Discussion in 'Homework Help' started by johncena, Aug 15, 2010.

Aug 15, 2010
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This is the problem...

Finding for the Voltage Input. I get 3.23v, but i know it's not the correct answer. pls guide me. thanks

2. mik3 Senior Member

Feb 4, 2008
4,846
63
Did you include the voltage drop across the LED in your calculations?

Aug 15, 2010
32
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i think the voltage drop of LED is neglected, as it only just need 20mA on Ic , The problem states that What voltage (Vin) does it need to produce that 20mA on Ic

4. mik3 Senior Member

Feb 4, 2008
4,846
63
The question doesn't say to neglect the LED voltage which can vary from 1.5 to 4 volts depending on the colour of the LED. Thus you have to include it since Vcc is just 10V and 2V across the LED can be quite significant.

Do you have the correct answer?

5. Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
Hmm,
Ib_min = 200uA so if safety margin is 2*Ib_min = 400uA
Vin = Ib*Rb +Vbe = 2V+Vbe
Or
Vin = 200uA*5K + Vbe = 1.7V----> 2*1.6V = 3.4v

6. hobbyist Distinguished Member

Aug 10, 2008
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I got Viin=2.32V.

But I used IB (min), for the calc.

7. Potato Pudding Well-Known Member

Jun 11, 2010
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Allowing around 3V for the LED Vf the Rc=300 Ohm and +Vcc of 10V will allow about 22mA with the transistor in saturation.

To allow exactly 20mA with 100Beta, if you double the volts as instructed Jony has it almost right.

Double only the volts across the resistor.

1 Volt across the resistor *2 +Vbe = 2.7 Volts

8. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
The problem is set up badly, but you know what?

You will get a lot more poorly thought out questions and proposals when you get out into real work. Know what you need to know and decipher the intended question, even when they don't know what they are trying to ask. It is a like a puzzle game sometimes, and if you learn to have fun with it and have a sense of humor about it you will find things hilariously fun instead of horribly annoying.