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I'm building a small fume extractor for soldering.

I have a leftover desktop fan I'll use. I was unsure about the voltage needed, so I hooked it up to my power supply and stepped up the voltage until it started spinning (around 6V but I think most of these fans are 9-12V nominal if I recall). It's drawing 30mA at 9V right now.

If I place my finger on it to arrest the movement the current draw drops to 0. When I release it the fan rotates quickly to some location which I assume is the armature aligning with the magnet. Now, though, it sits stationary for about 2 seconds before resuming spinning.

What's happening in those 2 seconds before it resumes spinning?

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  • \$\begingroup\$ is the label on the fan missing? \$\endgroup\$
    – jsotola
    Commented May 30, 2020 at 20:15
  • \$\begingroup\$ Yes, it's old and has been in my spare parts bin for ages. No idea where it originally came from. \$\endgroup\$
    – brenzo
    Commented May 30, 2020 at 20:16

3 Answers 3

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Compare a typical BLDC fan controller IC.

enter image description here

The delay before a restart attempt after 'mechanical lock' is detected and coil current is shut off (Toff) is nominally 2.8 seconds.

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The fan is protecting itself when you stall it. After you bump into someone while walking, do you immediately take a step forward? Or do you wait a bit and hope they get out of the way?

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  • \$\begingroup\$ I hadn't considered there might be a controller in there, I assumed it'd just be a DC motor. \$\endgroup\$
    – brenzo
    Commented May 30, 2020 at 20:20
  • \$\begingroup\$ It is not. It is a brushless DC motor which means controller mandatory. \$\endgroup\$
    – DKNguyen
    Commented May 30, 2020 at 20:21
  • \$\begingroup\$ @DKNguyen I think you need to emphasize "motor controller" here, because OP seems to be interpreting it as "microcontroller" \$\endgroup\$
    – Maple
    Commented May 30, 2020 at 21:17
  • \$\begingroup\$ @Maple Perhaps. It is not a microcontroller on there but a specialist motor driver IC that is capable of some of its own smarts as well as power drive. \$\endgroup\$
    – DKNguyen
    Commented May 30, 2020 at 21:20
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What's happening in those 2 seconds before it resumes spinning?

quite likely the firmware of the motor controller integrated into what is called "Brushless DC motor" detects the fault state and stops for 2 s for the fault to be removed.

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  • \$\begingroup\$ That makes sense, I hadn't considered there might be an MCU in there. \$\endgroup\$
    – brenzo
    Commented May 30, 2020 at 20:21
  • \$\begingroup\$ I am pretty sure there is no MCU in there, but rather specialized chip like TC642, for example. The behavior of "Fan Auto-Restart" function described in datasheet resembles the effect described. Other fan controllers most likely have similar functions. \$\endgroup\$
    – Maple
    Commented May 30, 2020 at 21:10
  • \$\begingroup\$ I agree, @Maple! (although, it's worth noting that the only difference between that TC642 and a microcontroller probably is the lack of possibility of the user to program it themselves.) \$\endgroup\$
    – Marcus Müller
    Commented May 30, 2020 at 22:05
  • \$\begingroup\$ I don't claim to know what's inside those chips, but it is definitely not something that can be programmed by anyone, it is just cost-prohibiting step for so cheap products. Most likely it is pure hardware solution, and quite simple at that. See block diagram in the datasheet from @Spehro Pefhany's answer. \$\endgroup\$
    – Maple
    Commented May 30, 2020 at 22:11
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    \$\begingroup\$ I referred to this chip from different answer. Nevertheless, the "algorithm" described in this datasheet is nowhere even close to say, I2C state machines in countless chips. And yet only few of them have actual software. The BLDC control (in the case of PC fan) is very primitive task, pretty much just a switch controlled by properly positioned sensors and a simple analog logic with maybe a few digital gates \$\endgroup\$
    – Maple
    Commented May 30, 2020 at 22:41

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