| Electronics > Beginners |
| Some noob questions |
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| rstofer:
--- Quote from: Mr D on July 18, 2018, 10:41:28 pm ---Ok, so i'm starting to get an inkling of understanding about this, after playing with my MM and some resistors. re: Ohm's law: 1 ohm of resistance is almost no resistance, right? Or is it in fact no resistance whatsoever? If that's not the case, how many ohms is, say, a 10cm piece of thin copy wire? --- End quote --- 1 Ohm is a very small number when dealing with electronics. It is a very high number when dealing with busbar inside a utility substation. All numbers are relative. When I think in terms of electronics, I think 220 Ohms for a bright LED from 5V, 330 Ohms if I want it a little dimmer and 470 Ohms as about as dim as I want to go (RED LED). I might use 330 Ohms from 3.3V if I want a particularly dim surface mount LED - something that might be used for diagnostics. Later on you will learn to calculate the resistor value for a particular LED current which will result in a specific amount of luminous intensity. Later on... I tend to think in terms of 1k, 2.2k, 4.7k and 10k for most transistor projects and 1 Megohm (plus 1 ufd) for op amp integrators (time constant = 1 second, this will come up later...). The problem with high resistance circuits is that they are easily swayed by noise. The problem with low resistance circuits it that they tend to use more power. Here is a calculator that should answer any specific questions you may have: https://chemandy.com/calculators/round-wire-resistance-calculator.htm There are MANY other calculators on the Internet. Some of them are more appropriate for industrial wiring. |
| james_s:
--- Quote from: Mr D on July 17, 2018, 07:21:33 am ---Why is it always necessary to explain via analogy? This is not meant as a criticism of your attempt to explain it. I just don't understand why it's never explained without resorting to analogy! --- End quote --- Well the water analogy works well for many people because you can see, touch and feel flowing water. Ok I suppose you can feel electricity too if the voltage is high enough, but that's not really a recommended way to learn about it! |
| Rick Law:
--- Quote from: Mr D on July 18, 2018, 10:41:28 pm ---Ok, so i'm starting to get an inkling of understanding about this, after playing with my MM and some resistors. re: Ohm's law: 1 ohm of resistance is almost no resistance, right? Or is it in fact no resistance whatsoever? If that's not the case, how many ohms is, say, a 10cm piece of thin copy wire? --- End quote --- 1 ohm is very little from the scale of resisters you can buy. You can get (commonly) resisters as low as 0.01 ohm (and may be lower still, but less common) all the way up to multiple giga ohms in one resistor. But 1 ohm is a lot if you don't want it - for example, on some of my cheaper switches, the contact is about 0.2 ohm to 0.5 ohm! That is, if I put the switch to the ON position and I measure the resistance at the solder legs - it is 0.2 ohms! Your DMM's probes has wires and so it has resistance as well. When you put it in resistance mode measuring ohms, and you short the the tip of your two probes, you are measuring the resistance of your probe's wire and the contact resistance between the probes, sockets, etc. You would probably read 0.25 ohm range - may be less, may be more. As to your question about 10 cm sheet of copper - can't answer that because resistance will depend on how thick. Take a look at this "wire gauge table" - it will show you how many ohms (or milli-ohms) per unit length of copper wire, as well as the cross sectional area for a specific gauge. https://en.wikipedia.org/wiki/American_wire_gauge |
| rstofer:
Suppose I had a 480V source and a load current of 1200 Amps (typical for a small unit substation). Now, assume I wanted to limit the busbar voltage drop to 1% or 4.8 Volts. R = V / I or 4.8V / 1200A = 0.004 Ohms or 4 milliOhms. That is 1/250 of 1 Ohm - a pretty small number. But it is typical of busbar for a unit substation. The bolted connections have to be done just right! This is in the world of electrical, not electronics but it does point out that 1 Ohm is not the lower limit of resistance, 0 Ohms is, although it can't be achieved without involving a superconductor. It's important to remember that Ohm's Law is a Law, not a suggestion. You can't treat it like a speed limit! And, yes, I am aware that there are boundary conditions on the Law but they are not relevant to this discussions. Google tells all about it but it is still irrelevant at this level. |
| james_s:
You don't need to be working on a substation for 1 Ohm to be substantial. I recently built a controller for the motor in a cordless electric lawnmower that can draw up to 40A from a 24V battery pack. If the mosfet had an on-resistance of 1 Ohm it would be trying to drop 40V which is more than the battery can provide. The result is the mosfet would be dissipating a huge amount of power and the motor would never be able to draw anywhere near 40A with only a 24V battery. Even if it is only 0.1 Ohm it would drop 4V and dissipate 160 Watts which is far too much. |
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