I don't understand how I am "calculating for the voltage drop"

  • #26
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What happens when I pull the resistor leaving only the L.E.D. across the 9V battery? I want to calculate both amperage & resistance, but it's a little confusing. I know there's a 2V drop across the L.E.D., but I also know at those same points there'll be a 9V drop across the terminals of the battery.
if LED always "eats" 2 V on every current, other 7 V will have to be dissipated on internal resistance. current will rise until IxR_internal=7 V. on battery terminals you will actually measure 2 V, same as on diode terminals.

this will be a large current which will destroy your LED in short time, LED will not be able to cool itself.

your battery schematics is "9 V source + internal resistance". battery resistance is serial resistor which can not be removed.
 
  • #27
sophiecentaur
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What happens when I pull the resistor leaving only the L.E.D. across the 9V battery? I want to calculate both amperage & resistance, but it's a little confusing. I know there's a 2V drop across the L.E.D., but I also know at those same points there'll be a 9V drop across the terminals of the battery.
If you remember, I wrote that the volts drop across an LED is constant over a range of currents. That range doesn't include the current that would flow when it's put across a 9V battery. I(t will melt, of course.

It's confusing because an LED is an active device and doesn't follow Ohm's Law.
.
Afaiaa, the term "active" refers to devices that use an external power source in order to work - e.g. amplifiers. An LED is just Non-Ohmic, with a particular V/I characteristic.
 
  • #28
analogdesign
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Afaiaa, the term "active" refers to devices that use an external power source in order to work - e.g. amplifiers. An LED is just Non-Ohmic, with a particular V/I characteristic.
A lot of people agree with you, but it is not standard. What would you call a diode-connect transistor? An active device connected in a passive configuration?
 
  • #29
sophiecentaur
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A lot of people agree with you, but it is not standard. What would you call a diode-connect transistor? An active device connected in a passive configuration?
I would not call an electric drill that's used to hammer in a nail, using its handle, a 'power tool' and I would not call a transistor which does not 'control' the flow in one terminal by the signal on another terminal, an active device. It's what it is doing that counts, surely; not the name on the side.

Can you quote me a credible example of the use of the word 'active' that doesn't involve a power supply plus some element of control? I am sure that there will be places where all sorts of jargon are used but that wouldn't convince me. Many so-called Engineers can be extremely sloppy when they get talking amongst themselves.
 
  • #30
analogdesign
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I would not call an electric drill that's used to hammer in a nail, using its handle, a 'power tool' and I would not call a transistor which does not 'control' the flow in one terminal by the signal on another terminal, an active device. It's what it is doing that counts, surely; not the name on the side.

Can you quote me a credible example of the use of the word 'active' that doesn't involve a power supply plus some element of control? I am sure that there will be places where all sorts of jargon are used but that wouldn't convince me. Many so-called Engineers can be extremely sloppy when they get talking amongst themselves.
I get it. You're a pedant. I'm a sloppy engineer. Fine. I'm sorry you didn't sleep well last night.

It is interesting in light of your signature how wrapped up you get in classifications.

The meaning of language is its use. We so-called engineers are too concerned making circuits that function as intended to worry too much about these kinds of classifications.

To the OP: I was wrong. An LED is a passive device, not an active one. You may hear diodes referred to in conversation as active devices but don't let that fool you. They are identifying themselves as sloppy engineers so avoid such people. Hopefully this clarifies your understanding of the circuit.
 
  • #31
sophiecentaur
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I get it. You're a pedant. I'm a sloppy engineer. Fine. I'm sorry you didn't sleep well last night.
I like to think that 'pedants' will be responsible for checking the aeroplane that I plan to fly in on Friday.
I can also be as sloppy as the next man, when conversing with colleagues - but, when I am trying to help someone to learn something that is new to them, I think I owe it to them to be as accurate as possible. That's only fair to them, I feel. Language is, as you say, defined by its use. But if terms are used inappropriately and their meanings are not defined and held in common, what happens to the communication? I would never carry on this way on a Ham Radio or Amateur Constructors' Forum but I thought PF had some standards.
Did you have any evidence about the term 'active' or are you too cross to give me any?
 
  • #32
analogdesign
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Did you have any evidence about the term 'active' or are you too cross to give me any?
I'm not cross. The issue here is you're not "more accurate" because there is no objective definition by a generally accepted body clearly defining the terms "passive" and "active" with respect to electronics. You pick a definition you like (often for sound reasons, then act like the definition came down from on high and was etched on stone thousands of years ago).

Many people, yourself included, subscribe to the definition that an active device has a power supply and a method of control.

Other people define active as a device that requires a source of power to operate. Often diodes require that. Sometimes they don't.

You have to set up an LED with a constant current to drive it. You can't just let the signal power it as a resistor or capacitor can (unless you've got a very unusual signal). To some sloppy people, that is considered an active device.

As I mentioned, I agree with you. Describing a diode as passive is probably more descriptive. But being so rigid in your thinking will just turn people off electronics as a hobby. Hell, it's turning me off from helping answer questions as a hobby.

Here are some examples of people describing diodes as active. The first one is a university. The second is wikipedia. The third is a tutorial from a company. The last is an MS thesis from MIT.

http://www.ami.ac.uk/courses/topics/0133_itc/
http://en.wikipedia.org/wiki/Electronic_component
http://sound.westhost.com/beginners.htm
http://dspace.mit.edu/bitstream/handle/1721.1/47505/40295442.pdf <-- calls PIN diodes active elements
 
  • #33
davenn
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http://sound.westhost.com/beginners.htm

Actually that one lists them as passive ... but a special case

Passive: Capable of operating without an external power source.
Typical passive components are resistors, capacitors, inductors and diodes (although the latter are a special case).
personally I have been told and understood all semiconductor devices to be active

Passive was for resistors, capacitors, inductors

Dave
 
  • #34
sophiecentaur
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http://sound.westhost.com/beginners.htm

Actually that one lists them as passive ... but a special case



personally I have been told and understood all semiconductor devices to be active

Passive was for resistors, capacitors, inductors

Dave
The old 'categorising' problem has reared its head again. If you try to put your finger on what exactly is 'active' then you need a definition and not just an arbitrary classification. If 'active' implies control of some parameter, by another (i.e. there has to be a source of power and an output which depends upon the value of an input quantity ) then where do you draw the line between a mixer diode and a full blown amplifier? If you are going to include all semiconductors in the category 'active' then the terms 'active' and 'non-linear' mean the same thing - which means the term is redundant.

If 'active' is taken to mean that a power supply (of some sort) is involved then, at least, there is less possibility of confusion. An 'active' loudspeaker system would not be expected to contain merely some diodes, for added distortion; we would assume there is a power supply somewhere, other than just the audio power form the amplifier.
otoh, a mixer diode (used as part of a larger circuit, of course) could be said to use the Power from a local oscillator to provide a frequency shifted version of an input signal. In that context, it could be active. But so could a piece of resistance wire where the current through it can be controlled by its temperature. It satisfies the condition of control of one quantity by another so it would have to b e an 'active' device.

You could lose a lot of sleep about this, if you were so inclined.
 
  • #35
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I think the total voltage drop in the circuit is = to the supply voltage in a simple dc series circuit like this. The led has a certain resistance, as does the passive resistor. The voltage drop across each will be equal to V=I*R.

The current that is flowing in the circuit will be equal to I = V/R (where R is the total resistance in the circuit) The voltage drop across each resistance, is according to the resistance of of that part of the circuit, and the current flow will be the same in all parts of the curcuit. Maybe neglible in most circuitry is the fact that the wires or other conductors like circuit board traces also have some resistance to the current flow.
 
  • #36
jim hardy
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Ohm's law is a law not a suggestion.

For semiconductor diodes and LED's though R is a function of both current and temperature, and maybe even incident light, so the arithmetic gets more complicated than for a simple ideal circuit element.
Opto devices excluded they're not active so much as they're simply nonlinear.

http://www.merriam-webster.com/dictionary/active
c of an electronic circuit element : capable of controlling voltages or currents
 
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