DC Motor speed control


I am trying to control the speed of a DC cooling fan by PWM. The system basically consisted of a PIC18 controller board and a mosfet H-bridge board. The controller handles the user interface (LED, buttons, etc) and generates the PWM signal to drive the H-bridge. The problem is that the controller is located 5 meters away from H-bridge & motor. This is necessary because the mosfet is mounted on large heatsink and it looks ugly.

The system works okay, I used shield cable to carry the PWM to H-Bridge but I am worry any large EMI (e.g. mobile phone, microwave oven) will switch on all the MOSFETs at once. Should I use a different amp on the H-bridge to reduce EMI noise?

PWM signal = 22 KHz

Cooling fan = 12v DC @ 10 Amp

Any help will be appreciated.

Reply to
john smith
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"john smith"

** Why the H-bridge when one direction drive is all you need?

A single, high current MOSFET will do the PWM job.

No risk of cross-conduction then.

....... Phil

Reply to
Phil Allison

There is no need for a large heatsink if done properly.


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Reply to
Rene Tschaggelar

Hi Phil

The H- bridge was a left over from a previous project but I am thinking adding reverse mode to extract cooking fumes when the misses frying fish & chip ;)

It doesn't need a heatsink but as I said it from a previous project. It's already bolted on to the H-bridge PCB, can't be bother to rip it off.

Reply to
john smith

Given that the capacitance of the MOSFET gate is huge compared to the EMI energy from mobile phones, microwave ovens, etc. you should have no problem with the FETs accidentally turning on, only a variation of a few nanoseconds for the turn-on or turn-off time.

Look for the gate charge plots on the data sheets for the MOSFETs and you may be able to convince yourself it's a non-issue.

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