Starting out with electronics - simple circuit, does not work

[philba]

yes, a single series of components all see the same current. however, LEDs will draw as much current as you can give them (and burn out if enough current). that's why you use a resistor.

yes, the circuit determines the current, not the power source. however, note that all power sources have resistance. In many cases it can be ignored, like this one.

Hfe is the transistor's DC current gain. a Hfe of 100 and a base current of 10mA will allow up to 1A of current flowing from collector to emitter. I say "up to" because load components (LED + resistor) will determine the current draw.

 

[Dotnet]

Hfe = Ic / Ib

Ic = Collector Current
Ib = Base Current


Also, let me reaffirm
please please please draw your transistors per convention emitters pointing down and +ve at the top

Dotnet

 

[JoeElectron]

I know I'm late but here are some basics:
Kirchhoff's voltage law:
The sum of the voltage drops around a DC series circuit equals the applied voltage. In this circuit, the VD (voltage drop) across the two transistors, the 820 ohm resistor and the LED equals 9 volts.
Kirchhoff's current law:
The current flowing to a point in a circuit must equal the current flowing away from that point. In other words, if two LEDs are in series, the current flowing through one must flow through the other, unless an alternate path is provided to draw the current around one of the LEDs.
Now, about those NPN transistors:
The base thru emitter current path is a forward biased diode. While the equation for current vs. voltage drop in a diode is complicated ( i=Is(e^(qv/nkT-1)) ) we usually 'assume' the VD to be about .6 or .7 v.
Now on the collector to emitter current path we take to be a current source. Here is where hfe comes in. For every electron going thru the B-E causes a cascade of hfe electrons thru the C-E pathway. So, if I have a hfe of 150, and a base current of 1 milliamp, I'll have 150 milliamp thru the collector and 151 milliamps thru the emitter (the current thru the base plus the current thru the collector). As I increase the base current I will continue to get this gain until 'saturation' which is when other limitations in the circuit prevent the C-E from being able to keep up.
Because the VD thru the base-collector is an exponential relationship, the gain is not linear with respect to the base voltage.
For the designer, the current thru the transistor is the first consideration. If you plan for 20 ma thru the transistor and the data sheet give a minimum hfe of 100, then you need a base current of 200 micro amps (200 micro amps X 100 = 20,000 micro amps or 20 milliamps). I usually double this to ensure saturation. The base resistor is critical. The easiest way to destroy a transistor is to allow too much current thru the base. With a 9v battery, a desired current of 100 micro amps and .6 v drop thru the base we have:
E= I/R
(9 - .6) = (400 micro X R)
8.4 / 400 micro = R
R = 21 k ohms
This represents the base resistance.
So, now that you have the transistors turned on, you look at the LED circuit. Specifically, the VD of the LED:
From Wikipedia:
Typical voltage drops
Although there are small variations between individual diodes, in general the voltage drop depends on the color of the LED and the brightness. The following table summarized some of these voltage drops for various LEDs.
LED Voltage Drops
Diode type Typical Voltage Drop (Volts)
non-high-brightness red 1.7 volts
high-brightness, high-efficiency and low-current red 1.9 volts
orange and yellow 2 volts
green 2.1 volts
bright white, bright non-yellowish green, and most blue 3.4 volts
bright blue and some specialized LEDs 4.6 volts
Most manufactures recommend 10 milliamps for the 430 nm blue diodes, 12 mA for the 3.4 volt types, and 20 mA for lower-voltage LEDs.
The LED resistor is:
E=IR
(9 – 1.7) = 20 milli x R
7.3 / 20 milli = R
365 = R
Try these values and see what you get. Usually I check the pins and hfe on a transistor just prior to installing it. It is easy to get different types mixed up. Lots of Luck!

 

[svk]

Thanks again for the help guys. You gave me a lot to digest...

 

[HiTech]

I want to applaud you for being genuine in your endeavor to experiment and try to understand your task at hand. It's refreshing to see someone actually try something on their own merits as well as inquire for assistance in an earnest attempt to learn something in the process. If you stay on track, you show promise of success.
Oh, and even though the circuit in question was a basic one, it wasn't one of wanting to screw up tv reception, jam cell phone communications, or knock out the power grid of the entire eastern US seaboard!

 

[Overclocked]

Well, I'm still in high school. I'll be entering college in two years.

However, my dad holds a degree in electrical engineering. But he's gone until 6 P.M., which is an awfully long wait for me. Besides, helping me out with electronics all-evening-long is not something he really craves... .

And even when I get to college, I'm not sure I will be able to afford to study electronics. I'm not sure what my major will be, but it's likely that half of my schedule will be filled with the required courses to qualify for that major. On top of that, if the college has any sort of a liberal arts tendency, there will be more required courses. And on top of all that, there are several other subject areas I have a high interest in. I'm not sure if there will be any room left... .

But I agree, some interactive help from a teacher would be nice. For now, I'll have to subsist on my dad. He's actually great at explaining some difficult concepts to me, but the amount of the time he can devote to me is quite limited.

Get a student Loan. You could always stay in college and get many degrees instead of one. I have to take courses (like you said) before I can get into electronics, but its worth the wait.

Do you buy any chance have an electronics (or tech) division at your school? You could try going to them.

 

[Screech]

Please note that the load ( Led and it's resistor) must be placed before all NPN type transitors.

4 wrongs with the below pic.

No wonder you are confused.

 

[Overclocked]

Please note that the load ( Led and it's resistor) must be placed before all NPN type transitors.

4 wrongs with the below pic.

No wonder you are confused.

I thought it didnt matter where the resistor on the collector/emitter goes, since Ic=Ie

 

[_nox_]

Please note that the load ( Led and it's resistor) must be placed before all NPN type transitors.

4 wrongs with the below pic.

No wonder you are confused.

Hmm nice circuit image! Might I ask , how the programm is called you used to create them ?

 

[Screech]

I thought it didnt matter where the resistor on the collector/emitter goes, since Ic=Ie?

With PNP type, the loads (Leds and it's resistor) are place after the transitor.

When you learn how to use your parts correctly, then you can start to use math formulas.

Hmm nice circuit image! Might I ask , how the programm is called you used to create them ?

I edited them with Microsoft Paint / Mspaint.