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Replacement transistor for c3203?

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I need to replace a transistor on the remote control for an old radio boombox.


Took it apart because the batteries in it rusted out, and had to clean the contacts. Saw that the center leg for the transistor was also rusted through.


It was a kec c3203 npn transistor.

http://oldwww.kec.co.kr/data/databook/p ... TC3203.pdf


I have a bunch of smd mmbt3904 npn transistors (smd versions of 2n3904).



Would I be able to replace the c3203 with the 2n3904?


From the circuit I am repairing, the important part is a simple transistor as a switch circuit.

Vcc -> IR led -> SMD 1R5 (1.5ohm) resistor -> Collector of C3203 transistor -> Emitter of Transistor -> Gnd

And the switch is (assumed)

Vcc -> Remote IC -> SMD 121 (120ohm) resistor -> Base of Transistor -> Emitter -> Gnd.



Assuming a 3v VCC due to using 2 AA, the ic to transistor is 25ma. Not knowing the IR led drop voltage, I can't tell what the current should be :(


(Side note, found http://www.qsl.net/wd9eyb/klunky/framed.html which let me build a simple schematic online. w00t)

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2N3904 is a bit weak for that application. PN2222 would be a reasonable replacement. The pinout is different, so you will have to carefully bend the leads to go to the correct PCB pad.


The Vf of an IR led is typically 1.2 to 1.5 V. Peak current could be close to 1 A in that circuit.


That throws me off too. Assuming a 1.5v vf, online calcs give that 1a rating. But the transistor has a max current of 800ma. The pn2222 is stated as max 500ma. That's probably constant current numbers instead of toggle, but neither look like they would fit... Actually, the 2222 is 1a max. With a similar hfe. Happy days. Now to find one. Radioshack?


I am using BC817 as an alternative to MMBT3904, same package, 500mA current.


See that's the thing. I suck at figuring out transistors, and I thought the hfe was the important part. The bc817 has a minimum hfe of 100 to a max of 600, while the 3905 has a min of 30hfe and a max of 300 at the same current/voltage of 100ma/1v.


Would these two even be used in the same circuit easily?



Oh, and another neat find.


Transistor cross reference search. Nice.

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That throws me off too. Assuming a 1.5v vf, online calcs give that 1a rating. But the transistor has a max current of 800ma.


The IR pulses have a aprox 50% duty cycle (typ) and come in bursts, so the average current will be under 500 mA. A transistor like the PN2222 that has 500 mA continuous / 1 A peak spec is common for this application.


The IR led is typically rated at 1.0 to 1.5 A peak for a short duration. The continuous current is much lower - typ 50 to 100 mA.


Hfe (beta) is important, exactly how important depends on the application. As a general rule, higher is better (but not always). An oscillator or amplifier is a more sensitive application than a switch.

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