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I recently took apart an HP LaserJet 1320 laser printer to salvage parts from it. Along with 3 solenoids, a fan, lots of gears, 83 springs, and 71 feet of wire, I got a large brushless DC motor with an integrated driver board with the part number RK2-0419 made by Nidec rated for 24V at 1.3A. Since I could not find anything useful by looking up the part number, I decided to look up the driver chip on the board (BD6761FS) and I found a datasheet. The connector on the motor has 5 wires: Vcc, FG, /DEC, /ACC, and GND. Going off of this EE Stack Exchange question about a similar motor salvaged from an HP printer, I decided to connect 24V from a switched-mode power supply capable of supplying 1.75A to the Vcc and GND pins of the motor. The motor immediately started drawing 28 milliamps from the power supply and the driver chip got slightly warm. I did notice that the motor shaft was very hard to turn by hand when power was connected. Following the datasheet and the previous Stack Exchange question, I connected the /ACC pin on the motor to the ground on my power supply. The motor did not move and still only drew 28mA. Nothing happened when the /DEC pin was grounded, along with grounding both the /ACC and /DEC pins.

How should I get the motor to work?

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  • \$\begingroup\$ Can you make it more clear whether you are using the whole controller_board+motor or just the motor? As far as I know a BLDC doesn't itself have "5 wires: Vcc, FG, /DEC, /ACC, and GND". Those are pins on the controller chip, probably brought out on a cable. \$\endgroup\$
    – Fizz
    Nov 2 '15 at 22:07
  • \$\begingroup\$ Yes, I am using the whole controller board as it is permanently attached to the motor and it uses a known controller chip. Also, if it is of any importance, the printer was fully working and it was disassembled per the directions given in the official HP service manual. \$\endgroup\$
    – 3871968
    Nov 2 '15 at 23:45
  • \$\begingroup\$ That sounds like it might be the answer. Should I connect it to 24V through a resistor or just short it? \$\endgroup\$
    – 3871968
    Nov 3 '15 at 0:19
  • \$\begingroup\$ No, absolutely not to the 24V, you'll fry it. The top limit voltage is given by the VREG pin (which you could measure, but 5V seems safe). \$\endgroup\$
    – Fizz
    Nov 3 '15 at 0:23
  • \$\begingroup\$ What would be the easiest way to get 2.2 to 5 volts there? \$\endgroup\$
    – 3871968
    Nov 3 '15 at 0:24
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Based on the datasheet, it seems to me the /DEC pin needs to be high (in addition to /ACC being low) for the motor to start. Anywhere between 2.2V and 5V seems fine for /DEC to register as high. Leaving it open-circuit won't do that though.

The reason why it does nothing with no voltage to /DEC is that this chip it has internal pull-down. Possibly other chips of this kind have internal pull-up instead.

enter image description here

And don't apply 24V, you'll fry it. The top limit voltage is given by the VREG pin (which you could measure, but 5V seems safe).

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    \$\begingroup\$ This makes perfect sense. If both /ACC and /DEC are pulled down internally, this would activate the brake function thus making the motor difficult to turn. \$\endgroup\$
    – 3871968
    Nov 3 '15 at 0:48
  • \$\begingroup\$ I tried it. The first time I connected it like this with 2 AA batteries with nothing connected to the /ACC pin, it worked fine and reached full speed and I let it run for about 15 seconds. Then I turned off the power supply. Then I turned the power supply back on. The motor reached full speed and in a few seconds, it suddenly dropped in speed, sped up again, and turned off. The the power supply shut down because of too much current draw. Now when I connect the motor it draws too much current even if the AA batteries are not connected. Any idea what happened? \$\endgroup\$
    – 3871968
    Nov 3 '15 at 15:57
  • \$\begingroup\$ Not exactly sure but generally it's not a good idea to apply voltage to the data/input pins without the [main] power supply. That can cause latchup and destroy an integrated circuit. So the correct power sequencing is always: main power on, then I/O input/voltage. For turn off, first remove the data/input voltage then cut the main power. \$\endgroup\$
    – Fizz
    Nov 3 '15 at 16:00
  • \$\begingroup\$ Yes, the batteries were disconnected before the power was turned off. There was no magic smoke or strange odor and the chip did not get hot. \$\endgroup\$
    – 3871968
    Nov 3 '15 at 16:01
  • \$\begingroup\$ When power is connected using a 24V power supply capable of more current, the chip does not get warm at all, however two 1/2 watt through-hole resistors on the front of the board get warm. It does not appear to be latch-up. \$\endgroup\$
    – 3871968
    Nov 3 '15 at 16:09

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