Failed Pololu A4988 -- Low Vref and Trimpot Wiper to GND Resistance

Ordered a new Pololu A4988 to replace a failed one and when I installed it I found that the torque on the motors was very low, regardless of how much I adjusted the trimpot. I tried setting the trimpot to get the same resistance from the wiper to GND (and thus the same Vref) as I have on my other 4 working Pololu's, but the measurement kept hovering around 20 Ω (not kΩ) no matter how I rotated the trimpot.

I then took the small trimpot off of this Pololu and replaced it with one from a failed Pololu that was showing the resistance I wanted from the wiper to GND. When I measured the resistance from this new trimpot, found that the value was still down around 20 Ω, when it had been 1 kΩ on the board I just took it off of . . .

So I realized something was either cooked on the A4988 or there was a short somewhere on the Pololu (less likely I would think). Problem is, this was a brand new Pololu, so the failure, if it is a failure, is disappointing to say the least. It's also a different failure from the one I've seen on another Pololu, since the resistance and Vref on that board was not bogus even when it would not properly drive the motors.

My question (finally, sorry ) is, has anyone else seen this behavior, and is there a way to fix the Pololu when it occurs? The Vref measured from the wiper of the trimpot to GND is close to 0V. When I set the trimpots on the other Pololu's to get resistance from the wiper to GND of 1 kΩ, I get a Vref of 0.05V, and the motors run well (at least in trials, will probably have to increase power when I begin printing).

Thanks!!!!!
Daniel

Vref = 0.05? That would be 125mA. Normally it is set to about 0.4V for 1A.

Connecting something into a circuit cannot increase its measured resistance, only lower it. Perhaps soldering damaged it.

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[www.hydraraptor.blogspot.com]

I do worry that soldering caused the damage, but I observed the failure after only soldering on the headers for inserting the Pololu into the RAMPS (and then later swapped out the trimpots with more soldering).

Are the Pololu's subject to failure from getting them too hot when the headers are soldered on?

You're right I will need to up the current once I get everything set up.

No soldering on the header should not be a problem unless the solder joints on the chip were dodgy to start with.

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[www.hydraraptor.blogspot.com]

Given that I can't get the Vref to adjust (just stays basically at 0V), is it safe to say that the driver is lost, or is there something else I should check? The cost is low enough that ordering a replacement isn't that painful, I just hate to waste another day or two waiting on a new one.

Thanks for the responses!!!!

Sorry I didn't realise you got 20 ohm, I misread it as 20K.

If Vref is shorted to ground the most likely cause is a short where the chip is soldered. If that is the case you might be able to clear it by adding flux and using a small tipped soldering iron clear of solder to wick some away, quite tricky with QFN packages.

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[www.hydraraptor.blogspot.com]

Thanks for the info. I looked closely at the boards but could not see any shorts or bad solder joints, etc., and ended up just replacing the driver with a new one. I have two spares right now on hand, so that gives me some peace of mind also.

The good news is that I got my printer going two days ago, and have been able to print out parts of at least recognizable shape and size

I'm putting pics, video, and info up on a blog--please comment if you see anything opportunities to give any more advice. There is so much to learn beyond just getting the motors to move and plastic to squirt out . . .

Here's the blog: [dmillerreprap.blogspot.com]