power transistors

Discussion in 'The Projects Forum' started by Jean-Karim, Mar 2, 2014.

  1. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    Hi folks,

    I was testing my new lipo 3S 1050mah 30C on few toy grade rc cars running on basic 9.6V packs, one of cars came to a dead stop after only 50 meters smelling burnt electronics, the car lost its forward motion and probably power transistor for forward motion, The actual refs are :

    two 2SC3420 transistor
    •Voltage: 50V
    •Current: 5A
    •Power: 10W

    and for the two 2SA1357

    •Volts: 20V
    •Current: 5A
    •Power: 10W

    So Understanding that I was running the truck with a Lipo 3S 1050mah, 30C Discharge, what numbers do I have to look at for the limit of those transistors?? And what more powerful transistors can I use to replace those ones? :confused:


    Thank you!
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    507
    Depends what Killed them. If I understand you, the change you made was going to a more powerful battery pack? If so, it may have increased the amount of current available to the motors which increased power dissipation in the transistors. I don't know if your drive electronics limits current? I doubt it.

    The most likely things that killed the transistors are:

    1) Too much current (exceeded max rating)

    2) Too much power dissipation, cooked the transistors

    3) Safe operating area violation.

    Without knowing why they died, hard to say what to fix.
     
  3. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    Yes I was trying to see what kind of performance gains I would get using a lipo 3S instead of the standard 9.6V Nicd pack, yes this car ran about 30% faster but only 50 meters.., as rear motion and steering still work and only forward motion died I am highly suspecting the power transistors controlling the forward motion to have died getting to much power but then how much and watt, amps, voltage??
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The transistors may be dead, the driver circuitry may also have died. I could hazard a guess if I saw the schematic.

    Be advised: running extra juice stresses the drivers, transistors and the motors too.
     
  5. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    I don't think that anything else has suffered as the whole circuit looks fine with no trace of burnt oberheating etc.. Only the smell of burnt elements is right on one or two power transistors.

    Do you guys have any refs for the most powerful transistors that could replace those ones ?

    Thanks!
     
  6. #12

    Expert

    Nov 30, 2010
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    What is the voltage of a LIPO 3S?
     
  7. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    About 12.6v fully charged
     
    #12 likes this.
  8. #12

    Expert

    Nov 30, 2010
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    Last edited: Mar 4, 2014
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I don't think it's voltage that killed them, it's the higher current available to the motors increased the power dissipation.
     
  10. #12

    Expert

    Nov 30, 2010
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    I don't think it's the voltage that killed them, either. It's just nice to know what the voltage is, and it was never mentioned before I asked.

    12.6 instead of 9.6 makes a ratio that tells me the current might go up by 31% and the power might go up by 72%.
    Then I go find transistors that are rated for at least 72% more power.
     
    Last edited: Mar 4, 2014
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The power diss problem could be related to the package type the transistor is in and what, if any, heatsinking is provided. may not have anything to do with the electrical specs.
     
  12. #12

    Expert

    Nov 30, 2010
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    In this case, and some IC's, the size of the silicon die inside the package accounts for better heat coupling to the package. The transistors I found are the same package with double the alleged power rating. Same for an LM350 compared to an LM317.
     
  13. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    Thanks guys!

    Comparing apple to apple a 9.6V pack nicd/nimh fully charged is in the 11.6V ballpark compared to 12.6V lipo. But then 30C discharge rate at 1050mah, would that be the numbers to look at?
    Are we dealing with a voltage , watt or amps problem??
     
  14. #12

    Expert

    Nov 30, 2010
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    amps and therefore, watts.
     
  15. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
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    So any idea what transistor could fit my application??
     
  16. #12

    Expert

    Nov 30, 2010
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    Not a clue. I don't know why I posted that matched pair in post #8.
     
  17. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    sorry.... so do you think that transistor in post 8 is the way to go? as I have 4 transistors , two PNP, two NPN Should I replace the 4 of them with 4 of those ones you have mentioned?
     
  18. DickCappels

    Moderator

    Aug 21, 2008
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    You might want to put a small value fixed resistor in series with the motor to limit the maximum current that can be drawn to a current lower than the transistor's peak current rating.
     
  19. Jean-Karim

    Thread Starter New Member

    Dec 27, 2012
    12
    1
    Being a toy grade rc with a brushed 380 motor do you think this motor would draw that much amps?
     
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