After i salvaged some stepper motors and read alot about them i finally decided to try them out... But before i did i asked: Simple stepper motor circuit safe?
in the above question you can find datasheet and circuits of the motor i used.
After finding the 2 coils and the center tap i tested with 5v using a battery.
As expected everything worked fine.
It was time to solder things together. I used 2 channels per chip To be sure to have enough sink and not overheat the chip. I tested everything again with 5v. Perfect.
Now, as the motor is a 12-24v motor i atached a 12v 1.25Ampere powersupply.
The green Light on the powersupply started to dimm as soon as there was current flowing trough the ULN2803. Anyway the motor just made some random noises and unexpected steps.
Even if the whole setup should consume less than 1.25Ampere i decided to power it with a bigger one. So i took a 12v 5A powersupply and turned the motor on.
After 3 erratically stutters the uln2803 exploded.
The only thing i could think of was wrong is the missing Diode between the powersupply and the common of the ULN2803 wich should be optional as the that chip already has alot of protection diodes.
Please help me to understand what i did wrong. I'm not an electrical engineer, i'm just trying to learn new things and i don't want to give up now. i have more uln2803...
here is the finished circuit http://imgur.com/a/tCdwk (2 images) and shematic