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Strange problem with transistors flipping current

Discussion in 'General Electronics Discussion' started by TaylorG, Jan 25, 2015.

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  1. TaylorG

    TaylorG

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    0
    Jan 25, 2015
    Using an arduino uno and 4 transistors as shown in the schematic I would like to flip the current direction across a coil of wire depicted by the solenoid here. The current also has to be fairly strong (a few amps). The transistors are all TIP3055's. http://www.onsemi.com/pub_link/Collateral/TIP3055-D.PDF

    The circuit seems to do perform ok up to about one amp. Above about 1.3 amps something changes and i'm not sure about what it is. The current seems to prefer one path to the other. I took some measurements while keeping the DC supply settings constant. Here is what I found: whenever the current flips the current changes between 1.3 and 1.5 amps. At 1.3 amps the supply also measures a much higher voltage.

    When Q1 and Q2 are "on" (current passes through them) and Q3 and Q4 are off, 1.5 amps is measured. When Q3 and Q4 are on and Q1 and Q2 are off, 1.3 amps are measured. When I try to increase the current using the supply Q1 and Q2 allow more current through but Q3 and Q4 still only allow 1.3 amps.

    I measured the collector-emitter voltages of each transistor as the circuit was running and here is what I found:

    On Off

    Q1 .94 1.61

    Q2 .42 2.08

    Q3 .69 2.25

    Q4 1.18 1.59


    I've made sure that each pin on the arduino is supplying the same voltage and I don't think there any inductance effects going on. And the base-emitter/ base-collector voltages remain the same as well. Could this be due to faulty wires or a bad breadboard? Or is there an obvious problem with the circuit? Taylor's current flipper_schem.jpg
     
  2. Rleo6965

    Rleo6965

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    9
    Jan 22, 2012
    You may want to replace your TIP3055 into MTP3055.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I don't see a schematic... Also can you post a photo of your construction.
     
  4. TaylorG

    TaylorG

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    0
    Jan 25, 2015
    Taylor's current flipper_schem.jpg
     
  5. TaylorG

    TaylorG

    4
    0
    Jan 25, 2015
    Sorry about that. I'll try to get a photo of it tomorrow
     
  6. Bluejets

    Bluejets

    3,779
    782
    Oct 5, 2014
    I'd be using relays or something like attached.
     

    Attached Files:

  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    OK, that circuit is called an H-bridge. It's normally drawn like the letter H for clarity - see https://www.google.com/search?q=h-bridge&tbm=isch

    The circuit you have there won't work properly because the two transistors with their collectors connected to the +9V rail are connected in the "common collector" aka "emitter follower" configuration and will not turn ON fully. Their emitters will only pull up to about 0.7V less than the Arduino's output voltage, i.e. about 4.3V if the Arduino is powered from 5V. They will also dissipate a lot more power than the other two transistors.

    You also need to consider the current gain of the transistors. The Arduino can only supply a few milliamps, and a typical (non-Darlington) power transitor has a current gain of about 50. How many amps will the load draw?

    There are various arrangements for H-bridges depending on your requirements. You can get H-bridge drivers, and complete H-bridge circuits including the switching devices, in IC form. Nowadays, MOSFETs are normally used instead of BJTs because they have lower losses and are easier to drive.

    Google H-bridge and read some articles about H-bridge operation and design. Also download some data sheets from Digi-Key's product selector for H-bridge drivers with external switching devices: http://www.digikey.com/product-sear...rs-external-switch/2556427?stock=1&quantity=1

    If you have any questions that aren't answered in the articles you find, please post them here.
     
  8. TaylorG

    TaylorG

    4
    0
    Jan 25, 2015
    Thank you very much.
     
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