There is the Maximum Beta, called the h FE. Is used to indicate "Beta"] is the ratio between the Collector-Emitter current, and the Base-Emitter current. Now, a note or two about something called Beta, and how that figures into the relationship between drive current and Base current. And, let's say we don't mind torturing our poor little Arduino, by forcing it to supply 40mA to the base of that transistor, then, the math would be as follows: So, back to biasing that transistor! Let's say your motor needs a half an amp to get it running. I, personally, don't like demanding more than 20mA from my Arduino outputs So, that would be the ABSOLUTE limit of using this transistor, with an Arduino, as a switch. So, well within the 2N2222A's 300mW power handling capability. That amounts to a power dissipation of: PD = 500mA * 210mV = 105mW So, first of all, I'm going to assume you meant a "2N2222A", which is rated a 625mW MAX, so rule of thumb, half that for, lets say 300mW.įigure 4, of the ON Semi datasheet, shows around 210mV for a Collector current of 500mA, when the base current is at 40mA, the ABSOLUTE MAX current an Arduino output should source. In your case, that will be whatever maximum current that motor will be drawing - and the maximum current will, invariably, be the start-up current. So, to figure out what current to apply on the "input", we need to start with how much current we want to switch. Like I said, before, a bipolar transistor amplifies current. OK, so what is the proper way to bias a bipolar transistor? Depends on how you want to use it, but lets just talk about using it as a "switch". If you try to put 5V across the Base-Emitter junction, you will either fry the transistor, or you will severely tax whatever is trying to supply that 5V - in other words, somebody gonna suffer! In fact, if you try to do that with an Arduino output, there's a good chance you'll burn it out! Thus, it's the current on the input, that makes it "go". The thing to know is that a bipolar transistor is a "Current Amplifier". If you pop it out, and turn it 180°, and plug it back in, it should be facing the right way.Īs for your "5V across" comment, when juxtapositioned with your resistor fiddling, it sounds like you have a misconception about how a Bipolar Transistor is "biased".
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#Pn2222 transistor threshold free
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