H Bridge with a PSoC

Discussion in 'The Projects Forum' started by dusttart, May 12, 2013.

  1. dusttart

    Thread Starter New Member

    May 12, 2013
    Hi, I am building an H Bridge to drive a motor. Attached is an image of my schematic. I used a LUT to send only viable inputs to the transistors; this part has been tested with LEDS. It, however, will not start the motor. Any suggestions or help would be greatly appreciated!

    Here are the data sheets for the PNP transistors and NPN transistors.
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Have you connected the emitters to the supply lines and collectors to the motor ?
    dusttart likes this.
  3. dusttart

    Thread Starter New Member

    May 12, 2013
    Yes I've verified the emitters are connected to the source voltages and the collectors to the motor. Attached is an updated sketch with details of the transistors.
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Why don´t you use the standard pnp and npn symbols? This is very hard to comprehend.
    Anyway the transistors seem to be connected correctly. But since the base resistors are only 1k, leaving 4.3mA flowing through the base, by the rule of thumb you will be only able to get ~43mA through the motor. And since the motors wants 320mA this will be a problem.
    Also are you sure the base signals are correct? Better use some leds for indication.
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    The 2n3906 transistor only handles a maximum current of about 100ma.
    That's not very clear in your 2n3906 datasheet. Other datasheets typically will have an absolute maximum rating section listing that spec.
    The 2n2222 is better but not enough.

    The motors appear to need an amp or more when running with a load. A 2 amp transistor rating might be OK, but 3-4 amps might be needed depending on the motor starting current which can be many times higher than no-load.

    So larger current transistors will be needed. As kubeek mentioned higher drive current is needed also.

    A solution would be darlington transistors.
    Normally mosfets are the choice, but with the low volts available and the motor needing only a few volts these should be ok.

    2 amp Darlingtons

    TIP100/TIP115 for example.
    Last edited: May 13, 2013
  6. ScottWang


    Aug 23, 2012
  7. dusttart

    Thread Starter New Member

    May 12, 2013
    It does seem that the 2N3906 are not providing enough current (I can get ~120mA) to run the motor. I used only P2N2222As for my H-Bridge and after a small twist the motor will run. The problem is, these parts were given to me for a class. I have posted on the course forums but I feel like the professor would know if the PNP transistor could not supply enough current to run the motor. I'm not sure why all the designs for an H-Bridge I see say to use PNP connected to Vsrc if the NPN transistors work fine. Any clarification on H-Bridges would be greatly appreciated.
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    Yes, you would expect to be given suitable parts for the project. You should bring this to the professors attention...hopefully the prof. is someone you can talk to.

    But, you can learn much more by solving problems with a circuit that doesn't work as expected than by one that works perfectly on the first try.

    Simple transistor PNP/NPN H-bridges are easier to control because you can tie the base resistors on a side together and control that side with a single control voltage and also get proper saturation to fully turn on the transistors.

    Now, since this is a school assignment, and based on the info yuo've been given here,
    what do you think would be needed to properly control an all NPN transistor H-bridge ??