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Ok, only one more question about transistors, I promise!!

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Ok, now that I've got the hang of what they're for, and things like DC gain and such, how would you design a circuit and figure out the specs for your transistor? For instance take this circuit for example:

How would I go about figureing out values for Q1? Would I use Ohm's law, Kirchoffs law, I'm really clueless... I'm sure I'd have to use Ohms law to figure out the amps going through R1, then use the DC Gain to get the current flowing through the Collector-Emitter, then put the proper resistor in front of D1 so that it doesn't blow out right? But then where does all this Vce, Ib, and Ic (and watever else there is) junk come into play? I'm really not sure I understand why you need those ratings  :-\

Thanks for helpin me out

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Hi f4,
1) Start with the LED, D1. You don't want to blow it out, so you don't want to operate it at its absolute maximum current. Choose 20mA.

2) Calculate R3. When the transistor is turned on, it will have about 0.2V from its collector to its emitter. The LED has 1.6V across it and therefore R3 has 7.2V across it. Ohm's Law calculates a value for R3 of 360 ohms. You can use 330 ohms as a standard resistor value.

3) Select a transistor that can conduct at least about 20mA. The 2N3904 is rated to conduct a max of 200mA so will be fine.

4) Calculate R1. The base-emitter voltage of the transistor is about 0.7V. Therefore R1 has 8.3V across it. The minimum current gain of a 2N3904 transistor at 20mA is about 80. Therefore the current in R1 is 20mA/80 = 0.25mA. Ohm's Law calculates a value for R1 of 33.2k. Since the base current is so low, use 10k to make certain that the transistor is turned on hard. The value of your R1 was 100 times too low.

5) Add a resistor R2 to make certain that the transistor turns off when the switch is open. Use a value 10 times R1 which is 100k. ;D


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ww i think he about covered it except for what those other specs are for ou usually don't have to worrry about them there stuff like how many nano seconds it takes to turn on and what temp. range they will work at etc. the 2 ones that you need are how much current it takes to turn on and how much current can flow through the transistor and what voltage it will work at i guess that is 3 :)

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