How do you find how much current an NPN transistor can safely draw through it?
Answer:
The manufacturer will specify a maximum collector current (Ic) for the device on the datasheet.
**However**, the max current for your particular circuit could well depend upon the voltage drop that your circuit will generate across the transistor, not just the current (the transistor will have a dissipation max specified in milliwatts or other power unit).
Whether or not you heat-sink the transistor, and how well you do it, can make a HUGE difference in what the part can tolerate.
All of this business has to do with not overheating the junctions and cooking the poor thing.
As an example: a 2N2222A has a max collector current rating of 0.6A. It also has a max Vce rating of 40V. While both values are nice, you can't have *both* at the max (or even close!) ... the part is only rated for a dissipation of 500mW.
You should *really* get hold of the datasheet on the transistor in question and work from there. If you'll add the transistor number and case style to your question, perhaps one of us will come back with an edit that will give you better detail.
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If with the word safe you mean that it should not risk damaging the NPN transistor, then just check the manufacturers datasheet and the power rating will be described therein. The current will be in there too.
This would be a fun experiment in the Physics Lab. Increase the current through the transistors until they burn up! Ha, ha. I think I'm going to go do this right now!
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