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identify and upgrade power transistorson toy rc car

Discussion in 'General Electronics Discussion' started by Jean Perret, Mar 1, 2014.

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  1. Jean Perret

    Jean Perret

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    Jan 3, 2013
    Hi folks,

    I was running some tests switching the standard 9.6V battery packs to lipo 3S , Over the 5cars tested one came to a dead stop after less then 50m..leaving the car without forward motion, electronics smelled burnt. Opened the car and nothing seemed suspect despite the burnt smell. The power transistors are the square flat with 3 legs screwed on a heat sink ( and by the way I'm surprised how fast this forward transistor burnt despite the heatsink!) ,refs are : Two are C3420 031 and two others A1357 031, Could you guys help me identify those transistors and maybe tell me which ones could replace those for higher performance ?

    Appreciate your help!
     
  2. jpanhalt

    jpanhalt

    426
    4
    Nov 12, 2013
    Smelled burnt -- that is not a good sign. That is ran for 50 minutes is in your favor.

    Can you post a picture of the board you want to turbocharge?

    John
     
  3. Jean Perret

    Jean Perret

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    Jan 3, 2013
    Thank you, no not 50 minutes!! Only 50 meters!!! I will try to post a pic
     
  4. Jagtech

    Jagtech

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    Feb 22, 2014
    Those transistors are C3420 = 2SC3420
    A1357 = 2SA1357
    Go to Alldatasheet.com for complete specs on these, and perhaps try to identify a similar one with a higher power (wattage) rating. I'm a little surprised that you fried these, they should be adequate operating at 9.6 V
     
  5. Jean Perret

    Jean Perret

    37
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    Jan 3, 2013
    Thank you! I will check, hope I can understand something.. They didn't fry with the regular 9.6V pack but with a lipo 3S...
     
  6. Jean Perret

    Jean Perret

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    Jan 3, 2013
    I found those infos on Amazon.com for the 2SC3420 transistor
    •Voltage: 50V
    •Current: 5A
    •Power: 10W

    and for the 2SA1357

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

    So Understanding that I was running the truck with a Lipo 3S 1050mah, 20-30C Discharge, what numbers do I have to look at for the limit of those transistors??
     
  7. Jean Perret

    Jean Perret

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    Jan 3, 2013
    sorry it's a 30C Discharge lipo, my bad.
     
  8. jpanhalt

    jpanhalt

    426
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    Nov 12, 2013
    A fully charged 3S should still be within its allowed discharge limit, even if it was only 20C.

    I did a quick parametric search on DigiKey and turned up about half a page of higher current and higher gain prospects.

    John
     
  9. Jean Perret

    Jean Perret

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    Jan 3, 2013
    Sorry, can you please clarify?
     
  10. Jagtech

    Jagtech

    43
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    Feb 22, 2014
    Those semis should be well within the limits using that battery. Are you sure the polarity was correct, or that there wasn't another wiring issue?
     
  11. Jean Perret

    Jean Perret

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    Jan 3, 2013
    No everything was fine as other toy grade cars were use for the same est and running that exact same battery without issues,only this car failed although it was running fine on the 9.6V pack. I am no expert on electricity but as this LIpo is a 30C discharge rate would that not be enough to fry those transistors?
     
  12. davenn

    davenn Moderator

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    1,882
    Sep 5, 2009
    the circuit will only draw what current it requires, regardless of the amp/hour capacity of the battery. The only time it will draw lots more current is if there is a fault in the circuit

    Dave
     
  13. Jean Perret

    Jean Perret

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    Jan 3, 2013
    But then what if the motor had more potential and used more power from that lipo then it could use with the standard 9.6V pack not strong enough to feed all what the motor could take? Then more power would have to go through the circuit board right? And maybe explain why those transistor could not keep up with the motor demand? (and on the run test the car did go faster with the lipo then it did with the 9.6V pack,, at least for 50 meters..)
     
  14. jpanhalt

    jpanhalt

    426
    4
    Nov 12, 2013
    I was about to say what davenn just said, so rather than repeat it, let me add a note of caution. The labeled discharge rate on lipo's is considered the safe discharge rate for that battery. Such batteries are capable of delivering much more current and have been known to self destruct in such cases. You may have heard of "puffed" cells. Be careful of them and don't try to re-charge them. Just discard them appropriately.

    As I recall, those transisptors are rated at 5A continuous, 8A pulse. That is at 9.6V. A fresh 3S lipo will put out something in excess of 12V. Just applying Ohm's law, without considering other factors, 12/9.6 = 1.25. So, if the transistors were at all stressed at 9.6V, they will be over-stressed by 25% at 12V.

    You also have to consider the drive capacity of the circuit for the transistors (e.g., base current). If that is inadequate to turn the transistors completely on with the higher current (transistor gain goes down with increased collector current), then the transistors will create even more heat.

    John
     
  15. Jean Perret

    Jean Perret

    37
    0
    Jan 3, 2013
    If we are comparing apple to apple a 9.6V pack fully charged is around 11.6V so 1V difference with lipo 3S. So if I find more powerful transistors would that do the trick?
     
  16. jpanhalt

    jpanhalt

    426
    4
    Nov 12, 2013
    Your apple to apple comparison hasn't identified what battery chemistry was used in the 9.6V batteries. Internal resistance of the battery, which may be the major current limiting factor, comes into play. Lipo's have very low internal resistance. I have done effectively the same experiment you did when I exchanged a 3S Lipo pack for a 9.6V NiCd pack with a small, brushed airplane motor. The motor sounded like it was going to self-destruct on the Lipo pack. I just kept the NiCd's.

    In terms of next steps, assuming you want to use this Lipo pack, go to your favorite supplier and use its parametric search to find potential substitutes (see post #8).

    John
     
  17. Jean Perret

    Jean Perret

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    Jan 3, 2013
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    look back at post #8 in this thread


    D
     
  19. Jean Perret

    Jean Perret

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    Jan 3, 2013
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
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