Over thinking the heater fan

[Delta Whisky]

First the background: smelled something burning while in flight a couple of days ago. Turned out to be one of the heater fan motors. (With about 45 minutes total time on the fans; so a fan respected for better reliability is in order.) Should be no problem replacing it - right? Well, not so much. But I have started the journey.

First issue - it appears that 1.5 inch (high) computer case fans are no longer made. That alone pretty much drives the decision to replace both fans - otherwise the mounting process becomes noticeably harder.

Second issue - it appears the available 12V fans shouldn't be run at more than 13.2 - 13.3V. Maybe driving a 12V fan at 13.9 volts is what did mine in - don't know. But if it was, I'm destined to smoke the second one and I'm not sure my heart can take another similar event. That helps me make the decision to replace both fans.

Third issue - by a rough estimate, about 90% of modern fans are of the PWM variety.

Forth issue - computer case fans are basically one of two varieties: standard and "high pressure". "High pressure" in home-assembled computer language means: able to push more air through a heat exchanger. (I'm so out of touch that I didn't know modern, home computers contained radiators and cooling fluid. What's this world coming to?)

Clearly I'm looking for something reputed to have a much better MTBF, is of the "high pressure" variety and is a direct fit back (120 mm) into the plane. It would be nice if it was reliable at 14V but that doesn't appear to be possible so:

Question one: what is the simplest, modern voltage control circuitry? Is it still the LM305 voltage regulator?

Question two - can someone confirm my suspicion that a PWM motor can be run full time at its nominal voltage?

I've pretty much found the replacement fan but would like the PWM answer confirmed first.

Update - Question three: If I'm reading the LM305 data sheet correctly, the input-output voltage differential needs to be 3.0V. This means it won't be a suitable answer. What would be a better solution? (Or, am I reading the spec sheet right?)

[Eric Page]

As you're probably aware, PWM -- pulse width modulation -- is simply a means of speed control by switching the fan ON and OFF at relatively high frequency with a variable duty cycle (ratio of ON to ON+OFF). Operating the fan at 12V DC is simply continuous 100% duty cycle. It's perfectly fine for the fan motor.

With regard to running the fans at 14V, it's probably OK, but might shorten their life in the long term. If you want to use a linear regulator like the LM305 you can, but it's not really necessary. I would just insert a resistor in series with the fan motor. If your bus is 13.9V and you want the fan to see 12V, then you need the resistor to drop 1.9V. The missing information is how much current the motor will draw; you can get that from the fan's name plate or data sheet.

For the sake of example, let's say your fan draws 350mA (0.35A) at 12V. Using Ohm's Law, we know that voltage / current = resistance. So, 1.9V / 0.35A = 5.4 ohms. Use the closest standard value resistor of 5.6 ohms. For standard resistor values, look at the "E24 values," here.

Power dissipated in the resistor = current squared x resistance. In this case, 0.35 x 0.35 x 5.6 = 0.69 watt. It's a good idea to get a resistor that can handle double your calculated power dissipation.

In this example, I would use a resistor like this, not because you need 25 watts of power dissipation, but because it will be very easy to mount it securely. Once you know your fan's current and have calculated the resistor value, start here to find the right resistor. Just select the desired resistance, click "Apply" and look for the cheapest one in a similar case style as my example.

Note that you can power both fans through one resistor; just double the current in your calculations and make these connections:

• If the fans' positive supplies are switched separately, then tie both of their ground leads to one end of the resistor and the other end of the resistor to ground.
• If the fans' ground leads are switched separately, then tie the positive supply to one end of the resistor and both fans' positive leads to the other end.
• If both fans are switched together, then the resistor can go in either the supply or ground circuit, whichever is more convenient.

This isn't a technique you would want to use for speed control with a DC motor that needs to maintain it's full torque output regardless of speed (an elevator trim motor, for example), as torque does decrease with voltage. In this case we're reducing a supply that's excessive, so the motor still sees its rated voltage. That said, if you want variable fan speed, you could add a PWM speed controller like this. You'll still need the resistor, as full blast on the controller will be 100% duty cycle, or nearly so (more than 12V).

Finally, Digi-Key also sells a wide variety of fans, and you won't end up with a low-quality imported knock-off. If you start here, I've narrowed it down to 120mm 12V axial fans. Since noise isn't a consideration, first find the widths that work for you, then I would sort by maximum air flow and maximum static pressure.

[avidflyer]

I'm going to jump in on this thread as well if you don't mind. I just today changed out the fan on my heater, and would like to be able to lower the speed if it gets to warm. In your responce Eric, you said you can use this PWM speed controller, but if you put up a picture or something I didn't see it. Here is the fan I just put in, they don't show the amps, 140x38mm 2 ball bearing High Speed 5200RPM at 308CFM, 12Volts

This is the controller I found, and wonder if it is likely to do the job for me if you don't mind giving your opinion. Please bear with me, I'm electroniclly challanged. (some would say in other ways as well) :-) JImChuk
Input supply voltage: 1.8V-15VDCThe maximum output power: 30W
The maximum continuous output
current :2A

[Delta Whisky]

Eric - thanks for getting me out of the computer sales sites and stores. Suffering from target fixation, I didn't think of going to DigiKey but that was the solution. You have to get into the product spec(s) but I found a fan rated for 6-14Vdc that has performance specs much better than the original, claims of a continuous duty life of 85,000 hours, only a slight increase in power drain and the need for any form of voltage control is negated. Again, thanks for the detailed response.

[Eric Page]

...I found a fan rated for 6-14Vdc that has performance specs much better than the original, claims of a continuous duty life of 85,000 hours, only a slight increase in power drain and the need for any form of voltage control is negated. Again, thanks for the detailed response.

Excellent, it sounds like you hit pay dirt! Glad to be of some help.

[avidflyer]

Thanks Eric, When I moved the cursor over the word, I saw it was a link. I would have guessed it would have been blue or something, but it looked just like all the other words. I did order one, never did find the specs on the fan I got for power draw. Thanks again, Jim

[DesertFox4]

Jim, I have the DPDT switch for my fans. That switch came from Kitfox Aircraft and is listed in their parts catalog on page 17.
It runs one fan on low ( on my Kitfox I wired the left fan for low) and both on high. Now during my build, I was testing the fan operation and somehow dropped a tiny screwdriver through the pilot side fan and instantly removed a fan blade rendering that fan in-op.
Went looking for a replacement at a big electronic store with damaged fan in hand and with store help quickly found a replacement. The new one was of course 12 volts and dimensionally had the same “footprint” as the damaged fan but was not the same thickness. I used some weatherstripping to raise the new fan to the same height as original.
The replacement fan did not quite put out the same flow volume but as it happens, it was perfect on the low speed setting when I just need that little bit of heat to make the cockpit comfy and don’t want to melt my tennis shoes.

[Eric Page]

I'm going to jump in on this thread as well if you don't mind. I just today changed out the fan on my heater, and would like to be able to lower the speed if it gets to warm. In your responce Eric, you said you can use this PWM speed controller, but if you put up a picture or something I didn't see it.

It was a link to an eBay listing. The word "this" at the end of that sentence is a clickable link.

Here is the fan I just put in, they don't show the amps, 140x38mm 2 ball bearing High Speed 5200RPM at 308CFM, 12Volts

This is the controller I found, and wonder if it is likely to do the job for me if you don't mind giving your opinion. Please bear with me, I'm electroniclly challanged. (some would say in other ways as well) :-) JImChuk
Input supply voltage: 1.8V-15VDC
The maximum output power: 30W
The maximum continuous output current: 2A

Not knowing how much current your fan requires, it's impossible to say for sure. As long as your fan doesn't draw more than 2 amps, it looks like it should work fine. You would have to put an ammeter in series with the fan to be certain. There are fans intended for servers and their power supplies that are quite powerful, so it's possible you have one of those.

[Flybyjim]

Glad to hear these work this way with the switch.
Thx

[Flybyjim]

I was looking at my heater instructions and there is an issue with the instructions and the parts supplied. Instructions state -use the supplied SPST switch but my switch is a DPDT with a off, L and H on the switch. There is no mention about hooking up the fans so one runs on low and both on high, anyone else have this switch. Seems like a good option but the instructions do not address this switch or fans.