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I'm using a BX35 for a robotics project. I'm stuck on the breadboard. I put together a standard H-bridge to operate two drive motors. The bridge consists of four NPN TIP31 transistors with 1k resistors. I am using a PNP TIP42 with a 2.2k risistor to drive each pair of NPNs. My power supply is a 12v battery. I'm using a 7805 5 volt voltage regulator to emulate the BX35 input signal to the PNPs. At first I didn't use the PNPs, I just tried to drive the NPNs with 5 volts, but it wouldn't work. So I drove them with 12 volts with the use of the PNPs being driven by 5 volts. My question is this; This set up seems to work fine on my breadboard to operate the motor, but when I used my meter to voltages throughout the circut, I noticed that when I powered up one PNP the base of the unused PNP was at 12 volts! This bothers me because this is supposed to be one of the output lines from the BX. It seems to me that this would fry the chip. I've studied my wiring over and over and it appears to be exactly the way I have seen it in my reference books. I thought current should pass through the base from the emitter or collector. Is this correct? Or what am I possibly overlooking? Any help would be appreciated. I've been over this so many times it's driving me nuts!

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Use a SN754410 Half H Driver you can drive directly with TTL inputs and its low cost. Mario

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