5 ms·

A resistor is usually to "resist" current on a certain component. If you've 10 volts in a circuit, you wouldn't want to connect a simple LED there, because you

A resistor is usually to "resist" current on a certain component. If you've 10 volts in a circuit, you wouldn't want to connect a simple LED there, because you wouldn't want that much current going to the LED. So you'd use a resistor of appropriate resistance in that instance to keep it from burning out. Though, in the example you linked to, there's no use of the resistor -- it's connected to the ground. It's just useless. Unless of course, I'm not understanding things right. (If I'm wrong anywhere, please point it out!)

mechanical_fish 18y ago
Well, no, you're not understanding things right. ;) The problem is that resistors have many uses. One of them is, indeed, to limit the current flowing through some bit of a circuit: If you have a nine-volt source, and you want some current to flow from nine volts to ground (because, say, you want to run that current through an LED and make light), and you don't want that current to be too large, put in more resistance. Another obvious application that everyone learns is: If you want to heat something up, strap a resistor to it and run a bunch of current through the resistor and it will heat right up. (Although you had better heed the wattage rating on your resistor or it will blow out right away.) This is a third application: Resistors convert a current to a voltage. Voltages are often easier to measure than currents. In this case, if you just hooked a 5 volt source across a thermistor, the voltage at the top of the thermistor is always 5 volts, and the voltage at the bottom is always zero by definition. (Because we're defining it as ground -- ground means "the point which we define as zero voltage in this diagram", although sometimes it also means "a point which we literally attach to the earth" -- darned EEs with their crazy terminology!) You can't use a voltage probe to measure anything interesting in this thermistor-only circuit. The resistance of the thermistor is changing with temperature, which means that the current in the circuit is changing with temperature, but you can't see that with your voltage probe. Unless you put in a second resistor -- the voltage across that resistor will be proportional to the current. That's what the resistor is doing here: Converting current to voltage so that we can see it. Of course, one of the reasons EE is tricky to grasp is that the resistor is doing more than one thing at once. For example, it is also limiting the total amount of current in the circult, just as you suggested. If you let the current be limited only by the thermistor you might draw too much of it and break the thermistor. Or, even if the thermistor has enough resistance to prevent that, there's a catch: If the thermistor draws enough current it will heat up, which will affect its reading. So you might want to choose a second resistor that is big enough to keep the current very small. But not too small, or there will be a lot of noise. Sooner or later you have to stop thinking and use good old trial and error! Learning this stuff is a little bit of thinking, then some math, then some more thinking from another angle, then some tinkering. Occasionally you have to melt some things -- that can help you learn too. It does take time. I was probably halfway through college before I could see this stuff, and I'm still blind compared to the real circuits guys.
- jimmyjim 18y ago
  Thank you very much for the elaboration. > ...the voltage at the top of the thermistor is always 5 volts, and the voltage at the bottom is always zero by definition. (Because we're defining it as ground -- ground means "the point which we define as zero voltage in this diagram", although sometimes it also means "a point which we literally attach to the earth" -- darned EEs with their crazy terminology!) Isn't what you're referring to rather a "common" than a ground?
  - blackguardx 18y ago
    Technically that is true, but any common reference point is usually referred to as a ground. If it does indeed connect to the earth, then it is usually called an earth ground.
    - mechanical_fish 18y ago
      Yeah, in an ideal world we would all use "common" for the one concept and "ground" or "earth ground" for the other. But in practice we use "ground" to mean "common" and "earth ground" to mean "earth ground". I remember, when I was a beginner, being completely confused about what all those ground symbols meant. That was one of my first big stumbling blocks in electronics. The answer is that they're usually nothing but notational: They save you drawing some wires (every ground symbol is assumed to be attached directly to every other ground symbol by a very-low-resistance wire [1]) and more importantly they define your reference voltage (once you've drawn a ground, you can say things like "this point in the circuit is at 3.2 volts", instead of "this point in the circuit is at 3.2 volts relative to Point X".) --- [1] There are times when the connections from ground to ground have too high a resistance, or when you pour so much current into ground that the ground-lead resistance can't be ignored anymore. Those are called ground loops, and are topics for advanced class. I have a nasty ground loop when I connect my stereo system to my cable TV. It manifests as the sixty-hertz hum from hell. http://en.wikipedia.org/wiki/Ground_loop_(electricity) http://en.wikipedia.org/wiki/Ground_loop_(electricity)
cosmo7 18y ago
Ohm my god.

mechanical_fish 18y ago

Well, no, you're not understanding things right. ;) The problem is that resistors have many uses. One of them is, indeed, to limit the current flowing through some bit of a circuit: If you have a nine-volt source, and you want some current to flow from nine volts to ground (because, say, you want to run that current through an LED and make light), and you don't want that current to be too large, put in more resistance. Another obvious application that everyone learns is: If you want to heat something up, strap a resistor to it and run a bunch of current through the resistor and it will heat right up. (Although you had better heed the wattage rating on your resistor or it will blow out right away.) This is a third application: Resistors convert a current to a voltage. Voltages are often easier to measure than currents. In this case, if you just hooked a 5 volt source across a thermistor, the voltage at the top of the thermistor is always 5 volts, and the voltage at the bottom is always zero by definition. (Because we're defining it as ground -- ground means "the point which we define as zero voltage in this diagram", although sometimes it also means "a point which we literally attach to the earth" -- darned EEs with their crazy terminology!) You can't use a voltage probe to measure anything interesting in this thermistor-only circuit. The resistance of the thermistor is changing with temperature, which means that the current in the circuit is changing with temperature, but you can't see that with your voltage probe. Unless you put in a second resistor -- the voltage across that resistor will be proportional to the current. That's what the resistor is doing here: Converting current to voltage so that we can see it. Of course, one of the reasons EE is tricky to grasp is that the resistor is doing more than one thing at once. For example, it is also limiting the total amount of current in the circult, just as you suggested. If you let the current be limited only by the thermistor you might draw too much of it and break the thermistor. Or, even if the thermistor has enough resistance to prevent that, there's a catch: If the thermistor draws enough current it will heat up, which will affect its reading. So you might want to choose a second resistor that is big enough to keep the current very small. But not too small, or there will be a lot of noise. Sooner or later you have to stop thinking and use good old trial and error! Learning this stuff is a little bit of thinking, then some math, then some more thinking from another angle, then some tinkering. Occasionally you have to melt some things -- that can help you learn too. It does take time. I was probably halfway through college before I could see this stuff, and I'm still blind compared to the real circuits guys.

jimmyjim 18y ago
Thank you very much for the elaboration. > ...the voltage at the top of the thermistor is always 5 volts, and the voltage at the bottom is always zero by definition. (Because we're defining it as ground -- ground means "the point which we define as zero voltage in this diagram", although sometimes it also means "a point which we literally attach to the earth" -- darned EEs with their crazy terminology!) Isn't what you're referring to rather a "common" than a ground?
- blackguardx 18y ago
  Technically that is true, but any common reference point is usually referred to as a ground. If it does indeed connect to the earth, then it is usually called an earth ground.
  - mechanical_fish 18y ago
    Yeah, in an ideal world we would all use "common" for the one concept and "ground" or "earth ground" for the other. But in practice we use "ground" to mean "common" and "earth ground" to mean "earth ground". I remember, when I was a beginner, being completely confused about what all those ground symbols meant. That was one of my first big stumbling blocks in electronics. The answer is that they're usually nothing but notational: They save you drawing some wires (every ground symbol is assumed to be attached directly to every other ground symbol by a very-low-resistance wire [1]) and more importantly they define your reference voltage (once you've drawn a ground, you can say things like "this point in the circuit is at 3.2 volts", instead of "this point in the circuit is at 3.2 volts relative to Point X".) --- [1] There are times when the connections from ground to ground have too high a resistance, or when you pour so much current into ground that the ground-lead resistance can't be ignored anymore. Those are called ground loops, and are topics for advanced class. I have a nasty ground loop when I connect my stereo system to my cable TV. It manifests as the sixty-hertz hum from hell. http://en.wikipedia.org/wiki/Ground_loop_(electricity) http://en.wikipedia.org/wiki/Ground_loop_(electricity)

cosmo7 18y ago

Ohm my god.