Power transistors for toy grade rc

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jean-karim

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Hi folks,

I was testing some few rc toy cars using a lipo 3S 1050 mah 30C instead of the regular 9.6V nicd pack and one of the car came to a dead stop after only 50 meters.., no more forward motion smelled burnt electronics, I think that the forward power transistors are fried , here the refs of those transistors if someone could tell me which other more powerful transistors I could use to replace those ones.
Actual one 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, 20-30C Discharge, what numbers do I have to look at for the limit of those transistors??


Thank you!
 

you might look at the TIP series, ie TIP33 and TIP34

**broken link removed**

The TIP33CG ratings are 10A 100V 80W NPN Transistor. That's double the maximum current of the blown transistors, but you might have to 'cherry-pick' for high hfe.
 
Last edited:

The Ni-Cad 8-cell battery was 11.2V when fully charged.
The Li-Po 3-cell battery was 12.6V when fully charged.

Therefore the voltage was 12.5% higher then the current was increased the same so the power was 27% higher.
 

Thanks guys,

So Voltage value is what I should look for my application? What about amps and watt dissipation?

What do you mean by "cherry-pick" for high hfe?
 

I showed you that when you changed the battery to one with a higher voltage then the motor drew more current which blew up the transistors.
With more output current then the transistors need a higher hFE to avoid blowing up their driver transistors.

When you pick cherries off a tree you only pick the best ones.
New power transistors have a range of hFE from low to high. You get whatever they have so you buy many transistors and test them all then pick the ones with high hFE.
 

I just realized that you are in Toronto same as me. Sorry if I sound like a newbie, but that exactly what I am.. Understanding that batteries are a mixed of Voltage, Amps and watts, then what is the limit number I am looking for a transistor? Voltage, amps or watts? As those transistors on the car are rated at 20 or 50V and the lipo battery only just over 12V fully charged why did the transistors blow? Or is it the 5 Amps limit? 10W?

Thanks lot!
 

You said the maximum allowed current for the Japanese transistors is only 5A. The car was designed for a 9.6V Ni-Cad battery but you used a Li-Po battery with a higher voltage. When the voltage is higher then the current is also higher so maybe the current in the transistors was more than 5A when the battery voltage was higher which destroyed them. Use transistors rated for 10A then maybe they will survive but cherry-pick them for a high hFE.

You said the Japanese transistors had a 10W maximum heating rating. When the power in the transistors is increased then their heating is also increased. Then the power rating of the replacement transistors should be more than 10W and larger heatsinks should be used.
 

As audioguru stated your main concerns are Amps and Watts.

First the transistors must be able to handle the higher current[Amps], and then survive the higher power [watts] being dissipated.

hFE is the gain [amplification]

Let's say the 'blown' transistors had a hFE of 100 - to reach a speed that required 5A they only needed 50mA of current

But if you replaced them with transistors that have a hFE of 50 - that same 50mA will supply only 2.5A to the motor. So you could well be using a 'bigger' battery but the car would have a lower top speed
 

If the hFE of the new output transistors is too low then the current in the driver transistors that drive them will be too high then the driver transistors might blow up.
 

Then would the TIP33CG transistors work?
 

jean-karim said:
Then would the TIP33CG transistors work?
Click to expand...
I did not lookup the hFE of the original Japanese transistors. If you use TIP33 transistors as replacements then the hFE must be higher at the higher operating current so that the driver transistors do not burn out.

The TIP33 transistors have a maximum allowed continuous current rating of 10A but then their hFE could be as low as only 4 and they perform poorly. They can dissipate a maximum of 80W with a perfect heatsink. But at 3A their hFE is from 20 to 100. You need some with the highest hFE.
 

any other replacements you guys can think of?

the TIP series are very common [and cheap], but may not be the best - I mentioned them as a starting point

If the OP is in Toronto, Canada, then that should open up many more alternatives

To the OP, many companies making transistors have a selection guide to help select a transistor model.
 

Then what about this one ? **broken link removed**,

seems the hfe is pretty high no? What would be its twin as a PNP??
 

that is a Darlington, so there would be a greater voltage drop {1.4V or so, rather than 0.7v), so you would not get the same power as with a single transistor
 

Do you mean because of the low 3 amps collector this transistor would also fry in this application?
 

Since your 5A transistors failed then this 3A transistor will fail sooner.
It is too bad that your circuit is too old to use modern Mosfets.
 

Then why don't I use some power mosfet like this?

**broken link removed**
 

We don't know what drives the output transistor. A Mosfet needs an input voltage that is much higher than an ordinary transistor.
Don't you also need a matching P-channel Mosfet?
 

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