Transistor H-Bridge overheating, what to consider?

Discussion in 'The Projects Forum' started by CranberryJuice, Dec 22, 2015.

  1. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    Hi All,

    I tried doing this circuit (as it is diagramed, swapped with a basic hand-wound solenoid ~ 60 ohm resistance). I notice if I were to keep the voltage supply running and the logic state at high (logic 1) low (logic 2) or vice versa, it does overheat the transistor (Q1). Could someone explain to me what the possible reasons may be and what to consider when doing this circuit?

    [​IMG]
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What are the voltage levels at I1 and I2?

    This is possibly the worst circuit you can use to make an H-Bridge.

    The issues with H-bridges are discussed in this long-running forum thread. Pay attention to Post #42.
     
    Last edited by a moderator: Dec 23, 2015
  3. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    For l1 = 1V l2 = 0.98V
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You have the NPNs and the PNPs reversed in your diagram.
    Also the logic signals need to go between ground and the supply voltage.
     
  5. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The H-bridge as shown is a perfect transistor melting device.

    The transistors are guaranteed to NOT be in saturation, and therefor dissipate lots of power.
    Look into using a MOSFET H-bridge, or find a better bipolar design.
     
  6. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    I'll look into a MOSFET H-bridge. Would this be a good one:



    I found this on the internet, so I have less of an understanding. Are you saying that the NPN and PNPs should swap places? In the case of the logic signals, then what are the resistors that are connected to the base get connected to?
     
    Last edited: Dec 23, 2015
  7. Colin55

    Member

    Aug 27, 2015
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    Are you saying that the NPN and PNPs should swap places? If you swap the transistor, you must be sure the input signal goes from high to low very quickly as both transistors will be turned on during the time when the signal changes from one state to the other.
    In the circuit above, the voltage across the collector-emitter will be 1v plus 0.7v plus a little from the 1k. This means you lose nearly 2v from each side of the H-bridge.
    A good H-bridge can deliver rail voltage minus as little as 100mV - not 4,000mV !!!!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Yes, the PNPs should be on top and the NPNs on the bottom.

    The input control signals 0V and V+ (+12V) are applied to the resistors.
    If you logic signals have a lower voltage, then you need to add a transistor to amplify the voltage to 0 to 12V to the resistors.
     
  9. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    @pnp/NPN: Okay, got it.
    The input control signals part, still don't understand. This is what I think I understand: L1 leads should connect to +12 and GND. L2, same thing.
    The resistors, on the other hand, I'm guessing R1,2,3,4 connect to +12 to the base?
     
  10. Colin55

    Member

    Aug 27, 2015
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    What is the value of the HIGH voltage on the input lines?
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Don't quite understand your question. :confused:
    The resistors stay connected to the transistor bases as shown in your diagram. They don't go to +12.
     
  12. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    Let's see if I could write my question better.

    In the diagram the resistors (let's look at the left-hand side) connect from Logic 1 to their respective transistor bases. If what you're saying is that the logic signals need to go between ground and the supply voltage, then how does the connection of those resistors change? They go from [where] to the base of their respective transistors?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    The resistors connection points don't change. They go from the 0V -12V control signal to the respective transistor bases.
    The only change is interchanging the NPNs and the PNPs.
     
  14. Colin55

    Member

    Aug 27, 2015
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    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
     
  15. CranberryJuice

    Thread Starter New Member

    Dec 22, 2015
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    Okayokay! I'll tell you how it goes.

    @Colin55: I didn't realize you were asking me a question. It got lost in the shuffle. If you're talking about L1, L2, I got about 1V for each.
     
  16. Colin55

    Member

    Aug 27, 2015
    191
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    It's not L1 but I1 - input ONE

    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?

    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?

    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?

    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
    What is the value of the HIGH voltage on the input lines?
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Well Colin55, we certainly can see that you know how to cut and paste. o_O
     
  18. Colin55

    Member

    Aug 27, 2015
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    "Well Colin55, we certainly can see that you know how to cut and paste"

    It's obvious you don't know anything about the initial requirements.
    You haven't even asked the most important questions before suggesting a change to the circuit.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    Well, thanks (not really) for your opinion on the matter but I don't need to know the requirements for a bridge application to know when it is not properly configured to work efficiently as a bridge. :rolleyes:
     
  20. Colin55

    Member

    Aug 27, 2015
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    You are entirely wrong with your suggestion.
    Changing the transistors produces "shoot-through" and you don't know the speed of the incoming signal. Sometimes you have to sacrifice some losses to prevent the bridge blowing up.
    You need to ask a lot more questions before suggesting a solution.
     
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