Go Down

Topic: A couple of questions on the 2N2222 transistor (Read 6 times) previous topic - next topic

CrossRoads

http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF

Look at the On Characterics on sheet 2.
With Ib of 15mA, the base-emitter voltage will be ~0.6V and the collector-emitter voltage will be ~0.3V.
Use those numbers to work the resister values.

(5V - 0.6V)/15mA = Base resistor
(5V - Vf - 0.3V)/20mA = LED resistor.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

westfw

Quote
The simulator reports 13.5 mA at the LED and 430 ?A at the base for an hFE of 32.

No, no.  hFE is a characteristic of the transistor, not the overall circuit.  You're looking at the current gain of the circuit.
A major point of designing a transistor circuit is that you minimize the effects of variations in the characteristics of the transistors, since they can vary quite a bit.  So Rbase is chosen to limit the current in the base/emitter circuit, and Rled is chosen to limit the current in the LED through the Collector/Emitter circuit, and everything will work just swell as long as hFE is "much larger" than Ic/Ib

oric_dan


Quote
The simulator reports 13.5 mA at the LED and 430 ?A at the base for an hFE of 32.

No, no.  hFE is a characteristic of the transistor, not the overall circuit.  You're looking at the current gain of the circuit.
A major point of designing a transistor circuit is that you minimize the effects of variations in the characteristics of the transistors, since they can vary quite a bit.  So Rbase is chosen to limit the current in the base/emitter circuit, and Rled is chosen to limit the current in the LED through the Collector/Emitter circuit, and everything will work just swell as long as hFE is "much larger" than Ic/Ib

Excellent description of the situation.

kf2qd


I already did  XD
This is from an upstairs window - can't really tell, but the undrifted snow is 20" deep, and the pile from the city plows at the end of the drive is 4 feet and 8-10' wide.


Gee - I remember those days... And then I moved to South Texas... Been a little chilly down here in the mornings of late. Temps around 70....

CrossRoads

Temps in the 70s... but it reaches the 'teens' in the summer, right?
115, 117?
Little warm for me 8)
Can always dress a  little warmer in the winter, but in the summer, its hard to dress cooler once you're naked!
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Go Up