# Thread: need help on sense resistor

1. ## need help on sense resistor

My friend gave me a stepper, bipolar rated at 5.3V 8.5ohm (0.625A / phase)

I use AVR + ST L6219 for controller http://www.st.com/stonline/products/...re/ds/1377.pdf

I have 0.22ohm 1w resistor then set vref to 1.65v since the formula from datasheet to calculate max current is

I max = Vref /RS * 10
= 1.65 / 0.22 * 10
= 0.75A -> a little bit over current

then I measure the voltage at RS, it 0.12v
I=V/R
= 0.12 / 0.22
= 0.64A

Am I on the right track here?
Is it normal for 0.625A stepper drived by 0.75A controller drain 0.64A ?

2. Originally Posted by xinzc28
My friend gave me a stepper, bipolar rated at 5.3V 8.5ohm (0.625A / phase)

I use AVR + ST L6219 for controller http://www.st.com/stonline/products/...re/ds/1377.pdf

I have 0.22ohm 1w resistor then set vref to 1.65v since the formula from datasheet to calculate max current is

I max = Vref /RS * 10
= 1.65 / 0.22 * 10
= 0.75A -> a little bit over current

then I measure the voltage at RS, it 0.12v
I=V/R
= 0.12 / 0.22
= 0.64A

Am I on the right track here?
Is it normal for 0.625A stepper drived by 0.75A controller drain 0.64A ?
The current in the coils is not DC but PWM'd to create the necessary varying drive level to step the motor. Therefore the voltage you measure will be an average voltage (roughly, depending on your meter) but not either the peak or true RMS voltage, (and therefore current) - in other words measuring the sense voltage tells you little about the actual current as its not a static value (except maybe when the motor is stationary). However its clearly in the right ballpark so unless your motor is getting excessively hot I'd be happy with it... but why not set Vref to 1.375v, or chage Rs to 0.27ohm?

3. Thank you for your comment & advice.

I'll change vref to 1.375v.
since there is no problem I'll rebuild the circuit from breadboard to pcb.