Tip31 as a current limiter

Discussion in 'General Electronics Chat' started by smokie, Jun 9, 2009.

  1. smokie

    Thread Starter Member

    Apr 15, 2009
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    That is not practical. The transistors act as on/off current switches. With a 12 volt source, you will need a 14 ohm resistor (13.63 exactly) to limit the current. The dissipation in the resistor will be just over 10 watts, so a larger resistor dissipation will be necessary.
     
  3. smokie

    Thread Starter Member

    Apr 15, 2009
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    Tip31 can also be used as an amplifier. I ran a test circuit with a 10 ohm, 10 watt resistor. Initially I was getting 1.2 amps collector current. I was then able to bring it down to .8 amps by reducing the emitter/base current, i.e., larger base resistor. Just don't know if there are glitches if applied to a stepper motor.

    Would like to avoid power resistors.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    You might get lucky and come up with the right limiting resistor in the base circuit. The TIP31's each then become a current limiting resistor and will dissipate about 10 watts, requiring heat sinks.

    A smaller supply voltage will minimize the losses caused by either active or passive current limiting.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Why not use common collector mode instead, the power out the emitter will be 1.2V less than the voltage on the base, which will be relatively low current drive into the transistors. The transistors will then take the heat (you WILL need good heatsinking).

    If you want to feed a constant current into the stepping motor you can put a low value resistor on the emitter and limit the base voltage, which will reduce the wattage the resistor will dissipate. The constant current will be out the collector.

    How familiar are you with basic electronics?
     
  6. smokie

    Thread Starter Member

    Apr 15, 2009
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    Thanks to you both.

    I'm no expert, just a hobby. Been dillydalling in electronics for many years. Can design a logic circuit.
    With the stepper driver I want to keep it simple. Since I'll need a 5v supply anyway will drive the base from 5 volts.
    Don't understand why I need a resistor on the emitter if I try different resistors on the base to get the .8 amps thru the motor windings. Will heat sink the Tip31.

    beenthere..... 8458 posts ... wow!
     
    Last edited: Jun 9, 2009
  7. Wendy

    Moderator

    Mar 24, 2008
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    Yeah, but I'm catching up with him... ;)
     
  8. AlainB

    Active Member

    Apr 12, 2009
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    Hi,

    I never saw any litterature about the way you want to do it. I suppose that if it was practical to do it that way it would be known. But then again, maybe it is possible!:)


    I am not an expert and the only ways I know how to use an higher voltage is to use a PWM chopper circuits or some ballast resistors.

    My way of calculating the values of the ballast resistors differ from the calculations of beenthere.

    If you want to run your motors only on the full step mode ( that is always 2 phases energized at the same time) a single resistor nearest value from 3,636 Ohms, and exceding 11,26 watts should be connected between the positive of the power supply and the red and blue wires from the motor, joined together.

    If you want to run the motor full step or half step or wave drive at your choice then 2 resistors are better. One on the blue wire and one on the red wire, not joined together. The nearest value from 7,273 Ohms 5,63 watts would be needed.

    Here are my calculations. Please correct me if someone think they are wrong. I am not absolutely sure about the voltage drop of the transistor:

    Power supply tension: 12,00 volts
    Minus tension needed: 4,40 volts
    Minus voltage drop from transistor: 1,20 volts

    This leave 6,40 volts to be used for the calculation of the value of the ballast resistor.

    So 6,40 volts divided by 0,88 A (the current of one phase) equal 7,273 Ohms and 0,88 A multiply by 6,40 volts gives 5,63 watts.

    This was for 2 resistors. For a single one you need to add up the current of 2 phases.

    Alain

     
    Last edited: Jun 9, 2009
  9. Audioguru

    New Member

    Dec 20, 2007
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    Transistors have a wide range of current gain. Some need a low base current for a high collector current and others need a higher base current.

    Transistors conduct more current as they heat up which needs additional parts and feedback to fix.
     
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