Unipolar stepper motor with ULN2003?

[irving2008]

Yes, you have found the reason, but the solution isn't a 0.5A power supply, unless you plan only to energise one coil at a time which is not efficient and means only full-step operation.

The correct solution to this is to use current limiting in the feed to each pair of coils. Your current limiter can be a big power resistor or an active limiter using a voltage regulator device. I've shown you how to do both approaches in the attached diagrams. Do the calculations for yourself. Which you choose to use will depend on whats available to you. Don't think you can get away with just a lower current power supply - at best it wont work reliably, at worst you will still end up frying your devices and motors...

The resistors in the first diagram are 18ohm 10W - they will get HOT, about 80degC surface temp so be careful - but they are designed to get hot assuming you use a proper power resistor. You can mount them on a heatsink to cool them if you desire.

In the second diagram the resistor are 2.4ohm 1W. The second option requires a heatsink for the LM317T devices of about 16degC/W, which can be done with a push-on heatsink or a piece of aluminium plate about 50mm x 50mm x 3mm thick (one each device).