Multisim LED exercise problem.

[mikekolas]

Hello , i have one exercise for my school and the question is *Which LED will not turn on and why?* The DC source is 7V and the resistor is 1kOhm with 1% tolerance. I must find also the Vf (v0lt) for LED_blue,LED_green,LED_ir,LED_orange,LED_red and LED_yellow with multimeter. Althought blue, orange and green do not turn on. The others do.

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Thank you in advance

 

[electroRF]

LED has fixed IF, and changing VF.

Voltage drop on LED = VLED = (Vsource - IF * R)

if VLED < VF, the LED will not turn on.

 

[MikeMl]

With 7V and 1k, they should all turn on, because all of them should have a Vf of less than 7V!

 

[JimB]

LED has fixed IF, and changing VF.

No.

An LED has a "fixed" forward voltage, which will vary slightly with forward current and temperature.

The forward current of an LED is what the circuit designer sets it to be. (Within the limits of the LED).

Regarding the OPs question:

Althought blue, orange and green do not turn on. The others do.

I am surprised that with a supply of 7 volts there are some LEDs which do not illuminate.
With a 1k current limiting resistor, they may only be a dim glow, but I would expect them to do something.

/CYNICAL_MODE: ON
But then this is just a computer game and not real components.
/CYNICAL_MODE: OFF

JimB

 

[mikekolas]

Thank you for your replies but how to find If ? The problem does not give it.

With 7V and 1k, they should all turn on, because all of them should have a Vf of less than 7V!

They have vf less than 7 all of them.