How do Transistors handle Amperage?

Hey, I'm working on a wireless power transfer system and I'm planning on running it on 12vdc with a 2n3904 transistor, but my power supply supplies 2 amps. Now the question is, will the transistor draw only the current that my transmitter coils draw, or will it simply draw it all for supplying the coils. I have not yet determined how many turns or what resistance they have yet, I'm still in early stages. I just don't want to ruin one of my few transistors. I know the absolute maximum current this specific transistor can handle is 200ma.

Ok a 2N3904 is a 625 mW transistor.

At 12 volts its max amperage is 50 mA amps not 200 mA.

If you put 12 volts at 200 mA you are applying 2.4 watts to a .625 watt transistor and it will burn out.

With transistors you take the wattage and divide it by the voltage you are going to apply and that gives you the max amperage for your circuit.

To not go over wattage at 200 mA you can only apply 3 volts.

Transistors work in flexible ball parks, to use the 2N3904 in a 12 volt circuit the collector emitter resistance should be 240 ohm, that is the coil reactance + resistor if needed. Then the circuit will only draw the needed current.

One thing you should understand about electronics. Components draw the power. The power supply doesn't push it out. Amperage ratings on Power supplies are the max amount those supplies can safely have draw from them before damage is done.

Now if your coil draws more than the amperage the transister is rated to handle then the transistor will fail.


Devices draw current (at a rated voltage) due to their "nature" (the amount of work the device is doing).

Also, a Bi-junction transistor like the 2n3904 is a current-in / current-out device. The amount of current that flows from C->E depends on the current applied to the Base (assuming an NPN in common emitter setup).

The 2N2222 would give you greater current handling, and are probably just as cheap...

steven48721 year ago

Assuming the transistor is set to run at full on or full off, the current will primarily be determined by the voltage and the resistance of the coil. You would need a lot of copper wire to get enough resistance to limit the current to 200ma. You will also need to know the voltage drop across the transistor and calculate the power dissipated by the transistor at 200ma. It is possible to overheat the transistor even if you limit the current to 200ma.