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Blew up a Pololu last night...

Posted by a kungfu mastah 
Blew up a Pololu last night...
September 13, 2011 12:58PM
I got my RAMPS working, I bought one from reprapworld (i think), lot of ABS too. Works great, So far I have the X axis working great, Extruder motor/gear working (broke drill bit in MIG tip :< ).
So I soldered together the harness for the two Z axis motors. They worked the first time, I adjusted the pot a little to the left, then all I got was shuddering, a bit of spinning and backward spinning, and didnt really get by that. I think the pots were not moving with the screwdriver. So the little screw spin thing got a little bent from needle nose pliers (but easier to grab). I moved it a little to the right, worked a little better so I killed power, took out pololu, adjusted a little more to the right. When I tried to use the motor it went up in flames.

I have since cleaned the PCBs and tested everything, Besides the driver everything is OK.

These are the motors I am using: motors

Arduino Mega 2560
Windows 7
RepSnapper at the time
Mechanical Endstops (Z was plugged in at the time)
Re: Blew up a Pololu last night...
September 14, 2011 01:15PM
I suppose a good place to start would be how I go about measuring the current the current pot settings are putting?


A kungfu mastah
Re: Blew up a Pololu last night...
September 14, 2011 02:31PM
Measure the voltage on the wiper of the pot and multiply by 2.5 to get the current in amps.

Re: Blew up a Pololu last night...
September 16, 2011 12:48AM
On all versions of the various Pololu type stepper controlers when used on a Ramps type board you can adjust the curent setting without the 12v -24v main motor/heater power suply being turned on.

I have found the easiest way to do this is to use a small metal instument screw driver that has the red wire of your meter conected to it while the black wire of the meter is connected to the 0V of the power input plug.

One other thing to watch out for hence the wording Pololu type stepper controlers the stepstick version has a 0.2 ohm current sence resistor not a 0.05 ohm current sence resistor as used on the Pololu board.

This means the voltage measured for setting up a stepstick stepper controller is not the same as required on a Pololu version.

The current formular remains the same for both Types of boards :-
MotorCurrent= VREF / ( 8 x RS) The voltage setting is significantly diffrent.

So for 1A drive on a Stepstick 1.6v / 8 x 0.2 = 1A compared to a Pololu board 0.4v / 8 x 0.05 = 1A.

On both types of board VREF is measured on the wiper.

Due to the values of resistors used on the VREF potential divider remaining the same as those used on a Pololu board and the new value used for RS (R Sence) the maximum curent avalible on a Step Stick is :-

Curent max = (10,000 / (20,000 + 10,000) / 5v) = 1.667 / 8 x 0.2 = 1.042 A

This makes the Step Stick virtualy imposible to ~Blow up as it uses the A4988 chip which has in built :-

▪ Thermal shutdown circuitry
▪ Short-to-ground protection
▪ Shorted load protection

Compared to the A4983 chip which only has

▪ Crossover-current protection
▪ Thermal shutdown circuitry


Please note:-

This does not mean you should abuse this extra protection..
You must still switch off the motor power supply and Audrino power supply (unplug the USB connector) before unplugging a stepper module or plugging one in or connecting/dis connecting a stepper motor. Also It is still better practice to adjust the motor current setting with the motor power supply switched off. Also Anti static precautions should be applied at all times when handling the Steper driver modules of both types.

Edited 2 time(s). Last edit at 09/16/2011 01:01AM by BodgeIt.

Bodge It [reprap.org]

BIQ Sanguinololu SD LCD board BIQ Stepcon BIQ Opto Endstop
BIQ Heater Block PCB BIQ Extruder Peek clamp replacement BIQ Huxley Seedling
BIQ Sanguinololu mounting BIQ standalone Sanguinololu or Ramps mounting Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]
Re: Blew up a Pololu last night...
October 27, 2011 01:47PM
Where on the SMD StepStick version can I measure Vref?

I have the same (green) version as described here: [reprap.org] and nowhere is described where Vref can be found (which is also a complaint on the Talk page).
Re: Blew up a Pololu last night...
October 27, 2011 05:05PM
On the wiper of the pot. Note that the ref voltage is difference to Pololu.

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