I am using NPN transistors in an H-bridge to connect a DC motor to ground. My problem is the base of the transistor seems to be grounding through the emitter pulling the value of my logic circuit to zero. This causes the logic circuit to jump to another phase.

Edit: It is setup almost identical to this.

Except there is a separate signal to each transistor coming from the logic circuit. I have PNPs on top (Tip42) connected to the voltage source and NPNs on the bottom (Tip120) connected to ground.

  • \$\begingroup\$ Posting a schematic would be helpful. What is "the" transistor? There are four transistors in an H-bridge. \$\endgroup\$ – Adam Haun Oct 15 '15 at 4:40
  • \$\begingroup\$ a schematic would show whether you have a base resistor! \$\endgroup\$ – Neil_UK Oct 15 '15 at 6:37
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    \$\begingroup\$ Show the exact schematic not something that is almost identical. Would a doctor expect to diagnose a problem in a patient when examining somebody else? \$\endgroup\$ – Andy aka Oct 15 '15 at 7:35
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    \$\begingroup\$ You do know that the base - emitter junction of an NPN transistor acts like a diode and will normally have just 0.7V across it (you can't make it go higher than this) - is this the 'grounding' you mean?. If so, you just need a resistor between your logic and the transistor base. \$\endgroup\$ – Icy Oct 15 '15 at 8:45
  • \$\begingroup\$ Do you see those little rectangles with "1k" written in them? You need to get some of them and add them to your circuit and it'll prob'ly start working. \$\endgroup\$ – EM Fields Oct 15 '15 at 9:49

I've included a capacitor from the 5 V source to ground with the leads from the same node connected to the PNP transistors. This seems to have solved the problem. Apparently it has something to do with the motors coil and the load varying substantially, especially after switching directions. The capacitor provides and extra current source which in turn prevents the base from grounding through the emitter.

Thank you to those that left constructive comments.

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