Re: Transistor Issue

by kculm on Sun Feb 24, 2013 10:58 am

Sorry to be a pain, but can you help me with one more thing.


What math do I use to figure out the resister size for the base pin on the transistor?

Thanks
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Re: Transistor Issue

by adafruit_support_mike on Sun Feb 24, 2013 12:33 pm

kculm wrote:What math do I use to figure out the resister size for the base pin on the transistor?


The short version is, "use a 1k, then adjust upwards".

For the longer version, it's easier to think of it in terms of current.. for every 100mA of current that goes through the collector, about 1mA of current needs to flow through the base. For simple 'turn it on/turn it off' applications, it's okay to provide more base current just to be sure.

The voltage between the base and emitter will always be in the 0.6v to 0.75v range, and we can safely round that up to 1v. IIRC, you were using a 5v supply, so the voltage across the resistor will be about 4v. To get 4v from 1mA of current, you'd need a 4000 ohm resistor.

Like I said though, when you use a transistor as a switch, it's okay to give the base more current than it actually needs. A 1k resistor will provide enough base current to drive about 400mA through the collector, but your load probably won't use that much. The excess will be 'waste current' that doesn't do anything useful, but doesn't hurt anything but your circuit efficiency calculations.

Like I said at the beginning, 1k is a good first guess. It will make the circuit work, and any waste current will be small enough to ignore. If you end up caring about efficiency, you can swap in larger resistors until the transistor doesn't give you enough current to drive the load. The "1mA of base current per 100mA of collector current" will give you a rule of thumb for how much you actually need.
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Re: Transistor Issue

by kculm on Sun Feb 24, 2013 1:03 pm

Wow mstone, Thanks for the detailed reply, very help full.
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Re: Transistor Issue

by adafruit_support_mike on Mon Feb 25, 2013 12:07 am

kculm wrote:Wow mstone, Thanks for the detailed reply, very help full.

* chuckle * Stick around and you'll learn I'm a compulsive explainer with a thing for BJTs. ;-)

Glad to hear the description helped though. May you have much good blinky.
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