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Multimeters and my lack of understanding thereof
FF
Once Upon a TimeIn OaklandRegistered User regular
Once Upon a TimeIn OaklandRegistered User regular
I've got this multimeter and I'm trying to measure resistance with it. I've followed the instructions included with the meter. The instructions only provide for using the first setting I'm mentioning here (RX1K) When I measure the resistance using the RX1K setting (OHMS, multiply reading by 1000) I get what appears to be a 0.2 on their scale. Multiplied by 1000, I've got 200Ohms of resistance. So far, so good.
However, when I measure the same two contacts using the RX10 setting (OHMS multiply reading by 10) I get a 4 on the scale. Multiplied by 10 and that gives me a reading of 40Ohms of resistance.
So, what's the deal here? Since the range I'm looking for is basically, anything higher than 200Ohms is good, anything under 200Ohms is bad, should I be using the RX10 scale?
However, when I measure the same two contacts using the RX10 setting (OHMS multiply reading by 10) I get a 4 on the scale. Multiplied by 10 and that gives me a reading of 40Ohms of resistance.
So, what's the deal here? Since the range I'm looking for is basically, anything higher than 200Ohms is good, anything under 200Ohms is bad, should I be using the RX10 scale?
Huh...
FF on
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http://www.thelostworlds.net/
Have a look here and see if it agrees with your instructions:
http://www.wikihow.com/Use-an-Ohmmeter
For example, you would use an ohmmeter to check the resistance of a potentiometer at a particular setting by putting one probe on the center terminal and the other probe on one of the outer terminals. You wouldn't e.g. disconnect the pot entirely and hook the ohmmeter up where the pot used to be, because then you're measuring something bizarre like the resistance across the entire rest of the device.
Can you maybe be a bit more specific about what you're trying to test the resistance of?
http://www.thelostworlds.net/
The instruction includes a picture of the terminals and also tells me that anything about 200ohms is good, anything below 10ohms is bad.
I'm guessing that from my halfassed testing that it's above 10ohms, but I want to be sure I'm using this thing correctly.
Just stick to 1k setting and you're fine (I'm actually taking automotive electronics right now so this isn't just out of my ass)
This is a digital ohmmeter we're talking about, right...?
Really? I was taught the opposite - start with the highest setting and work your way down, since setting the multimeter to too-low of a setting and measuring a high value can potentially damage it. Of course this doesn't apply to measuring resistance, or to auto-ranging multimeters, and I can't -really- see it being a problem with the values you'd find in a PC, but still.
The next time I hear we're purchasing tools at work, that's first on the list.
It's not uncommon for analogue ones to have different scales for different settings. Make sure that you're reading off the right scale. And what are you trying to measure the resistance of?
Oh man, I should learn to read.
Anyway, it seems that what they're trying to get you to do is test for the presence of a short circuit.
Below 10 ohms means that something in the circuitry has shorted (and is thus borked). Anything well above 10 ohms demonstrates that a short circuit is not present, and thus not the problem.
Based on your testing, you do not have a short circuit where you're measuring.
Does your scale for RX10 go to 20? If not, then you cannot realistically use the RX10 setting if you expect it to be 200 Ohms.
Never the less, it does look like I'm getting resistance across the two terminals so it looks like the pwer supply is good, meaning it's the motherboard.
(For future reference...or something...What I'm working on is a G5 Power Mac. The troubleshooting steps are from the service manual. Yes, I am aware that I probably should already know how to use a multimeter. Als, I do not. Well, did not.)
If so, each curve of the gauge indicates a separate scale - most of them have 2 or 3 settings for range, and you read off the appropriate color.
Meh, everyone's got to start somewhere.
But if you'd like a more detailed explanation of why you're not getting consistent results, then:
* The results from ohm-meters/multimeters are only guaranteed to be consistent if you have a pure resistor (I admit, fringe cases exist, but I'll simplify for now).
* This is because the multimeters put a known current through the probes (my old analogue meter puts 150mA at RX1, 15mA at RX10, 0.15mA at RX1K), and measures the resulting voltage across the probes to determine the resistance using Ohm's Law, V = I R
* The terminals that you're being asked to measure across have all manner of components - inductors, capacitors, transistors, perhaps other integrated circuits connected to them
* Depending on the setting you've turned your multimeter on to, you're feeding different amounts of currents into all those components, which might be enough to, say, turn a transistor half on, or charge up a capacitor to such and such a voltage. It's hard to say without a circuit diagram.
* As a result, feeding, say, 100 times more current isn't necessarily going to give you 100 times more voltage (The RX10 will feed 100 times more current than RX1k) because 100 times more current might turn a transistor on which reduces the output voltage. Or you might force a diode to conduct or something.
* Thus, you will get inconsistant readings between using RX10, and RX1k modes.
An exception to this is if you have a short circuit - where the resistance is 0. In which case, it doesn't matter what current you feed in, because V = I x 0 = 0V. This is why you can only really test for the presence of a short circuit in complex circuits.
Thanks to all of you actually.