On Wed, Dec 21, 2011 at 9:13 PM, Ethan Swint <eswint DOT ramu AT verizon DOT net= > wrote:

On 12/21/2011 12:11 PM, = Rob Butts wrote:

My application has two 12 v dc motors that I want to operate in both direct= ions with speed control. I have two potentiometers to determine speed contr= ol for each motor. I have one push button and three to five switches for st= ate control.

I initially didn't plan on speed control for each motor and I had a PIC= Kit 2 so I opted for the Low Pin Count development board with the setup for= the pic16F690 microprocessor. =A0The documentation for that microprocessor= says the pmw can be set up for one, two or four channels but it has just o= ne pwm module.

The processor that microchip suggests has more pins and I'd have to pur= chase more development hardware. =A0I have the code completed and working f= or the five input state controller and the two adcs monitoring the motor sp= eed potentiometers. =A0Does anyone have suggestions for speed and direction= control of the two motors where I don't have to purchase new developme= nt hardware? =A0I thought about two micros in a master slave configuration = but that would make development and prototyping on my desk difficult.

Thanks,
Rob

The four pins will be sufficient - you'll need a gate driver. =A0Put ea= ch motor in an H-bridge configuration, i.e. totem-pole two mosfets on each = side. =A0The low-side transistors will be your direction selectors - turn o= n the left for CW, turn on right for CCW, low-frequency, run a GPIO pin fro= m the MCU. =A0The hi-side transistors will take the PWM signals (2 for each= motor, 4 total) - the gate drivers will have a boot-strap power supply to = drive the hi-side gate. =A0I'd suggest a toggle switch for direction &a= mp; running and the pot just to set the speed - otherwise finding 'zero= ' speed on the pot will be hard. =A0You can just take the max raw ADC v= alue and scale that to your max PWM counter value, a simple multiply-by-con= stant. =A0PWM frequency of 15kHz won't annoy most people over 30yrs, bu= t 20kHz would be better.

On your layout, keep the loop tight from +12V across the 2-FET totem to gro= und, bypass with a few hundred nF of ceramic caps so that the parasitic ind= uctance on the PCB won't over-voltage the FETs and toast them - typical= ly 10x capacitance of a FET's capacitance, given on the spec sheet. =A0= Several caps in parallel will get their ESR lower so that they handle the t= ransient more effectively, too.

The flyback current from the motor will go through the body diode of the FE= Ts, but make sure you have sufficient time of zero PWM on the top when reve= rsing, e.g. >50uSec. =A0If you accidentally turn on the top FET while th= e bottom is turned on (or the body diode reverse recovery), you can over-cu= rrent the FETs and toast them in one PWM cycle.

No capacitors across the motor - you effectively have two mismatched voltag= e sources in parallel and the current spike will kill the FETs. =A0No resis= tor in series with the motor is needed, either - that just wastes energy.
-Ethan