Hey Guys,

I am making a stepper motor controller to control a stepper motor rated @ 1A 3.15ohm using the UCN5804(www.allegromicro.com/datafile/archive/5804.pdf) chip. The motor operates fine if i use 5ohm resistors @ 5volts (L/R Drive as shown on page 5 of the datasheet). What i would like to ideally do is limit current using a MOSFET instead of a resistor, currently i am driving the MOSFET gate with a 20KHz signal @ 63% duty cycle which i think should give me 3.15V but this does not seem to work; on putting this circuit together i find that the motor windings are energised but the motor shaft does not rotate, it is as-though the motor is not getting enough juice. Is it possible to limit the current in this manner; what am i doing wrong??? suggestions...

 

What MOSFET are you using? What are the voltage levels of your gate drive?

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Ron