Stepper motor problems

I'm trying to get a stepper motor to replicate the movement of a second hand on a clock. so i have wired up a unipolar stepper as in the two wire diagram on : http://arduino.cc/en/Reference/StepperUnipolarCircuit except without the dioide back into the com pin

I've also done a simple piece of code to get it to tick.

my problem is that the ULN2003A I'm using gets very hot quickly and then starts to fail. I'm running the stepper on the VIN power (9v from mains or battery, both tried).

is it likely to be the lack of diode causing this?

What current is being drawn by the stepper? Do you know the voltage/current rating?

The voltage at each output of the ULN2003 will be about 1V or so when on thus any current going into this pin will cause the ULN2003 to heat up according to the power dissipation P=VI (where V is about 1V). So a stepper whose coils each conduct 1A when 9V are applied will cause the ULN2003 to dissipate 1W per pin which, yes, is going to cause it to get really hot!

It's all about the numbers.

The maximum total current that can be flowing through a ULN2003 at any instance of time is about 650mA. That is adding up all the current on all the pins. Any more and it will get so hot that it will soon destroy itself. See:- http://www.thebox.myzen.co.uk/Tutorial/Power.html and also the power examples where that situation is described precisely.

I'm going to test the current it is drawing over the weekend.

but assuming that it is excessive (say 1A)

what can I use instead of a ULN2003A Am I just back to a set of transistors? if so what kind spec do you think would be approriate?

The spec for tghe transistor depends on what current you need, so when you have your meassurements just find a transistor that can handle the current required. Some of the transistors in the TIP XXX series can most likely handle your needs.