H-Bridge Schematic Correct?

Discussion in 'General Electronics Chat' started by tracecom, Nov 15, 2011.

  1. tracecom

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    Attached is an H-bridge schematic that I snagged from the web and redrew. I plan to prototype it, but before I do, I would be grateful for someone to check it and point out any errors. Thanks in advance.

     
    Last edited: Nov 21, 2011
  2. praondevou

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    Are you going to drive a motor with that? At what frequency are you planning to use it?

    I would prefer the UF4001 over the 1N4001, they are much faster.

    I would prefer MOSFETs anyway....:rolleyes:
     
  3. SgtWookie

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    Agree with praondevou on the UF4001's or Schottky diodes like 1N5817 thru 1N5822

    If you are planning on driving this bridge with logic level inputs, your bridge Vcc can't be higher than your logic levels, or you will not be able to turn off Q2 or Q4.

    You don't mention what your expected load current is.
    However, assuming your inputs are 5v logic level, and the bridge Vcc is the same, then your maximum base current will be ~(5v-0.8v) / 1k = 4.2/1000 = 4.2mA; for 42mA collector current.

    In order to keep the transistors in saturation, you need to supply the bases with 1/10th the desired collector current. That can be a bit tricky to do.

    You really need to give more information about what your intended load is, what your Vcc will be, and what you are going to be using for controlling the bridge.
     
    Last edited: Nov 19, 2011
  4. tracecom

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    I have some BAT85 Schottky diodes; would they be a better choice?

    I plan to drive the bridge with μC output (5V.)

    I have never used an H-bridge and am building one just for the learning experience. I planned to scavenge a little DC motor somewhere and use that as the load. I also have never worked with a bipolar stepper and planned to try that as well.

    I don't have a practical application in mind; I had the parts on hand and thought I would build it up and see what it does. I plan to control it with a PICAXE of some sort. Each PICAXE output can source up to 20 mA.

    Thanks.
     
  5. tracecom

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    So, to stay within the PICAXE source 20 mA current capacity, I should make the resistors 220Ω (19mA) which would mean a collector current of 190mA?
     
    Last edited: Nov 20, 2011
  6. SgtWookie

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    Those will be fine as long as the average current through them won't exceed 200mA.

    OK, then your bridge VCC needs to be 5v as well for this circuit.

    That's all well and good. :)

    OK, you could work with a small bipolar with this circuit.

    You can also operate some small unipolar motors as bipolars if they have 6 leads. You just don't use the center taps.

    If you have an old 3-1/2" floppy drive kicking around, they have a really small bipolar stepper motor that drives the head carriage.

    Yes, that's right. Keep in mind the package current limit as well. See the electrical specifications in the datasheet; it varies with the uC.
     
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  7. SgtWookie

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    That will work.
     
  8. tracecom

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    In an effort to understand Hfe, I built the attached circuit and made the following measurements: base current is .14 mA and collector current is 8.9 mA, which would indicate an Hfe of 63.6 (I think). From previous posts, I inferred that the PN2222A Hfe would be about 10. What am I missing?

    Thanks.
     
  9. praondevou

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    From the datasheet it looks like the minimum Hfe for 0.1mA is 30. A maximum is not specified for this base current.
     
  10. tracecom

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    Actually, it's the base current that is .14 mA; the collector current (Ic) is 8.9 mA, which according to the datasheet gives an Hfe minimum of 75 at 10 V.
     
  11. praondevou

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    Yes, right. Then, since you have only 8.9mA instead of 10mA the Hfe is also lower. At 1mA Ic it's 50. So it should be ok...
     
  12. Audioguru

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    It is not Hfe. Instead the DC current gain is called hFE and hfe is the AC current gain.

    hFE is used when the transistor is a linear amplifier that has plenty of collector to emitter voltage (10V on the datasheet), not as a saturated switch. You should look at the "maximum saturation voltage" spec on the datasheet which says that the base current should be 1/10th the collector current.

    Since your transistor had a collector current that is 63 times its base current then either it is poorly saturated or you are very lucky that it has very high current gain.
     
  13. Wendy

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    Gain is not fixed in BJTs. If you are going into saturation on a BJT use 1/10 the current that is flowing through the CE on the BE. In other words, if you are lighting a LED with 20ma you need 2ma base current.

    Opps, saw AG's response after I wrote the above.
     
  14. praondevou

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    Oops, yeah what a difference these letters make...:rolleyes:
     
  15. tracecom

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    Thanks.

    I woke up this morning with the idea that I could control the current through the LED by means of adjusting the base resistor and thus no longer need the LED current limiting resistor. After removing R2 (see the previous schematic,) I tried it and the results are in the attached table.

    It seems to work, but I guess there must be reasons why it's not good practice. What are they?

    In addition, I noted that there is a very definite, consistent relationship between the base current and the collector current. If it's not hFE, what is it called?

    And finally, I am not really clear on when a transistor goes into saturation. I know that it is when is fully on, but when is that? In the attached table, is the transistor ever "saturated?" From your previous post, because Ic is always (much) more than 10 times Ib, is the transistor always saturated?

    Thanks.
     
    Last edited: Nov 19, 2011
  16. Audioguru

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    1) Transistors have a RANGE of current gain. Some have high gain but others have low gain even with the same part number.
    2) The gain of a transistor changes with temperature changes.

    A transistor is saturated when its collector to emitter voltage is very low and adding more base current does not reduce it. It is saturated when its base-collector becomes forward biased.
    Your chart does not show the collector to emitter voltage so we don't know if it is saturated.
     
    Last edited: Nov 19, 2011
  17. tracecom

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    I measured the collector to emitter voltage (with R2 omitted) at R1 values of 1m to 100k and it ranged from from 9.14 to 8.85V, which told me the transistor was not fully turned on. Correct? Which means I was using the transistor as an amplifier and not a switch? Which means that the ratio of Ic to Ib that I previously observed was actually hFE?

    I couldn't further reduce R1 without overdriving the LED, so I put R2 (1k) back in the circuit and measured the collector to emitter voltage as follows.

    R1 1k, .02V
    R1 10k, .05V
    R1 51k, .09V
    R1 68k, .10V
    R1 82k, .11V
    R1 100k, .12V
    R1 220k, .54V
    R1 330k, 3.24V
    R1 470k, 4.99V
    R1 1m, 7.22V

    So, I conclude that the transistor is (essentially) saturated with a base resistor of 100k, which produces a base current of .114mA. Correct?
     
    Last edited: Nov 19, 2011
  18. SgtWookie

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    What kind of load did you have on the collector?

    Was it a constant current type of load?

    If not, how do you know what the actual collector current is vs the Vce?
     
  19. Audioguru

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    Which means I was using the transistor as an amplifier and not a switch? Which means that the ratio of Ic to Ib that I previously observed was actually hFE?[/quote]
    No.
    The LED was connected from +12V to the collector.
    Without R2, the voltage across the LED was 12V - 9.14V= 2.86V so the LED was holding 9.14V across the transistor.

    In your LED test circuit, your transistor saturates fairly well with a 100k base resistor. But a weaker (but passing) 2N2222A transistor needs a 12k base resistor.
     
  20. tracecom

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    The schematic is as shown in post #8.
     
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