interesting Pololu burnout

I have come across an interesting failure mode on the a4988 Pololu drivers. The driver clamps Vdd to ground and shorts circuits the logic power supply. It has happened to me twice and I have no idea what caused it. Does the A4988 drives have some special Vdd Vmot powerup requirements?

After this the driver stops being a stepper driver and starts being a quite expensive resistor.

I am quite puzzled, because I do not have the faintest idea, what caused it. I have another 4 Pololu's plugged into a RAMPS board and the do not seem to be acting up.

ESD causes that. In the winter sometimes when I pull an object off the bed of my Mendel it latches up one of the Allegro drivers. The VDD current is limited by a 78L05 regulator so they survive and just need a power off to recover. If they are connected to a high current supply like the 5V rail from a PC PSU I think they would be destroyed.

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[www.hydraraptor.blogspot.com]

I was powering the VDD from USB. It might be that USB is enough to fry them. If I remember correctly the smallest 78L05 is 100mA the usually people use the 500mA version, but 500mA is USB also and that was enough to kill them.

So I suspect 100mA is the number to aim to. So feeding them using a 50 ohm current limiting resistor should work. Spec sheet say's that the logic uses about 8mA so that would reduce the VDD from 5V to 4.6V in normal operating conditions.

I wonder if any of the current reprap electronics limit the VDD current to the Pololus?

I have used a small series resistor to prevent latch up on other devices in the past. Obviously you need to put the decoupling after the resistor.

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[www.hydraraptor.blogspot.com]

Have exact same problem. Ordered 4 stepsticks from reprap.me and 2 of them work fine, the other 2 have the same issue as you describe they short my logic.

If I put them into my sanguinulolu the power led does not even go on with the broken stepsticks, the other 2 work fine.

Also this is my second printer the first printer I just slapped the stepsticks onto an arduino, hot glued them inside a box, didn't fit heatsinks + soldered some resistors directly on the headers from ms2 to vdd to get microstepping etc. and those puppies have been working from januari without any problems 'rock solid'.

For the second printer I got a nice proper sanguinulolu board, some brand new stepsticks and 2 fail to work for me on arrival



stevep Wrote:
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> I have come across an interesting failure mode on
> the a4988 Pololu drivers. The driver clamps Vdd
> to ground and shorts circuits the logic power
> supply. It has happened to me twice and I have no
> idea what caused it. Does the A4988 drives have
> some special Vdd Vmot powerup requirements?
>
> After this the driver stops being a stepper driver
> and starts being a quite expensive resistor.
>
> I am quite puzzled, because I do not have the
> faintest idea, what caused it. I have another 4
> Pololu's plugged into a RAMPS board and the do not
> seem to be acting up.

I'm working with a big easy driver BED and want to set my current limit for my stepper. The stepper is 1.7A and wall work rated for 12V . I set the current using the TPI pin on as per the guide book but it shows two differnet sense resistors. I've the same settings as POLOU A4988 but can't imagine what I have.

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