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Topic: TIP31A Transistor Help (Read 1 time) previous topic - next topic


If you ever want to drive high voltage and high amps see about getting a few of these as a sample.

I've used them and they're pretty damned tough. It's a low side driver like an NPN transistor but it is designed for logic levels and it's more efficient due to the very low on resistance.
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I hadn't come across those before. It looks to be a nice device, although not suited to PWM applications because of the slow switching speed.
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base wired to suitable series resistor (200-1,000 ohms), other side of resistor wired to arduino output pin


Sorry I don't mean to hijack your thread, but this question seems relevant to the topic;

How do you choose a resistor value for this application? I have been using N-channel MOSFET for low-side drivers. Example here:


But they are kind of expensive. I have lots of cheap 2N2222 transistors that I could use for lower current projects, but I'm not sure how to choose the resistor on the base.


but I'm not sure how to choose the resistor on the base.

Choose it such that the base current is at least the collector current divided by the transistor's gain. Then give it a bit of margin by halving it.


How do you choose a resistor value for this application?

technically you can calculate the base resistor value required by determining the load current requirement and then dividing that by the current gain of the transistor, it's beta value, and then size the base resistor to that amount of base current value or more. It's easier to just use a resistor that limits the base current to somewhat less then the maximum arduino output pin current rating (20-30ma) or the maximum base current rating for the transistor being used (whichever is less), that will insure the transistor is turned on to maximum saturation, with minimum voltage drop from emitter to collector.


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