So I have been researching on how a bipolar NPN transistor work. Here are my understandings:
- In order to turn on a transistor, you need to apply a voltage (from base to emitter) to overcome the depletion region, that voltage is usually 0.7Vdc
- Once the transistor is "on", as the base current increases, the current flowing from the collector to emitter will also increase proportionally by a hfe constant
Armed with this knowledge, what I should expect to see in a data sheet should be the following:
- The minimum current/voltage to turn the transistor on (voltage needed to overcome the barrier)
- The maximum current allowed at the base to avoid damaging the transistor
How do I navigate the data sheet to obtain this data?
Finally, there is a DC current gain chart in the data sheet, here are my questions
- Why they used the collector current instead of the base current? shouldn't this makes more sense to use the base current? because you would think "if I have x amount of current in the base, I will get x amount of current from collector to emitter?
- What if your Vce is larger than 1.0Vdc? say 5Vdc? what would the gain be then?
Circuit I am trying to solve
As you can see from the diagram, the relay will only turn on is if there is 5Vdc and 71.4mA across it. Knowing this fact, how do I determine the current I need at the to achieve this?