A little H-Bridge help.

Discussion in 'Analog & Mixed-Signal Design' started by jgreene44, Jan 14, 2017.

  1. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    Hello all,

    Thanks for being here in advance.

    I have put together many H-bridges. Very familiar with the Schematic. Putting one together so far has not been hard at all.
    Getting one to work proper is. The one in my hand has 4 2n3904's running it. they are all npn and i see some schematics are
    calling for the negative end of the bridge to be pnp. I have tried both was and this seems to be working. However it is very underpowered.
    i have:
    The Arduino plugged in via usb ---> laptop
    The hc-05 plugged into 5v, Rx ---> Tx, Tx ---> Rx
    The positive side of bridge feed from 12v seperate supply
    All three commonly grounded

    Test results.
    Arduino output: apprx. 4.5v
    Hc-05 input: apprx. 5.1v
    Hot rail breadboard: 9.6v

    Top left 2n3904. (positive input is north):
    C: 9.6
    B: 4.5
    E: 3.8

    Top right:
    C: 9.6
    B: 0.0
    E: .007

    Bottom left:
    C: 3.8
    B: 0.0
    E: 0.00

    Bottom right:
    C: .004
    B: 4.5
    E: .001

    Test point @ DC motor:
    -: .005
    +: 3.79

    325ohm resistors are at each base. germainium dioide from collector to emitter, negative pointing to collector.

    All readings where with the H-bridge active and under load. I have no clue... i have a feeling this is somehow related to the fact that the bottom two transistors are not pnp. the only pnp transistors i have laying around are c8550's and im thinking they might not be suitable since they have that 2w audio spec? i have tried using them. acts like a dead short. i have 4 and they all seem ok outside of the bridge. im also thinking it might be the forward voltage/current from the arduino?
     
    Last edited: Jan 14, 2017
  2. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    recently heard the term "emitter follower" is this the problem?
     
  3. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    hi 44,
    Could you post a circuit diagram.?
    Using 4 , 2N3904 NPN transistors in a H Bridge will give a few problems.
    E
     
  4. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
  5. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    hi,
    Got the circuit OK.
    What are the A and B driving voltages and phase relationship.?
    Also the supply voltage.?
    E

    EDIT:
    Looking at your first post voltages for Drive and Supply. you should get this result
     
    Last edited: Jan 14, 2017
  6. BobTPH

    Senior Member

    Jun 5, 2013
    1,214
    243
    Yes.

    To make a proper bridge for a 12V motor driven by a 4.5V Arduino signal, you will need PNP transistors for the top two, and a level translator to drive them.

    Bob
     
  7. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    BobTPH: ok wow! i looked at a bunch of h-bridge diagrams when i first started learning of these. somewhere along the line i got it in my head the pnp's where on the bottom. so that is where i applied my s8550's. did not work. (of course) hrm thats really great you pointed that out for me. seems like a simple fix. testing now!

    the word level translator is pretty new to me. my arduino is supposed to use one for the 3.3-5v on the tx or rx i forget (i dont use it like im supposed to. seems fine). either way. thats about the extent of my use and knowledge. would really like it if you could explain to me a little better. i did do a quick google on it but, im not sure at all how to apply this to my current circuit.

    is the level translator for 12v ---> 5v

    ok i added the two pnp's to the top side in a breadboard. dead short. put the two npn's back in. works fine again? i have 4 8550's from salvage they might not be reliable... when mixed them all they all acted the same...

    ...more investigation. seems if i focus on one side of motor feed. i get instant on. no 5v to base. just running. checked pinout again. is fine. i know why it acts like dead short now at least. not clue why these pnp's are acting the way they are. they had been tested by simply turning an led on and off before i stored them.
     
    Last edited: Jan 15, 2017
  8. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    hi 44,
    What was the point in you posting a circuit web link that does not represent the actual circuit you are using.???

    E
     
  9. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    im not sure i follow i think all is the same minus the fact i used 2n3904's?
     
  10. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    Hi,
    As Bob points out if you use 4, NPN transistors, powered from a 12v rail, the top two will not be fully turned ON with a Base voltage of 5v.

    A level shifter is required, usually an additional transistor level shifter for the top pair.

    You could use PNP for the top pair, but a level shifter is still required, but it is a simpler shifter.

    E
     
  11. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    hi,
    Look at this very basic circuit showing level shifting.
    What are the specifications of your motor and power supply.?
    E
     
  12. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    is the level shift at points A and B? if so i have that. power supply running at 9.6v.
     
  13. ericgibbs

    Moderator

    Jan 29, 2010
    6,180
    1,171
    hi,
    The A and B are the bridge drive from the Arduino output pins.
    In the program, ensure that A and B are NEVER at 5v at the same time.!

    The Vsupply of 9.6v is OK.
    You have not said what is the spec of the motor, Volts , Amps .???

    E
     
  14. dannyf

    Well-Known Member

    Sep 13, 2015
    2,196
    421
    it can be on the bottom, or the top, or nowhere, depending on the design.

    the use of a pnp here is that you may not be able to shut it off.
     
  15. ScottWang

    Moderator

    Aug 23, 2012
    6,411
    991
    I'm not sure how much current the motor needs, so all the calculation is according to your original value as Iin = 25.3mA, Iout = 253mA.
    HbridgeMotorDriverWithTwoVin_ScottWang.gif
     
  16. jgreene44

    Thread Starter Member

    Dec 8, 2016
    108
    4
    thats awsome! i got it. adding the remainder of the circuit did it. thank you all soo much!
     
    Last edited: Jan 15, 2017
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