Help me understand some basic concepts

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thenendo
 
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Help me understand some basic concepts

Post by thenendo »

I'm new to electronics, but having a blast!

Some really basic conceptual points I'm currently confused on:
  • My physics textbooks tell me that the term "voltage" or "potential difference" only makes sense in reference to a pair of points in a circuit. Yet it seems like electronic engineers are constantly talking about the voltage at a single point. For example, from the wikipedia page on op amps:
    V+ is the voltage at the non-inverting terminal, V- is the voltage at the inverting terminal, and AOL is the open-loop gain of the amplifier.
    So what do they actually mean? What is the implicit second point in each case?
  • This tutorial about LEDs on ladyada.net was extremely helpful, but I want to make sure I really understand why the method of calculating the current through the LED makes sense. The tutorial seems to imply (but without quite saying it) that the voltage drop across the LED is constant (equal to its "forward voltage") at any current -- does this mean that LEDs, or diodes in general, are made of some magical substance whose resistance varies as necessary in order to keep a constant voltage across the leads? That's the only way I can reconcile what the tutorial says with Ohm's law.
Thanks for any help :)

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chatham
 
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Re: Help me understand some basic concepts

Post by chatham »

Part 1: You're right, voltage is a measurement of potential, and you need two points to measure it. We do always use an implicit second point, though, which will be your circuit's ground.

Part 2: Basically, diodes aren't resistors. I don't know very much about the actual science, so this explanation is almost certainly wrong in some regards, but as I understand it, basically you're getting electrons to jump across a one-way gap, and it needs to have a certain amount of potential to jump, which gives you that constant-voltage drop characteristic - if it's got enough potential (the forward drop voltage), the charge can make the jump, and it goes on it's merry way, minus the potential it used to jump. Kinda like a tollbooth, in a way.
http://en.wikipedia.org/wiki/Diode#Curr ... acteristic

Hope it helps!

uoip
 
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Re: Help me understand some basic concepts

Post by uoip »

Alan's right. I'll jump in and add more.

Ohm's law only holds for ideal perfect resistors. In reality, there is no such thing as an ideal perfect resistor, though plenty of things are close enough that, for all practical purposes, we can pretend they're ideal resistors, at least over a broad range of voltage and current.

I've heard people say that a scientific "law" is something that has been proven to hold true in all cases. That most definitely is NOT what Ohms law is. Ohm's law is a description of ideal behavior. In this way, Ohm's law is like the ideal gas law or Hooke's law. In real life, it's never quite exactly true, but it's often close enough to make the law very, very useful. Ohm's law can be considered to be the definition of a resistor, or the definition of resistance. When a circuit element follows Ohm's law, we call it a resistor. When something doesn't behave according to Ohm's law, we say it's non-resistive, or sometimes we say its resistance varies.

Diodes are most definitely non resistive, and that's why we like them. A resistor conducts equally well in both directions, while a diode, to a first approximation, only passes current in one direction.

Yes, a diode, to a reasonable first approximation has a fixed forward voltage across its terminals when current is passing through it in the forward direction. If you measure really closely, you'll find the forward voltage varies a little bit with current, but not very much, and certainly not the way Ohm's law would describe. For many purposes, you can consider a diode to be a fixed voltage source, as long as you supply it with enough current to conduct in the forward direction.

thenendo
 
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Re: Help me understand some basic concepts

Post by thenendo »

Awesome, thanks, Alan and uoip, that was very clear!

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zener
 
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Re: Help me understand some basic concepts

Post by zener »

Yes. That is why resistors are called linear devices (voltage proportional to current) and diodes are called non-linear devices (voltage not proportional to current). Now if you take some EE courses like 331 you will find out that for a 10x increase in current, the Vf increases by about 60mV.

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