H-bridge gets too hot - help

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi there,
I breadboarded this h-bridge circuit: http://www.jfunk.org/extras/labs/BipolarHBridgeSchematic.gif
with tip125's for the PNP's and tip120's for the NPN.

The h-bridge is connected to an arduino and a separate 9v battery to power a small dc motor. The h-bridge works, however the motor only receives about 2 volts and transistors get really hot.

The other problem is that when the h-bridge is connected to the power supply and the arduino is off the transistors still heat up.

Any help would be appreciated. Thanks.

2

What signal are you putting to the transistor's bases. It needs to be as high as the power supply going to the bridge or else the top PNP transistor won't turn off. It sounds like it is always on and so drawing current.