Easy driver current sense resistor.

[dr pepper]

I have a couple of easy driver stepper motor drivers that use the ad3967 ic.
I'd like to use these for 1/8 microstep, which they can do.
On a fat stepper I have they work fine, however I want to use a small motor with little load on it, to get this to work I'll need to reduce the current limit on the board, no problem I though pull the 2 sense resistors and put bigger ones in, but I cant find any r's less than 1k.
I assumed there would be sense resistors as there is a current limit pot on board, which currently is 150 to 750ma, 15 to 75 ma would be more appropriate for me. Maybe it current limits through other means.
The board does work on a little stepper but the movement is very rough on anything other than full or half step.

 

[Wade Hassler]

If this is your schematic, change R5

 

[dr pepper]

Ok then.
Mine is version 4.4 but it looks similar.
R1 and R2 are the current shunts, the driver is at home right now I'll have to have a look later, I'm thinking maybe these resistances are pcb traces.
For now I'll try shorting R5 along the lines of what you mentioned, that will get the reference i/p voltage down to be able to limit low currents, long term I'd like to replace the shunts too.

 

[Wade Hassler]

The current sense resistors are those two 1206's(?) at the right of the IC in the V4.4 layout

 

[dr pepper]

Excellent thanks.
I dont think theres any markings on those devices on my board.

 

[dr pepper]

I changed the current sense resistors to 10 ohms.
I can adjust the current down to the point where the motor wont turn, turning it up to about 50ma gives reasonable torque from the motor, however it appears to miss the last 2 steps from the 8 step sequence, or perhaps the last step from one sequence and the first step of the next.

 

[dr pepper]

Turns out the easy driver wont really do this kind of motor.
Being chopper control the easy driver requires low resistance windings, my motor is 185 ohms, so the current control doesnt get chance to work.
If I wind the supply right up to 40v I can get the thing to step reasonably smoothly, but the lm317 smd isnt going to last at that.
I found one of the limitations of the easydriver.

 

[dr pepper]

Found these on ebay, they call them a pololu driver:

**broken link removed**

And also these, that can do 45v at 2.4a, and will do 1/32 step, pretty good:

**broken link removed**

And they both come with heatsinks.