Thermally controlled fan

[jbsaxman]

All,

I am working on a project but am not the most proficient with designing electronic circuit. I'm hoping to bounce this idea off you and see if I'm close to the right idea.

The background is that I want to create a thermally control fan switch for my truck. Right now it had a mechanical fan clutch and I don't like it. I have a couple of fans that draw a lot of CFM, more than sufficient to keep my truck cool.

I would like this to be thermally indexed, meaning the fan will kick on at a certain temperature, but only at 50 or 60% of full power, then increase as the temperature increases up to 100% power. (I want to do this to avoid any unnecessary stress to the alternator as it powers on.) The fans are both 30 amp fans off of a Volvo truck.

My thought is that I will need some sort of thermistor for determining the temperature. I was going to use a simple engine coolant temperature sensor for this.

To control the fan speed, I was thinking a MOSFET would work for pulse width modulation to start the fans. Or would an SG3525 be better?

Really, this is as far as I think I know.

So here are a few questions right out of the gate:
- Do I put a variable resistor in line between the thermistor and PWM to control when the fan kicks on?
- As the temperature decreases, will the fans behavior be the exact opposite of the temperature increasing?
- 30 AMPS is a lot of current. How can I safely pass this current to the fans without burning any of my electronic components up?

If this will work, I have found a schematic that I think will work, but I still wonder how I can do this without burning up my components.

If anyone has any suggestions, I'd appreciate any help or feed back.

 

[NickS]

Hello and welcome. I like that you have put some thought in before posting.

I need to think about your situation a little more but off the cuff I would say since you are looking for a range of operablility perhaps a thermocouple would suit for sensing. you should get a definable current for given temperature and the sensing tip is very small.

Are you comfortable with microcontrollers(uC)? The most direct option seems to feed the thermocouple information into a uC via built in ADC then you could easily control your duty cycle based on a lookup table.

From there the uC output will need to feed a FET driver that will in-turn drive the gate of the MOSFET. You want a FET driver because you need a strong driver for the kind of MOSFET you will need. Speaking of which 30A is do-able for a FET ...BUT... you will want a serious heat sink for it(I would recomend a part in a TO-220 type package since heat sinks are pretty common for these). Your MOSFET will need to have a very low Rds on to keep internal dissipation down to a minimum. And you will probably need a snubber circuit at the drain to keep spikes from the motor at bay.

Even though there are not a lot of parts for this kind of circuit there are many subtle details that make the difference between success and massive failure. I say massive because at those power levels when something goes wrong it may involve exploding parts(I have been shot in the face with the end cap of an inductive power resistor). High current paths need to be short and heavy gauge. Parts need be adequately over rated. And I would recommend trying it out on a lower power non-inductive load and work up to the fan. By the way you may find a light bulb makes a great test load.

Good Luck.
-Nick-

 

[jbsaxman]

Nick,

Thanks for the reply.

I have done a bunch of research on this over the weekend and I think that with the cooling requirements of my truck, I might have to stick with the stock mechanical fan.

The stock setup flows 10K CFM of air through the radiator and over the engine. The two fans I have would flow a combined 6K CFM. I haven't been able to fine any electric fans that could flow more than that and give any improvement over the engine performance of the stock setup.

That said, I have another project I want to work on using a 555 timer:

I would like to create a glow plug timer that will, after I turn the key in the ignition, illuminate the Wait to Start light in my dash, then energize the glow plug relay for 30 seconds, turn the Wait to Start light off after 15 seconds while keeping the glow plug relay energized for the remaining 15 seconds. I'd also like to have a switch that I can flip to maintain the same light illumination period, but increase the time for the relay to 1 minute.

Or better yet, use a rheostat to increase between 30 seconds and 1 minute. (For cold weather.)

I'm not sure I can do this with a single 555 IC, or if I'll need two.

Thoughts, of course, are appreciated regarding this.

 

[NickS]

555's are a hot topic around here. It seems like you should be able to make them cycle at any period you want but in reality they become problematic at large time delays due to small current flow and capacitor leakage current. A micro controller is perfectly suited for the type of thing you want to do

If you are not the micro controller type then I think you should take a look at this Timer/Counter by Maxim-IC ICM7242IPA+. It touts timing ranges from micro-seconds to days. And it is accomplished by dividing down the result of the RC time constant so it is much more reliable than just using bigger part values. The part datasheet also talks about using a thumb-wheel to adjust the delay(just like you wanted). And finally it is available in DIP packaging for the DIY'ers.

You can find the datasheet on the page I linked above.

Good Luck
-Nick-