Equivalent transistor for 2SC3420

[jean-karim]

Hi folks,

I need to replace a burnt 2SC3420 transistor from a 1/10 Eztec RC Toy car that runs on a 9.6V/1000mah nicd/nimh battery pack, brushed 380 motor for some kind of reason the prices of those specific transistors are insane ( for what it is) so looking for a replacement part I came across a forum where someone was saying that you could safely use the transistor BD139 to replace the 2SC3420 in this application.
What do you guys think?

Thanks!

 

[Audioguru]

The battery current is not its maximum current. It is how much current it can produce for one hour (1A). It can probably produce 10A for a few minutes. Check the datasheet for a 380 motor at 9.6V to see its stalled current.
If you compare the Japanese transistor with a BD139 then you will see that they have the same case but have completely different ratings. Look at the datasheets:

The BD139 works poorly above a few hundred mA and its max continuous current is only 1.5A. The Japanese transistor works well above 4A and its max continuous current is 5A.

 

[jean-karim]

If we consider the Mabuchi RS380SH , The nominal voltage is 6V but a 24A stall current?? Then it would fry the vast majority of those transistors..?! Not sure I got this right..

 

[Audioguru]

A DC electric motor is stalled when it begins being powered and when something stops it when it is still powered.
The transistor saturation voltage loss reduces the voltage to the motor that reduces its current. Wiring has resistance that also reduces the current.

 

[jean-karim]

OK so ballpark, I am looking at a ratio 20 (stall motor)/5 (transistor) A? I found this transistor to replace that 2SC3420, The 2SD826, the numbers are quite close to the original, should be a correct fit .

 

[Audioguru]

I doubt that a 5A transistor will survive 20A. In addition to the transistor and wiring reducing the current, the connectors and battery also reduce the current.

 

[jean-karim]

It's probably not this exact motor on the car as the original transistors are those 2SC3420 so also same 5A, could be the RS380PH or other with lower stall AMPS.

 

[Audioguru]

Were you simply guessing the voltage and current ratings for the motor? Maybe it is a 12V motor that uses much less current at your lower battery voltage.