Problem with transistors

[Kaar3l]

Hello
**broken link removed**
I followed that scheme. One difference is that I used TIP122 transistors. I am using 12v power supply.
When I connect power supply and without computer the transistors should be at "off" state, but they aren't.

Could anyone help me?

 

[MikeMl]

PC parallel ports are notoriously bad at sourcing current in the high state. They are very whimpy in the high state; they are lucky to source 1mA@3V . OTOH, they can sink about 20mA when at 0V.

The TIP122 (see here) has resistors between base and emitter. With the base open, they should be off. With the base open, if yours are on, then you have them wired wrong.

 

[Diver300]

Well as the TIP122 is a darlington pair, you shouldn't have any problem with providing enough current.

However, there are no freewheel diodes in that schematic. There will be a huge voltage spike as the stepper is turned off, which has blown the TIP122.

They could also be wired wrong.