Motor Shield w/Reprap motors doesn't like my 12V 1A power supply

[sendhb]

I have an Atmega 1280 with a Adafruit Motor Shield.
I'm using these motors: http://www.ebay.com/itm/5-Stepper-motor ... 748wt_1413
The comments say "I have had RepRap owner state to me that they run these at 12 volts and .8 to 1 amp."
Current per Phase: 1.2A unipolar .85A bipolar

When I power these using the stepper demo using the USB they move nicely (and honestly unexpectedly - I wouldn't have thought the USB was enough to move these at all).

When I remove the PWR_EXT jumper and hook up a 12V 1A power supply, the motor ICs get very hot and the motors move erratically.
I've tried substituting the L293D with SN754410NE which also work fine at USB levels and erratically and too hot at 12V 1A.

What I am I missing? Is that power supply incorrect?

H.B.

[adafruit_support_bill]

Do you have them wired as bipolar or unipolar? 1.2A is still too much for a SN754410NE.

You might consider piggybacking with a heatsink:

[sendhb]

They are six wire motors. I believe I have several options to wire them
http://www.xylotex.com/Windings.pdf
http://www.sxlist.com/techref/io/steppe ... ctions.htm

They're definitely wired as bipolar (four wires) but I'm not 100% sure which of the above configurations I wired them right now. They were 6 wire motors originally, so I believe it's bipolar-series (.85A).

I can try the piggybacking. I did put a large heatsink on it.

Can you clarify why a 12V 1A would be too much for the chip based on http://www.ti.com/lit/ds/symlink/sn754410.pdf?
The 1.2A would be if i were using unipolar?

H.B.

[adafruit_support_bill]

It is not a matter of what your power supply is rated for. It is a combination of what it can really provide (it may be over or under-rated) and what the motor tries to draw from it. Motor specs also vary in practice with variables such as PWM frequencies and driver design. The bottom line is, your bridge chips are getting warm and are showing classic signs of thermal shutdown.