Which transistor for ne555 output?

I have a simple circuit based on an ne555, the power supply is 15 volt provided by a robust ac adapter. On the pin 3 of the ne555 i need a max current of 4000mA. I was thinking to use a BD439 transistor, would be ok? And about the base resistor? Would be ok a 100k resistor or i need a different value?

thanks a lot.

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rickharris2 years ago

That's 4 amps. Will the BD439 pass that much current?

well the data sheet says yes BUT it only has a current gain of 140 so the input (base current) will need to be 4000/140 mA, Collector emitter voltage needs to be 60 volts.

You can work out the resistor using ohms law.

akopelumenuscu (author)  rickharris2 years ago

Hi rickharris, thank you for the reply.

However I'm not very skilled/expert, so I have no idea about the proper value of the base resistor. I can use, instead, a BD437 which should be more robust. If i attempt to calculate the base resistor here

http://www.petervis.com/GCSE_Design_and_Technology... i got a value of 180Kohm, but I'm not very sure of the result.

I aim to get the more current/boost possible from the output.

explain a little more about what your trying to do, in view of your electronics skills this may not be the only way forward.

akopelumenuscu (author)  rickharris2 years ago

I aim to amplify a triangle wave shape, to feed an oscillator: if I feed the oscillator directly from the NE555 output, the triangle wave disappear (from the NE555, I can only get about 200mA, if i recall correctly); so I was thinking to boost the NE555's output using a transistor on the output.

Many thanks.

OK but why 4000mA that's 4 AMPS! are you intending to drive something with the triangle waveform?

Are you going to get 4 amps from your supply?

If so then


shows lots of options for driver circuits from a 555 timer. You may need to experiment a little to get the best results. Running the BD439 at 4 amps is at the limit and it WILL get hot. You will need a heat sink.

Good luck.

akopelumenuscu (author)  rickharris2 years ago


As I've said in my first post, I feed the circuit (the ramp/wave shape generator already exists) using a robust ac adapter/power supply: I have defined it "robust" because is able to provide 10 amps :)

I just need an help to determine the base resistor for the transistor, since the circuit is already functional and the power supply is able to provide me a lot of amps. Obviously I will not bring the transistor to the limit.

Nominally between 470 ohms and 1000 ohms then - You will still need to experiment and monitor performance be prepared to blow a few transistors.

4 amps IS it's absolute max and it will get very hot This is especially true if the transistor isn't turned fully on i.e. in saturation.

akopelumenuscu (author)  rickharris2 years ago

Thank you.

Anyway, let me know if I properly understand: if the value of base resistor is high, then I will got an higher signal from the transistor?

I don't want - obviously - damage the transistor; considering the fact that the BD439 is able to provide 4 Amps max, would be fine achieve around 3 amps.

If I try again to calculate these values here (previously I did wrong calculation):


I obtain a value of about 100 ohm (RL=5 Vcc=15 Vi=13)
Would be realistic?


explains how the transistor works.

The value can be very flexible. Essentially your using the gain of the transistor to estimate the base current. The collector current / by the gain gives the base current.

Using ohms law knowing the voltage at the base (approx 15 volts if your driving the 555 with 15 volts) and the base current you can calculate the base resistor to give that current.

Ohms law says Resistance= Voltage / Current.

Resistors have a wide tolerance and fairly large changes will not make a lot of difference to the operation of the transistor.

In essence your trying to drive the transistor full on and full off. With an NPN transistor driving the base to 0 volts will turn it off. driving it positive turns it on (like a switch). To make sure it is fully turned on it is common to set the base current to 2 or 3 times the calculated figure.

3000 mA with a gain of 140 = 2.14 mA base current - x 3=6.4 mA

So R=15/6.4 mA = roughly 2343 ohms.