuceEFI]

It would be better to use a relay to switch that much current. All my cars have a coolant fan relay under the hood similar to the Fuel Pump relay, that way you don't have that much current on your PCB.

Just my $0.02

[Fred]

Relays don't allow variable speed and progressive temperature change and lack of noise that come with that :-)

I intend to PWM the fans on my truck and the fuel pump also. I just don't need 900hp of fuel at idle ;-)

The main thing is NOT to try to do this through the ECU box as the noise will be horrendous.

http://www.a1electric.com/Merchant2/mer ... FAN-PWM-V3

I can't find the ozzy one right now, but I came across it from a link on this forum, possibly from you oden.

Fred.

[Peter]

I'm going to have to agree with Fred on this one. It's kinda ridiculous when your truck is sitting around idling, and the engine loses a couple hundred RPM, because the turbines fired up. My dad's truck that has a winch on it has this problem, because the 200Amp alternator is pulling so hard on the engine. Plus if you don't have a large alternator it probably drains the battery at low RPMs.

[DeuceEFI]

ok, I see your reasoning now.
I haven't been concerned about the electric cooling fan coming on at idle as the only time it comes on is on a 98F day sitting in the sun and it only runs for a few minutes then shuts off. Besides, I can't hear the fan over the headers anyway, LOL

Another thought, if the A/C is running then the fan should be running at full speed to keep air moving past the condenser... just saying...

[BenFenner]

While a common automotive relay would downgrade the functionality of the system, can we assume that style of implementation is at least compatible? Quite a few cars are set up with a simple relay from the factory, and it would be nice to know if that is supported. Thanks. =D

[Peter]

You could always drive the factory relay with the transistor. Which kind of gives me the impression that you don't have a really good understanding of what a transistor does. Transistors are "basically" just a fancy relays.
http://www.mechatronics.colostate.edu/lab_book.html
If you go to the third video on Lab 5, it does a pretty good job of explaining what some differences are between relays and transistors. It's a windows media player video. If you don't want to download it, it shows the outputs of a transistor and a relay at higher frequencies. The mechanical components in the relay can't react to the signal as fast as the transistor, and the relay stops working when they jack the frequency up high enough.
Which kinda leads into Power Width Modulation(PWM) where you turn the transistor on and off really fast so that the average output is a fraction of the input.
Somebody smarter could probably explain it in better detail, but that's the way I think about them.

[Fred]

Ben, whatever drugs you're on, I want some! :-) Of course normal modes will be supported. I just plan to support fancy ways too. And I don't plan to have it be a specific feature, just generic code that does GPIO. Time will tell how that works out, but it should be awesome. Slated for 0.5.0 but will be sneaking in in various ways before then, and has already begun. Tacho output can be on ANY port/pin via config :-)

[oden]

A programmable fan controller would be the simple way of resolving the problem, but where is the fun in that. Would much rather build something

Designing a uC that will adjust the fan speen using PWM and speed up when the AC is on. It will also provide CTS readings to FreeEMS and my dash as I only want to have one CTS in the engine bay and sharing sensors does not work well I am told. Found a digital pot that will do this nicely.

Despite trying to learn as much about FET's from google, I have questions on how to use this part
1) How far away can the flyback diode be from the fan? Everywhere says as close as possible to the motor, would 30 cm be to far?
2) From what I have read a flyback diode should be fast recovery, same current as the motor or more (20A) and support high reverse voltage, so would this diode work? http://www.vishay.com/docs/94167/94167.pdf
3) What should the value of RG (gate resistance) be? From what I have read there should be a graph on the data sheet that explains this, I dont understand which on it is. My Vin will be 5V TTL from a uC.
4) Page 9 figure 5 shows an 85 ohm resistor on the source pin, why?