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Feb 13, 2015, 12:15 AM
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A linear ESC

After a sleepless night of using the travel fan, it obviously needed to be completely silent. PWM would never do. An ESC like none other needed to be built. It needed to use linear voltage control. It was finally a practical reason for the lifelong dream of building an ESC from scratch.

It needed to run on linear voltage down to 1V. It would be fed by a 6V BEC with some way of adjusting voltage without PWM. It would need all N MOSFETS, because the high voltage wouldn't be high enough to turn on a P.

There was technical appeal in making it out of BJT's. It would be the only one of its kind. There's a massive supply of BJT's in the apartment. They would take some voltage drop off the regulator. But to be a travel fan, it needed to be compact, so MOSFETs won out. There was still a massive supply of MOSFETs from the days of trying to launch spaceships with magnets & burning them up instead.

The 1st attempt was a through hole failure. It used P MOSFETS which would't turn on because the high voltage was too low. A more complicated circuit with N MOSFETS, a full boost converter, high side BJTs would be required.

A luxurious PCB was built up & reworked.

The mane challenge was wiring the N MOSFETS on the high side required putting the current source on the drain & the load on the source. Current must always flow from the drain to the source.

Imagine how much progress could have been made if current flowed in the same direction as the electrons. Instead, many a future genious was stymied by the notation being reversed. The next problem was retlw on the PIC18 not working like it was supposed to. A charge pump properly set the high side gates to 13V.

After all that, a simple test in stepper mode made just as much noise as an ESC with PWM. The noise was from commutation rather than PWM. The simple, linear ESC was a myth. The next chance of making it silent would be supplying a sine wave to the gates. It would be hideously complex.

The BJT which amplifies the microcontroller to 13V to turn on the high side would be lousy at generating a sine wave. It would require more like an audio amplifier. It's more practical to get a big computer fan than try to repurpose RC parts.
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Feb 13, 2015, 01:57 AM
JulietKiloMike's Avatar
Nice work! We built an ESC of sorts at school using Hall Chips and some transistors for a scratch-built DLRK motor a while back, but I didn't get a chance to build anything using back EMF for motor position rather than hall chips. Neat!

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