electricity to a transistor

E

Thread Starter

Emre OZTURK

using pascal program Im sending electricity with intervals of 1 second from parallel port to Bc 107 transistor. Before transistor there is a 5,6 Kohm resistant.But the transistor makes the led light or low light. I wish to make led open or closed. Does anyone has an idea what my fault is? Shold I increase the resistants ohm value? or lower it?
Thanks
emre
 
Emre,

If your transistor is an NPN type, sounds like your transistor [if connected properly] appears to be operating in the linear region. To make it like a switch you need to operate it in the saturation region. Try this: connect a resistor between the power supply and LED anode to limit the current [say 500 ohms for 5V supply]. Connect the LED cathode to the transistor collector. Connect the Transistor emitter to supply return [ground]. Connect a base resistor Rb between tranistor base to your parallel port. The value of Rb is chosen to put the transistor into saturation. Rb < [Vb/Ic ]x hfe
where Vb=base voltage, Ic = collector current ~[5V-led Vdrop]/500 ohms,
hfe = transistor gain see data sheet. Remember that your transistor is going to have a saturation voltage or probably 0.2V or bigger depending on the collector current.
As a rule with NPNs, you need to go lower on the
base resistor for saturation. You might want to try a MOSFET. A MOSFET is a voltage controlled device and saturation does not depend on current but on gate voltage.

+5V
|
500 ohm
|
LED
|
c
\
b---Rb--paral. port
/
e
|
gnd

Good luck.

Oly
 
J

Johan Bengtsson

I would suggest wireing it like this:


|+5V
|
/
| /
signal |/
--------|
|\
| _\|
\
|
---
| |
| |
---
|
-----
\ / ->
-----
|
|GND


Make the resistor minimum (4-Vf)/If 4V is (very approximately) what you could expect after the transitor when the output is high If is the maximum current your LED allows and Vf is the voltage drop

For typical LED values a resistor in the range 150-330 ohm would be suitable. If you don't want to calculate the value use 330ohm since it is
quite "safe"


/Johan Bengtsson

Do you need education in the area of automation?
----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 Hässleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------
 
C

Curt Wuollet

Hi Johan

I was assuming that he is using an LED lamp with a limiting resistor built in. This is a fairly safe assumption as otherwise the LED would have expired long ago.

Regards

cww
 
J

Johan Bengtsson

Well, could be (I didn't think of that actually), well then the resistor below wouldn't need to be there at all. (it if is kept there the led will not be as bright as it could be)

However, if it is solved as you suggested (I was about to suggest approimately the same, but of course, with a limiting resistor) or as below is mostly a matter of taste anyway.


/Johan Bengtsson

Do you need education in the area of automation?
----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------
 
Top