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Motor Control app ...how to sense current non invasively?

I'm gonna be driving DC motors in the range of 24 to 100+ volts using an H bridge. Due to the potentially high voltage components, i'm isolating the drive circuit from the PIC microcontroller with optocouplers.

However, I need a way to sense the motor current. I'm thinking of putting a current sense resistor, and the traditional op amp, etc, and feeding this to a voltage to frequency converter chip. The output of the VtoF chip would then go into the optocoupler input, and out to the Capture/Compare pin on the PIC.

The above looks too tedious and has too much parts count. Is it possible to use a "current sense transformer" or even so called "hall effect sensor" from Allegro, where I would thread the motor cable through and it gives me the voltage? I would be clocking the PWM at 32 khz to 64khz.

THanks, Mike

Reply to
Mike V.
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That's not high voltage, really now! Think analog and get a data sheet for the IL300. IIRC it has full circuits for just such as this.

Sensing would be fairly slow. All your output devices could be dead before the PIC heard them squeal.

Paul Burke

Reply to
Paul Burke

This is way too fast for your pic to do reliably, why do you need to chop so fast anyway? As a general rule you should run at the lowest frequency that you can get away with. You can sence the drive current with a resistor/amp, or use a transformer, coupled to a comparator and fed back to 1 of your pic interrupt inputs. Be aware that using software to directly control the mosfets can lead to problems/failures if you have a crash/bug. Most people use hardware to do this sort of thing, you can find lots of circuits all over the web.

Reply to
CBarn24050

Does your PIC have an ADC ? The best (although not inexpensive) solution is an isolated closed loop hall-effect current transducer

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You'd also probably want hardware overcurrent comparators to quickly disable the FET's/IGBT's in the bridge in the case of an overcurrent

Reply to
Gary Pace

If you can manage to stay below 55V then the LMD18245 from National Semiconductors may be your ticket. It has an integrated current sense amplifier, output readily available.

Rob

Reply to
Rob Turk

There is on opto device that transmitts voltage levels across the isolation barrier. CP Clair perhaps. You could put in a current sense resistor then this opto and have an isolated voltage proportional to current.

George

Reply to
George

Thanks, can't use this though, because the supplied motor may be one rated at 120V. So i'll be rectifying the winding from a 100VAC transformer.

-Mike

Reply to
Mike V.

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