| Electronics > Beginners |
| Multimeters and Resistors |
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| cowana:
--- Quote from: Kohlrak on November 24, 2017, 01:52:24 pm ---Though, when my results varied considerably i found it strange. especially when the leads were directly touching the leads of the resistor. By touching the leads of the multimeter the resistance went down which confused me. I would say it was the result of oil from my skin, but when i pinched 1 end together and touched the other lead of the multimeter without touching the resistor, it still went down which does actually confuse me. --- End quote --- Your body has a resistance too - so when you hold (the metal of) one multimeter probe in each hand, it will display the resistance of *you*. If you're trying to read the resistance of a resistor at the same time, you'll get the parallel combination (which will be less than the resistor on it's own). The higher the value of the resistor, the more pronounced this effect. |
| Kohlrak:
--- Quote from: cowana on November 24, 2017, 02:42:17 pm --- --- Quote from: Kohlrak on November 24, 2017, 01:52:24 pm ---Though, when my results varied considerably i found it strange. especially when the leads were directly touching the leads of the resistor. By touching the leads of the multimeter the resistance went down which confused me. I would say it was the result of oil from my skin, but when i pinched 1 end together and touched the other lead of the multimeter without touching the resistor, it still went down which does actually confuse me. --- End quote --- Your body has a resistance too - so when you hold (the metal of) one multimeter probe in each hand, it will display the resistance of *you*. If you're trying to read the resistance of a resistor at the same time, you'll get the parallel combination (which will be less than the resistor on it's own). The higher the value of the resistor, the more pronounced this effect. --- End quote --- I thought the resistance of my body was too high to have the parallel effect (just like how air doesn't count). But, now that i think about it, it does make sense. The resistance is still present across the same circuits, but if i can let even the slightest amount of current from that 9v through me, it'll take some of the resistance away. I still do wonder what the threshold for current flow is. In a pure math world, everything's constantly conducting in the slightest amount (too small to measure), but in the real world that's not the case (at least by our understanding). There's a noticeable threshold for when an arch happens and when it does not. Presumably, this should be the case for all materials. So at what current value (according to ohm's law) does current actually flow? |
| cowana:
--- Quote from: Kohlrak on November 24, 2017, 03:53:15 pm ---There's a noticeable threshold for when an arch happens and when it does not. Presumably, this should be the case for all materials. So at what current value (according to ohm's law) does current actually flow? --- End quote --- Most materials have a voltage at which they break down (ionise), and a much larger current (arc) can flow. For air, this is somewhere around 3kv/mm. For humans, a very high voltage (around 500v) can cause dielectric breakdown of the skin, allowing very large currents to flow. Before this point, the skin will present a modest resistance (a few 10k ohms) - meaning a small current can flow. Even applying 0.001v across your hands will cause some (very tiny) current to flow across your chest - it's just so low you don't have to worry about it. Generally you can consider around 50v to be the threshold where things could get dangerous. |
| David Hess:
--- Quote from: Kohlrak on November 24, 2017, 08:14:04 am ---Anyway, the bane of my existence and learning experience is these resistor color codes. I'm not color blind, but half the time i'm looking at these things, I can't tell yellow from red from orange, red from brown from orange, you get it. --- End quote --- With experience, you get an idea of what values are invalid although that is not much help distinguishing 2.2k from 22k. But reading the later backwards reveals 3.2k which is not a standard value. |
| GreyWoolfe:
--- Quote from: IanB on November 24, 2017, 10:36:10 am ---Here's a picture of the meter measuring a 1.8 k resistor (1800 ohms) on the 2000 ohms range: --- End quote --- This should be framed--a HF meter that actually works. I had 2 of them, new in the package with new batteries added, that measured a 30R resistive element at over 250R. My Aneng 8008 reads it correctly. They were free so I obviously got what I paid for. One of my co-workers loves HF and their free coupons. After him giving me the 6th free meter, I had to ask him to stop. |O |
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