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• posted on March 29, 2009, 5:32 pm
I need to buy a multimeter. I noticed that for the resistance some have 2000k and some have 20M, which I believe are totally different. Which one is better to have? Or does it even matter? I'm mostly interested in testing something that's around 2M. So I don't know if it matters which multimeter I get.
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• posted on March 29, 2009, 6:10 pm

Partly it may be matter of what you will use it for! For household electrics the cheapest one may be fine. It's also a matter of knowing how to use it; practically not theoretically. If you are measuring batteries, home electrcity etc.it doesn't matter much. If you are tinkering or repairing with electronic circuits, it may be desirable to have one that has a high resistance so that it affects what one is working on the least. For example if you put a meter of 2,000 kilohms (That's 2 megs by the way) across a circuit that itself has resistance of say 2 megs you will get a wrong reading (a very wrong reading). That's why high quality bench testers can be 50 meg-ohms, so they have virtually no effect on what they are measuring. If testing 12 volts DC for the car, or whether an electric heater of around 10 ohms is OK, or 115 or 230 volts AC it won't matter! Megohm = 10^6 or one million ohms Kilohm = 10^3 or one thousand ohms Ohm = one ohm All are measurments in units of resistance to the flow of electric current.
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• posted on March 29, 2009, 8:38 pm
stan wrote:

So a Gigohm is 10^9 and a Teraohm is 10^12 I suppose.
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Van Chocstraw
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• posted on March 29, 2009, 6:40 pm

A low impedance meter actually works better for home electrical testing. For electronics testing the high impedance meter works best. The higher the impedance the less the meter itself will affect the circuit being measured. If you are measuring outlets in your home you would want the meter to reject currents that are induced into adjacent wires. If measuring a resistor in-circuit on a PC board in an electronic device you would want a high-impedance meter so that the meter itself has little effect on the thing being measured.
An analog VOM meter would be an example of a low-impedance meter, most DMM's are high impedance (like an old VTVM meter).
(I'm assuming you are asking about the rated impedance of different meters and not the choices on the selector knobs since you mentioned a 20meg ohm vs 2k ohm meters).
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• posted on March 29, 2009, 6:59 pm
On Sun, 29 Mar 2009 10:32:59 -0700 (PDT), svu geek

It all depends on what you want to do with it. If you are just looking for something to work on things around the house 20k/v is plenty and may actually be better for "power" circuits. (they do not have the phantom voltage problem). The very high impedance meters are for working on electronics like CMOS where circuit loading gives you bad readings. Unfortunately they get sold to homeowners and would be electricians who will show up here asking why they have 30-40v on a disconnected wire.
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• posted on March 29, 2009, 10:41 pm
"svu geek" wrote:

For resistance measurements, more is better. A short circuit is ideally zero ohms, and it goes up from there.
Practically speaking, you might never need to measure more than 2 MegOhms, which is a high enough value for most uses.
Jon
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• posted on March 30, 2009, 1:42 am
On 3/29/2009 2:41 PM Jon Danniken spake thus:

>

You're confusing the *resistance* range of a meter to its *impedance*. Both are measured in ohms. The impedance has nothing to do with how high a resistance the meter will measure.
--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
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• posted on March 30, 2009, 12:50 am
David Nebenzahl wrote:

Hi, On digital meters input impedance is pretty high already. High Z gives more accuracy by not loading circuit under measurement but some times it is not useful for it's high sensitivity. That is why I still have old tank Simpson 260, Fluke, and even an old VTVM when dealing wth high frequency circuit. On top of that o'scope if I need to see something when numbers don't help. Some times also simple test light is enough.
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• posted on March 30, 2009, 2:30 am
On 3/29/2009 4:50 PM Tony Hwang spake thus:

But we weren't talking about that aspect of meters, so you're just further confusing the subject.
--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
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• posted on March 30, 2009, 1:54 am
David Nebenzahl wrote:

Hi, If I were buying one now I'd go to eBay, look for used Fluke for a good price.
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• posted on March 30, 2009, 2:57 am

The real question is: What is the use of the multimeter? If you were an electronics pro, you wouldn't be asking this question. If you just want to fix stuff around the house, go cheap. Maybe not \$5 Big Lots cheap, but say \$15 or \$20 cheap at Walmart or the big box hardware chain stores. I'd pay more for digital, myself, just for the convenience.
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• posted on March 30, 2009, 9:22 pm
Bert Byfield wrote:

The \$3-4 harbor freight cheap is OK too.
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• posted on March 30, 2009, 10:34 pm
I own several of the cheap HF ones. Far as I can tell, they work just fine. They go high enough voltage to do AC line current. And enough DC amps to test flash lights. Works for me.
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Christopher A. Young
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• posted on March 31, 2009, 3:59 am

first one I bought read a 1.5v battery at 1.8V,very inaccurate. the 2nd tested better.
plus,they are only 1 Megohm input impedance;most DMMs are 10 Megohm,less circuit loading.
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Jim Yanik
jyanik
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• posted on March 31, 2009, 9:42 pm
Jim Yanik wrote:

Mine does that when the battery gets low. So does my Fluke.
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• posted on March 31, 2009, 12:26 am

No, the OPs question is concerning the upper limit in resistance measurements, *not* the meter's impedence.
I think you might want to reconsider who is confused here.
Jon
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• posted on March 31, 2009, 9:32 pm
Jon Danniken wrote:

I was thinking that several people were confused.
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• posted on March 30, 2009, 5:59 am
On Sun, 29 Mar 2009 10:32:59 -0700 (PDT), svu geek

The resistance you plan measure, 2M ?, had nothing to do with the resistance of the meter. All mulitmeters can measure a wide range of voltages, probably enough for anything you'll want, unless your intrests broaden a lot. (For example, you need a special probe to measure the 25000 volts on a color tv picture tube, but you will never do that. And if you need to see if there's voltage to a picture tube, you can just put a little neon bulb in a soda straw, and hold it near the thick wire. If it lights up, you have voltage. :) )

Just for the record, on an analog meter and iiuc on a digital meter, it's 2K or 2meg ohms/volt, ohms per volt. I think that means that the resistance in the meter circuit is 2K or 2M times the greatest value at the right end of each analog scale. That is, the resistance changes for each dial setting. Not sure what it means in digital, but something parallel.
I hate to like new-fangled stuff, but I do in this case. I think you can easily learn to ignore phantom voltages in AC current and that is pretty much the only drawback to high impedance meters. While there are high-impedance analog meters (Like FET-VOMs) they are quite uncommon. Most are digital and the advantages of digital are auto-polarity (no need to get the red and black right when measuring voltage), auto-zeroing and auto-maximum for resistance measuring, and for some slightly more expensive meters, auto-ranging (no need to set a dial to the 2, 20, 200, or 2000 volt scale, etc. It figures it out)
Analog don't and can't afaik have any of these things.
OTOH, analog have the advantage that you can put the meter on resistance and watch a capacitor charge or discharge. After a while, I felt I could tell a good cap by the way the needle moved. You can also watch a discharge with the meter on DC voltage, but iirc for all but really small caps, it takes too long.
It's nice to have a continuity tester with a buzzer built-in, so you don't have to look at the meter for this simple task. The very cheap Harbor Freight meters don't have that, and maybe some others don't, so if you want that, check if it has it.
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• posted on March 30, 2009, 1:01 pm

So: With that overload of technical information by some very knowledgeable posters it comes back to; "What do you want to use it for?".
If just to test household stuff a reasonably cheap digital display meter under \$20 probably OK.
Learn how to use it, keep the internal battery (replaced) although they last a long time. Although some/many digitals will shut themselves off after a few minutes to conserve the battery, anyway.
Check that it has, or make good test leads. The sort that have substantial rubber over a spring clip are good, so fingers don't have to touch metal clips contacting possibly live wiring. An extra lead and/or extension can sometimes be helpful and can be stored in the same case with the meter.
Make sure electric power to a circuit IS turned off before using the ohmmeter scale .... that's the one where the meter is most vulnerable and is using it's own internal low voltage battery to test a 'dead' circuit. You don't need to hook that setting of the meter, inadvertently, to say a 115 volts wall outlet and blow the proverbial you know what out of the meter!
However many do have a special small protective fuse (usually a few milliamps) inside and some even provide a spare fuse! Resist the temptation to 'fix' the fuse with Al foil (unless very temporarily and you know exactly what you are doing) because next time one makes a mistake (and we all do occasionally!) It's very easy perched on an awkward ladder with cramp in the bad left leg to forget that you were just testing for continuity with the ohmmeter setting; and then touch it on 115 or 230 volts ...................... darn!!!!!!!!! That may very well damage the meter!
Congratulations to the OP for asking though.
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• posted on March 30, 2009, 9:26 pm
stan wrote:

In my experience, testing on the amp ranges is where the greatest risk to the meter comes in.