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does LED work in a passive circuit or only in active circuit

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Aug 9, 2009
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Maybe a dumb question


I'm admittedly new to this, so if this is a dumb question, I'm really sorry.

Can you put an LED in a passive circuit and have it work? Or does an LED only work in an active circuit?



Maybe a dumb question

A passive circuit is all ok. Only passive components in use. That is stable voltage supply and a resistor in series with a led.
Then you don't have active components such as transistors etc. . .
Re: Maybe a dumb question

The LED is a transducer. The output power of the photons are less than the DC input power. And by a very low efficiency. Only low frequency mains transformers are near 99% efficinecy. Electronic amplifiers are somewhat near 75%. Light sources are lucky if they are 30% efficient.
Re: Maybe a dumb question

Thanks for the replies. I want to make sure I understand. I want to put an LED into a circuit that is a passive circuit with a potentiometer and a dpdt pushbutton switch. Currently, the circuit simply goes throuugh the switch, with one route as a bypass for the signal, and the other through the potentiometer. I would like to add an LED so that when the signal goes through the potentiometer, the LED would light and when it goes through the bypass side, it would turn off. Can I put the LED in series with the pot and have it light? It doesn't have to be bright. I just want a visual signal to know when it's on and when it's off, and I'd rather not have to add additional power to the circuit.

Thanks again.

Re: Maybe a dumb question

first of all let us understand what is active and passive circuits and/or elements.

Passive circuit is a circuit which is not capable of generating energy(power gain) bud does consumes energy.
Eg: A resistor consumes energy so is passive.
A transistor i caable of amplifying power, so is categorized as active element.
A circuit which amplifies is called active.

Deductions from the definitions are....-
*A circuit whose output is less than the input power is called as an passive circuit.
*A circuit whose output (signal) is greater than the input(signal) but obviously not greater than the DC power/signal running it (observing laws of conservation of energy that energy can neither be created nor be destroyed) is called as an active circuit.
In crude explanation i might say a microphone amplifies the signal (output>input) but not greater than a certain limit which is decided by the battery you are using.

An LED requires just a voltage difference of +0.7V between its two terminals. Anode or +ve terminal should be 0.7V greater than cathode and LED starts emiting light. Which goes brighter and brighter a you increase the difference till certain point after which the brightness remains constant.

So you can put led anywhere (active/passive ckt) where you can have the conditions mentioned above.

In your experiment. put the LED's +ve term. in NON-BYPASS line and cathode grounded. you will get the light. Make sure the diff is 0.7 volts and not very very large either.

Thanks. I hope it helped you.
Re: Maybe a dumb question

akshay_d_2006 said:
An LED requires just a voltage difference of +0.7V between its two terminals.
It's more likely to be around 1.7V and above. You had regular diodes in mind.
Re: Maybe a dumb question

Yes, very helpful. Thank you all very much. I'll let you know how it goes.

Added after 3 minutes:

"In your experiment. put the LED's +ve term. in NON-BYPASS line and cathode grounded. you will get the light. Make sure the diff is 1.7 volts and not very very large either."

Sorry. I am about to go do some soldering and I thought I should get clarification on one thing. Please explain what I need to do to make sure it is cathode grounded. Cheers,


Maybe a dumb question

When you purchase an LED, on the upper glass/plastic coat you will see a plat edge at the lower rim/circumference of the glass. That lead is the -ve term. i.e it is cathode. Othe way to know is If it Is brand new, then the one lead which is longer in length is cathode. cross check it with the flat edge techniqut.
Just make sure you connect it to ground by checking whether ground and that cathode lead are shorted or are at the same potential(Which means shorted)...Good Luck
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