# Stepper Motor current question

Hey guys....

I'm currently trying to figure out the Easy Driver V4.4 from Schmalzhaus to control a Stepper Motor. Okay, so I bought a Easy Driver V4.4 and a Stepper Motor.

The Motor needs max 280mA but the Easy Driver supports up to 750mA I think. To adjust that, there is a potentiometer on the driver board.

My question:

I need to measure the Voltage on Pin TP1 to calculate the current on the pot but can I destroy the Motor if the pot ist set to high from the beginning??? The documentation says that the pot direction can be different because sparkfun used a few different pots...great ^^

GeoSurveyor

according to the schematic, Vref controls the current limit. You shouldn't need the motor connected to check that. Power the shield with the 5 volt supply and measure Vref (at tp1). If my calculation is correct 1.8 volts on Vref will limit current to 280mA.

Ah okay, but how do you get 1.8 volts?? I found this equation: I_max = Vref / 8 * Rs

rearranged:

Vref = I_max * 8 * Rs => 0,28 A * 8 * 0.75 Ohm = 1,68 volts

Look at the schematic (a link in the link for the board). The note above pin 1 of the A3967 chip states that at Vref 1.0V current is 166 mA, 3.3V Vref = 550mA 5.0V = 833 mA. This is pretty linear, so 280/166 = 1.68. Yeah I dropped the 6 when I typed, oops. So either way your 280 mA will be at 1.68Vref.

Awesome =) thanks for quick answers!!

I'll test it tomorrow!

One point that may not be obvious is that a chopper-driver like the Easy Driver benefits from a supply voltage thats at least twice the nominal winding voltage, in order to get reasonable response at speed (in other words it will be able to overcome the DC resistance and the back-EMF and keep the current at the preset limit). I'd suggest that 12V might be a little low if you were assuming that.

If you are needing really fast stepping, that's not the ideal motor since the winding resistance and inductance are fairly high - but otherwise it makes no odds. Higher supply voltage (upto the Easy stepper's max) will get faster stepping.

If you overdrive the motor with too much current it will get too hot - with older stepper motors there was also a risk of de-magnetising the permanent magnets, but modern steppers use rare-earth magnets that are far less prone to demagnetisation (but not to overheating). Note that steppers do get hot, and 80C isn't an uncommon design temperature at rated current...