H Bridge trouble

Discussion in 'General Electronics Chat' started by amsim, May 24, 2009.

  1. amsim

    Thread Starter Member

    May 24, 2009
    I am building a CNC router. I have the whole thing essentially done, except the control of power to the motors. To this end, I am trying to build H-bridges, which I have never done before. I acquired some MJE3055T power transistors, and I've looked at the basic literature. I constructed the standard H-bridge, but it isn't working. Many of the schematics I've seen for power H-bridges have both PNP and NPN transistors in them. Is this a requirement? The data sheet says that the gain should be between 5 and 100. A few that I have are around 250. Does the gain have to match all around? Is the low gain going to be a problem? I tried to use a single transistor to turn on a motor. When I arc the base to positive, the motor will run when it is below the transistor, but not above it. I am so close to finishing this. I am short on time and money, and this is to supplement the family income. Please help.
  2. AdrianN

    Active Member

    Apr 27, 2009
    You should post your schematic. An H-Bridge can be driven in switched mode, or active region, depending on power and noise (smoothness) requirements. PNP and NPN in the same bridge is not a requirement, as long as the driver matches the bridge topology.
  3. hgmjr

    Retired Moderator

    Jan 28, 2005
    It sounds like a possible connection problem. A schematic will help us help you.