The title says it all. I'm doing a project with a spinning HDD disk with a slot cut in it, RGB LEDs, and precise turning on and off of the colors.

I'm planning to glue a magnet and use a hall effect sensor, but just opened another HDD and this one has 4 pins instead of 3. I applied voltage to all 4 pins (5V)so I hope I didn't fry the sensor, but I've identified 3 that have 8 ohms, 1 has 4 ohms. So I want to know, can I forget using a magnet and use the sensor already in the motor?

If so how would I wire it to receive pulses on each rotation, or magnetic pole switch?

Any insight on this is greatly appreciated as I've googled and cannot find one example of how to wire it.


1 Answer 1


Your '4 pin BLDC motor' is probably just a normal 3 phase motor wired in Star or 'Y' configuration with the center tap brought out. If your motor has phases that are 4Ω each then it should measure 4Ω from center tap to each phase, but 8Ω between phases.

Some drivers use the center tap to help measure back-emf for commutation. However a 'virtual center tap' can easily be generated externally, so most modern drivers only need 3 wires. The TDA5144 is an example of a driver IC that is designed to work with 4 wire 3 phase motors.

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  • \$\begingroup\$ Oh that would make sense why it's half the resistance from the center tap vs the other windings. I guess I will stick to my original plan of the magnet super glued to the platter. Can you explain how to make an virtual center tap out of curiosity? I'm having a hard time visualizing that. \$\endgroup\$
    – Matt
    Aug 19, 2015 at 2:35
  • \$\begingroup\$ Virtual center tap - 3 equal value resistors one connected to each phase, and their other ends joined together. \$\endgroup\$ Aug 19, 2015 at 2:48

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