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general question about pwm and brushless fans

I have a fan I want to use with continuously variable speed. It is a brush less type (specs on this one say 7-12 volts) should work. And it does . . . but won't take PWM directly, it wants to see steady voltage not chopped. I tried frequencies from 20 kHz to 1 kHz

  1. Is this typical?

  1. Anyone else with experience, of speed control of generic brush less fans?

I tried my circuit with two other junk box fans and they also ignore pwm, but like steady voltage for variable speed.

Right now I have my picaxe starting at 50 ms on, and 150 off, (ersatz pwm) up to steady on, in four ranges - which gives me the variable cooling I want, but it sounds like it is panting and is moderately annoying.

Plan B:

I really want to continue using an N channel logic level MOSFET and don't want to disturb the gizmo that's working and in-service for a variety of reasons.

Seems to me that since my mosfet is pulling the fan's minus connection to ground, it should be possible to just turn it into a switching supply with steady DC output. Hook an inductor to the drain, freewheel diode to the mosfet drain and Plus, and put the fan (load minus) to the inductor's other terminal (and electrolytic cap across the fan).

sort of a switching supply, but upside down and backwards

  1. Does it sound feasible, or am I missing something?
Reply to
default
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It sounds OK to me. The fan has a bit of circuitry in it to make it go, and that circuitry is probably confused as hell in the presence of a PWM'd power supply.

You're still constrained by the fan's input voltage range; I assume you're OK with that.

--
www.wescottdesign.com
Reply to
Tim Wescott

Yes. A "brushless DC" motor is a synchronous AC motor which either has a built-in AC generator or is designed to be driven by an external AC generator. Fans normally have it built in.

Some fans filter the input voltage (to allow PWM) and adjust the rotational speed according to the filtered voltage. Others don't. If the input isn't filtered, it isn't going to like PWM as the built-in controller will be continually losing power.

Yep; low-side buck converter; just like a normal buck converter without the need to drive a high-side switch.

Should work fine. Just ensure that you maintain the minimum supply voltage at all times so that the built-in controller isn't getting continuous power-out/brown-out resets.

Reply to
Nobody

Filter it after chopping it.

Grahama

Reply to
Eeyore

Cap filter holds the peak value. LC filter might work.

Reply to
default

All the other advice you got here is valid, but I have done exactly what you are trying using a 555 based PWM and to get it to work, with the brushless fans I had on hand, I had to go WAY up in frequency from where you are trying. If I remember correctly I'm in the 8KHz area. My biggest problem wasn't getting it to work, it was getting it to work QUIETLY. All my fans would squeal at the PWM frequency when set to low speeds, except when I went above 5KHz or so. Then they would quiet down and run smoothly.

Reply to
WangoTango

You are using direct PWM? (fan terminals have 8 KHZ PWM on the power terminals?) That's what wouldn't work - fans just acted like they were a high impedance, or just not connected.

8 Khz is right in the middle of audio band so I'd expect it to be noisy.

Yeah, at 200 ms I'm just turning the fan on and off and can hear it start and coast, over and over.

Reply to
default

Fan + lead to +12V - lead is being pulses by a TIP102 transistor. All 4 of the brushless fan varieties I have here work fine. It is possible that I just lucked out and happened across of selection of PWM tolerant fans, but that's not usually my luck. :)

No noise at most speed settings and just a little when the fan is stalled and trying to start up.

I can email you a PDF of the schematic, nothing special just a run of the mill 555 circuit.

Reply to
WangoTango

I'm going to be bread boarding my circuit again and will give it another try. Could be the 555 is swinging more voltage and has a higher current than the 4.5 volts and 20 ma my pic is able to. Suggesting my mosfet was not really saturating fully.

Reply to
default

It might work if his PWM was from a totem pole, but the OP is using open-drain.

Reply to
Nobody

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