Problem with transistors(overheating rapidly)

Discussion in 'The Projects Forum' started by haelstone, Jan 23, 2010.

  1. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    Hey all,

    please if anyone could help me, im having problems with my transistors-2N4401(for driving the stepper motor)when i solder the coils to the emitter pins and switch on the 12v dc current it over heats rapidly.my stepper motor is a 6v..first it was coil 1 which over heated, so as a test changed the coils then transistor 3 heated up rapidly...would appreciate any help i'll add a schematic for better understanding:)

    http://www.filedropper.com/4_74 << schematic(i tried "Go advanced" method, but did not work)
     
    Last edited: Jan 23, 2010
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The full schematic would be helpful. Yours shows the emitters already connected to circuit ground. Circuit values and the specs on the stepper are also needed.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    We don't know the resistance or the current of the motor coils.
    The emitters of the transistors are grounded so maybe you have the transistors connected upside-down (collector and emitter reversed).
    If the transistors are powered from a 12V supply (to the collectors) and are emitter-followers then they will get extremely hot when the base signals are only 5V.

    Please attach schematics here (use the Go Advanced button), instead of over at FileDropper.
     
  4. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    specs of stepper are 1.8 degrees 6V im having some trouble atm figuring out the resistance of the wires of the stepper,my multimeter just died on me today, though i found out the 2 common center taps, 6 wires,iv added a 1k resistor to the base of the npn,which other circuit values do you need?
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    You will have to be more forthcoming by providing the stepper motor part number. We need more info to help you out.

    hgmjr
     
  6. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    Servo stepping motor, KP6M2-001 6V 1.8degree, japan servo NO.AE22391, thats all the info i found on the stepper motor
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    Yes, that is the place i ordered it from, they did not provide any data sheet :(
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Not a single word about the current it uses.
    Measure the resistance of its coils and post it here.

    Are the coils connected to the collectors of the transistors or to the emitters?
    Is the other wire of the coils connected to +12v or to ground?
    What is the value of the base resistors?
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    I saw one reference to the motor that indicated the coil resistance was 0.7 ohms. If this is accurate then it is easy to understand why your transistors are getting :eek:hot:eek:.

    hgmjr
     
  11. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    Resistance of the coils will take some time because my multimeter isnt working properly.

    The coils are connected to the collectors of the transistors
    The common wires of the stepper motor are connected to the pin 1 of the 5 volt regulator(7805)so + side
    the value of the resistors are 1K
     
  12. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    hmm.. any easy way to rectify it then?
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    You will need to look into using suitable power MOSFETs in place of BJTs to drive your motor.

    hgmjr
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    If the resistance of the coils are 0.7 Ohms, your coil current will be I=E/R, or 5v/0.7 Ohms = 7.1428 Amperes! :eek: No wonder your poor 2N4401 transistors are getting toasty; they can only sink a small fraction of that current.

    You will need to operate the motor using a lower voltage, and higher rated transistors.

    I'll make a guess that about 2A current would be appropriate.
    So, 2A * 0.7 Ohms = 1.4v for the supply. You won't be able to use a linear regulator to get that voltage and current; it would simply burn up.

    If you had a spare ATX form factor PC power supply, you could convert it to a bench supply. Lots of info on converting these available on the Web; Google "ATX bench supply"

    Then you could use the 3.3v output and a current limiting resistor.
    3.3v - 1.4v = 1.9v
    1.9v / 2A = 0.95 Ohms. 1 Ohm is close enough. Now you need to calculate the power dissipation in the resistor.
    P = E*I, so 1.9*2 = 3.8. Double it for reliability; 7.6 Watts. They don't make 7.6W resistors, but they make 10W resistors.

    Oddly enough, Radio Shack carries 1 Ohm 10W power resistors:
    http://www.radioshack.com/product/i...0&filterName=Type&filterValue=Power+resistors

    That would work for your current limiter.

    If you're converting an ATX power supply, you'll also need load resistor(s) on your 5v supply. You could use a couple of these:
    http://www.radioshack.com/product/i...1&filterName=Type&filterValue=Power+resistors
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    2N4401s are called "small signal transistors" for a reason. You need "Power transistors", mounted on a large heatsink!
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Pin 1 of the 7805 regulator is 12V so the 0.7 ohm coils are trying to draw 17A each!
     
  17. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    Hey thank you for the reply

    but im kind of new in electronics, can you elaborate a bit more..im really sorry,right now im using a 12v(DC) for the supply..basically i need to reduce the volt to coils to 1.4v?and the resistors wont apply?or just replace the resistors and transistors with an 10ohm resistors,I want to fix it the easy way since i have no knowledge of ATX power supply.
     
    Last edited: Jan 23, 2010
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Your multimeter: You probably were trying to measure current, but it was too high and blew the internal fuse. Open it up, and replace the fuse with the same type and current rating, probably 250mA.

    If your stepper motors really are rated for 6v and have a coil resistance of 0.7 Ohms, that's nearly 8.6A current.
    6v/0.7 Ohms = 8.5714...Amperes. You will need a very capable power supply.

    What are you using for a 12v supply?

    In your case, trying to drop 12v to 1.4v at 2A current:
    R=(12v-1.4v)/2A = 10.6/2 = 5.3 Ohms; I'll round that to 5 Ohms.
    I = E/R, so 12v / 5+0.7 = 2.1A
    Now recalculate voltage across the resistor:
    E=I*R, so 2.1*5 = 10.5v
    Now calculate resistor power:
    P=E*I, so 10.5v * 2.1A = 22 Watts, double for reliability = 44 Watts.
    You'll need a 50W resistor, and most of the power will be wasted in the resistor instead of being used in the motor.
     
  19. haelstone

    Thread Starter New Member

    Jan 23, 2010
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    I am using an AC/DC adapter for the power supply, therefore giving out 12VDC..and i need to add the 50 Watt resistor to the base of the 2N4401 transistors or to the coils of the stepper??:confused::confused::confused:
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    Don't you understand the datasheet for the little 2N4401 transistors?
    Their absolute max allowed current is only 0.6A but your motor uses 8.6A at 6V.
    Your little transistors need a base current that is much too high to be fed from a micro-controller. You should use logic-level Mosfets instead of the little 2N4401 transistors.

    Don't use a weak little 12V AC/DC adapter. Instead use a 6V power supply that has enough power.
     
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