I keep killing A4988's please help!

Hi all,
I have a home built 3d printer. It uses Arduino mega, ramps 1.4, and A4988 step controllers.

things have been going along nicely until last weekend.

I was doing a print that took about 24 hours, and when the print was finished, I homed the print head.

x and y worked great, but when the z started down, it was very erratic, and then it just stopped.

I ended up swapping out A4988's and then it worked fine for about 10 minutes, then it too died.

I swapped it out again, and this time I made sure that the pot was turned down to limit the current.

I have printed two pretty big projects since then, but this morning the Z was again dead.

I swapped out my last good a4988 and made sure the pot was turned down. I also put an amp meter inline.

my 2 Z motors were drawing 120mA. I homed the Z and it moved about 2 inches and then died.

does anyone have any ideas as to what I'm doing wrong?

I know that prior to the first controller dying I had a Z crash. ( homed outside of the bed too far) but it ran for a few days after that.

any ideas on what I have hurt?

thanks.

I may have figured it out.

I swapped out ramps boards. problem went away.

Also might want to make sure that the cable and connectors that connect your Z motor are solidly connected and not intermittent in some way. Also check the solder joints on the RAMPS board for the sockets that connect the drivers to the board, and the connector for the Z headers.

If the motors become disconnected while the unit is powered on, you're more likely to damage the drivers, especially if you do this repeatedly. If you've got a loose connector or bad solder joint and the machine vibrates during movement it could make-break this connection a lot causing the driver to fail.