Help : New build Ramps 1.3 possibly 1 faulty Pololu A4988

I've just built a RAMPS 1.3 with 4x A4988 black edition drivers and I'm testing the drivers one at a time
(printer frame not assemblied yet). All have been set 0.4v.

configured and running marlin, endstops are working correctly.

3 of the drivers work great, motor run smooth in x,y and z locations (I have
tried E0 but can't run the motor because temp too cold, heater and thermistors
not connected)

1 of the drivers cause the motor to run rough, movement is jerky and noisey.
I tried adjusting the voltage, have been upto 0.8v with no discernible change
in the motor, voltage range tried is 0.2v - 0.8v with same result. The driver is
the same in either x,y, or z RAMP locations and with 2 different stepper motors
(other 3 drivers are fine in same configerations).

Motors are only run for a few seconds so I don't think its a overheat problem
(heatsinks dont feel that hot). At no time have drivers or motors been
connected/disconnected with power on (either 12v or 5v usb).

Does anybody have any ideas on other tests I perform to try and locate the
problem ? (only have a multimeter)

or have I got a faulty A4988 that needs to be replaced ?

Thanks.

Hi,

Just did some more testing.

using x-axis on ramps, I removed the jumpers that set the microstepping, so now it's full stepping only. Voltage is 0.4v as before.

Using pronterface, if I hit the foward/backward '1' ring the motor moves as before i.e. feels rough sounds noisy (motor is only moving in small increments, should it do this with microstepping jumpers removed ??)

if I hit the '10' ring or x-axis Home , the motor stalls and makes a whining noise.
(when i hit home with the microstepping jumpers installed (all 3 jumpers = 1/16) the motor runs, its just noisy and jerky, so
the stalling is a change in behaviour)

I then adjusted the pot to 1.0v (microstepping jumpers still removed) , and did the above again using ponterface, with the exact same result.

So it seems to me that this A4988 is not providing enough voltage or amperage ? to the motor, no matter where I set the pot on the driver board to.

motors are pretty standard , 2.8V , 1.68A (other 3 A4988's run the motors fine).

using the simple schematic on the pololu site Lower down on the page does anybody know how I can test the A4988 , using a multimeter. (my electronics knowledge doesn't extend to me being able to look at those things and know what to test, not without summoning forth smoke)

thanks.

Edited 2 time(s). Last edit at 06/10/2013 05:57AM by Masnachu.

The easiest thing to do is to swap motors and/or drivers to see if the problem moves or not.

Just remember to remove power before changing anything. I destroyed 2 of my drivers when I made wiring changes inadvertantly under power. Replacing drivers is nor a huge problem as they are only $10 direct from pololu. In fact when I was replacing my bad ones, I purchased two spares.

yep the problem driver is the same wherever I plug it into RAMPS i.e. x,y or z. It's also the same whichever of the 5 stepper motors I use.

The 3 good drivers all work properly, either on their own or alltogther in x,y and z , they're also fine with all 5 motors.

So when you say you destroyed 2 of them, were they completly dead ? or were they still working just not properly.


Thanks.

Did some more testing with the multimeter this afternoon.

voltage via POT set to 0.4v (is this vref ?)

Faulty driver first.

VDD = 4.88v
VMot = 12.26v

then tested the coils between driver and motor.
1B = -0.07A
1A = +0.07A
2A = -0.50A
2B = +0.46A

Think I've found the problem, as suspected driver isn't suppling enough current (coils are unbalanced), to make sure did a 2nd test and measured again.

1B = -0.04A
1A = +0.10A
2A = -0.50A
2B = +0.50A

which confirms it.

I then tested a known good driver , again voltage set to 0.4v

1B = -0.50A
1A = +0.55A
2A = -0.50A
2B = +0.50A

So definitely a faulty Pololu.

Thanks.