Fix an rc toy car mainboard power transistors

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jean-karim

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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!
 

Those would be 2SC3420 and 2SA1357 which without looking them up are probably NPN and PNP comlplimentary types.
I very much doubt you would get "higher performance" by changing them from the ones the manufacturer selected. Transistors are not graded by performance but by their maximum ratings. The remainder of the circuit decides how close to the ratings they are used and putting ones with higher ratings in will not make them do more work. It would be like saying "If I buy bigger pistons for my car engine it will go faster" when in reality they wouldn't fit in the engine block at all.

Brian.
 

Why are there multiple threads about this toy car?
In another thread he was told that more powerful output transistors need the original drivers for them also replaced to produce more power. Now he wants to replace the transistors with Mosfets without making the circuit be able to use them.
 

Actually, I am not trying to make more power as stated at the beginning, I am trying to find transistors able to handle the use of a a lipo 3S instead of the 9.6V pack. Again I am a NOOB please understand. Again if the actual circuit board of a toy grade car can't get any upgrade power transistors or mosfet and I have to simply replace the bunt ones by the exact same ones and keep using the 9.6V pack only, please be clear and let me know.
 

Back to basics then. The circuit is designed to work with the original power source. A battery with higher capacity means it can hold it's voltage stable under a heavier load, it doesn't mean it WILL provide more power, only that it is capable of it. The circuit decides how much power it draws and it doesn't depend on the ratings of the parts as long as they are adequate for the job they are asked to do. There is no magic 'part' that will make it more efficient or more reliable, that depends on the design as a whole so there is no point in looking for upgraded replacement components.

I find it hard to believe that power transistors on a heat sink would burn out in normal use when the battery is changed. I suspect something else went wrong and it would be more useful to investigate the reason for failure than trying to make it more robust.

Brian.
 

A lipo 3S is 4.2V per cell when fully charged which is a total of 12.6V and can provide that voltage when heavily loaded. If the motor has a max current of 5A with a 9.6V Ni-Cad battery then its power to the motor is 48W.
The current is 6.56A with the Lipo battery and the power to the motor is 82.7W which is MUCH MORE POWER so of course the original transistors failed.

You can completely re-design the circuit to use Mosfets or you can replace the burnt parts with the same parts and use the original battery.
My Electric radio-controlled airplanes use powerful lipo batteries. Their circuit is smart and shuts off the motor when its current becomes too high. Simply reducing the throttle to zero resets the shutoff then the motor runs again when the throttle is increased. Some pilots put a 3S battery (12.6V fully charged) in their airplane designed for only 2S (8.4V fully charged). Then the circuit and battery survive and the airplane flies like a rocket.
 
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Appreciate your help guys! 'm not planning to go crazy about this project as I am simply not competent for it.. Just trying to find if there's an easy way to run safely this truck on lipo 3S changing the actual power transistors for stronger ones ,if it wont work and needs much more work, then I will simply change them with the same ones and only run the 9.6Vpack.
 

By the way would a Darlington pair work in this application replacing a PNP/NPN pair as I have two pairs of those for power transistors on this board?
 

jean-karim said:
By the way would a Darlington pair work in this application replacing a PNP/NPN pair as I have two pairs of those for power transistors on this board?
Click to expand...

they would - I looked and looked but it seems very difficult to find a transistor that would handle the increased current, have sufficient hFE and has the same/similar case.

But I was thinking the darlington's would likely have increased voltage drop, so it is unlikely there would be any significant increased speed.
 

Since darlington transistors have a much higher voltage loss than the original transistors then they will overheat.
 

OK then back to basics and simple transistors.. I will try to find one at least a step above in Voltage and Amps from what is now on the truck , TIP, others, what do you think??
 

your main concerns are the package, hFE, amps, and watts

i looked and looked but came up empty, but maybe you have will better luck
 

If I look on this page **broken link removed** it seems that some transistors could handle the job no? High voltage and amps at least? Like one showing NPN SI POWER 140V 40A TO-218
 

jean-karim said:
If I look on this page **broken link removed** it seems that some transistors could handle the job no? High voltage and amps at least? Like one showing NPN SI POWER 140V 40A TO-218
Click to expand...

hFE is low
 

The original 2SC3420 made by Toshiba is in an old TO-126 case and has a maximum allowed current of 5A. Its minimum hFE at 4A is 70.
Don't you want a replacement transistor in the same case as the original so it fits? But not from Sayal.

I do not know an electronic parts distributor that has lots of transistors to select from that are in that old case.
 

Is hfe related to the total amount of power such as High amps , means if you have a transistor rated at let's say 100V and 20amps but low hfe is it still a higher hfe then another transistor with a lower rating of 50V and 10 amps but higher hfe??? Or any hfe number can be compared no matter the other characteristics of the transistor??

- - - Updated - - -

oh boy.. here's something else.. what is this transistor?? http://www.sayal.com/STORE/ActionIndexP.asp?ID=233590
 

hFE is DC current gain. The original Japanese transistor had an hFE of 70 at 4A so its input is 4A/70= 57mA. If you use a replacement transistor with less hFE then whatever drives it might burn out.

Your newest part from Sayal is an SCR (Silicon-Controlled-Rectifier) that is not a transistor.
 

And what about those toy rc using 2 or even 3 relay switches instead of transistors?? Like this one **broken link removed**, can you put more power through those?
 

jean-karim said:
And what about those toy rc using 2 or even 3 relay switches instead of transistors?? Like this one **broken link removed**, can you put more power through those?
Click to expand...
The relay is on all the time so the car goes full blast or it is off and the car stops.
Doesn't your RC car use Pulse-width-Modulation at a high frequency for speed control? A relay cannot DOO DAT but transistors can.
 

Still on my quest for the correct transistors and fix this toy board, I was given those transistors parts numbers by Digikey tech support , KSE45H and KSE44H , what do you guys think will they be better then the actual ones or worse?..
 

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