something confusing me with LED

[baby_1]

Hello
the 120 resistor it is connected to a pin of micro (pic18f452) when i set it high( set 5 volt at the resistor terminal) it works fine

but when i changed the 1kohm situation at the collector of bdx53 led won't work!!!!!

i tested it by BC337 and BDx53 and two led but it doesn't work at the second circuit.why?!!!!

 

[barry]

You've obviously hooked something up wrong. It makes no difference which way you do this.

 

[Sloth]

Measure from both ends of R2 and D1 and the Q2 collector. You should have 9 V, ~1.6 V, ~1.6 V, 0V and 0V. This won't be the case though, because you've connected something wrong.

 

[baby_1]

Hello Sloth
VBE=2
V(resistor 120ohm)=3.1 ~ 3.2
VCE~=1.5

 

[D.A.(Tony)Stewart]

If the LED was ever put in reverse the 8V reverse voltage would damage the junction and short it out. Vr=5V max.
What is the LED diode drop now?

 

[Sloth]

Hi,

VCE seems high. How about the R2 and D1 ends? I think something is not connected there.

 

[barry]

Vbe is way high. Should be about 0.6V. Something bad happened to your components.

 

[D.A.(Tony)Stewart]

If the LED is hot and not illuminated, my theory may be correct. Also LED's are ESD sensitive.

 

[FvM]

Vbe is way high. Should be about 0.6V. Something bad happened to your components.

BDX53 is a darlington. Vbe of 1.2 to 1.4 is O.K.

 

[fredd]

Hello
the 120 resistor it is connected to a pin of micro (pic18f452) when i set it high( set 5 volt at the resistor terminal) it works fine
but when i changed the 1kohm situation at the collector of bdx53 led won't work!!!!!


i tested it by BC337 and BDx53 and two led but it doesn't work at the second circuit.why?!!!!

did you do both simulation & actual testing?

 

[Joeisi]

No need. He already got it to work in a similar arrangement. I think he killed a part.

 

[fredd]

No need. He already got it to work in a similar arrangement. I think he killed a part.

ah i see.
to add, initial study must be done to ensure proper connections and setup.

 

[thannara123]

LED's act as load .then what is the collector current ?
LED's Current ?