How To Use A Multimeter To Test

Agreed.....the code will vary from location to location and inspector to inspector even. We should stick to teaching how to use a multimeter. Even then, perhaps we should decide whether to go strictly with a digital, or include analog meters. There are advantages and disadvantages to both. Digitals also often are capable of basic tests of capacitors and some semiconductors, whereas analog meters do not offer those.
 
Agree with Tony; digital and analog meters are different aminals. And, analog meters can do simple go/no go tests even on capacitors and semiconductors.
Mark S.
 
What kind of multimeter? What kind of power? The probes are typically insulated. Keep your fingers only on the insulated part. Set-up the meter correctly so you don’t smoke it. Very high voltage is a different animal/story.


Here is the cheap one I have:

41EXf-0qupL._SY400_.jpg



After I get the hang of it, I will move up to a better one. Are there fused models, so that if I smoke it I can replace a fuse?
 
In keeping with our usual practice, I think Nelson wants us to tell him how WE use our meters, not just send him somewhere to watch videos. We're here to teach each other, and there is probably more trust in what our members tell or show him than simply refer him to someone else. If we did nothing but point out what NOT to do, preferably with pictures, as always, that would mean more to him. Besides, this is interactive instead of just watching an infomercial by Fluke.


Correct Tony. Show me what YOU guys do, for example to check a motor with one , check your wiring on a VFD, check for continuity on a lathe or mill.
 
Here's a really good site that seems to cover the basics.

How to use a Multi-Meter

Might get you started.

Main idea is to set it to a range higher than what you are expecting to read, unless it is auto-ranging. Analog meters (with a needle) are often better for some tests like intermittants, and they are more rugged electrically than digital meters, important for sparky- motory type stuff.
Most cheap meters have poor probes/test leads, too short and with points not sharp enough to penetrate varnish and corrosion.
I like probes that can accept screw-on alligator clips, very handy and safer.
Mark S.


 
I wouldn't spend the money on a tossmeter, but when you get a good meter, a lifer, spend the money on good probes. My favorites, and I'm sure every serious meter user has theirs, are those from Oldaker. They make aftermarket probes that are very good. Especially if you like or need the insulation piercing type. Pamona also makes great test leads, but I have grown attached to my Oldakers.

http://www.testleads.net/

no connection other than a very satisfied customer
 
For me personally, I would only want a Fluke brand mulitimeter. And yes they are internally fused. I find used Flukes at my local fleamarket all the time. And have had VERY good luck with them still working. They are very reliable. A few with a blown fuse, dirty or broken female pin sockets and missing display segments. But that’s not common and all fixable. Maybe around $60-$125 for used “current” model Fluke mulitimeter. Do not get the older all gray cased models. And only get it with the original Fluke probes/wires (that have not been spark tested:eek 2:)…Good Luck, Dave
 
Nels -
I'd be willing to betcha anything that your meter is indeed internally fused. Shucks, even the Harbor Freight giveaway meters have a fuse inside. You can find it by removing the back cover (not just the battery cover). And don't sweat that you don't have a Cadillac class meter. Any multimeter that can find AC and CD voltages and resistances can be used to diagnose a motor. I do own a good Tektronics meter, but use a Harbor Freight giveaway for most of what I do. And of course, once you get started with the meter you have, you can draw your own conclusions about upgrading.

Disclaimer - I haven't looked at the videos, so I may be terribly redundant here ... but then again, we're talking about some pretty basic stuff.

First, be SURE the motor is isolated from all possible sources of power. Then use a screwdriver or something else with an insulated handle to short across any capacitors.

Switch to a resistance (ohm, Ω) scale. For simple shorts testing, the highest scale is best. For checking windings & continuity, use the lowest scale. Some meters are auto-ranging, so just set to ohms.

SHORTS
Set the meter to the highest available scale. Note what indication the meter gives when the test leads are not in contact, so you'll recognize it during testing. We'll call this "infinite resistance."

Now hold both leads against a bare metal part of the motor frame. You should see something like zero resistance. If not, you may need to scrape through paint, clear lacquer, or crud to make good contact with the metal.

Keep one lead against the metal (or even better, held in place under the head of a screw). Now touch it to each the wires and terminals you can find. You should NOT see zero or small values of resistance anywhere. If so, you have a dangerous short to ground.

Capacitors can be given a very basic "proof of life" test (AFTER THEY'VE BEEN SHORTED to be sure there's no high voltage juice left inside). Place the test leads across the terminals. A good capacitor will very briefly show a somewhat low resistance, then go up to infinite resistance. What you're seeing is the test current from the meter charging up the capacitor. As the capacitor is charged, it no longer accepts current. No current flowing in the leads = high resistance. You can repeat the test a few times by shorting out the capacitor between tests.

OPENS/CONTINUITY
Set to a low resistance scale. Hold both leads together. Note the indicated resistance. It should be a small value or zero. If not zero, this indicates the resistance of the leads and internal stuff in the meter. Note this "lead resistance" for later reference. If far from zero, something is wrong.

Now touch the leads across the ends of the various coils of the motor field. This is best done after disconnecting all wires from their terminals (you DID draw a diagram of what went where, didn't you?), so that you're looking at just one coil at a time. You should get relatively small resistance values (couple ohms at most). If necessary, you can "correct" these readings by subtracting the previously measured lead resistance. Lower quality meters will also have a random factor in the equation, but you can usually get a good approximation of what's going on. Run coils may well have different resistances than start coils, etc.

If the motor has a centrifugal switch, check the resistance across it, both with the switch closed and with it (manually) opened.

Brush-type motors can be given a few additional tests. Check the brushes themselves by placing the leads on the wire leading to the brush and on the commutator segment the brush is touching. You can check the rotor windings by connecting the test leads from one brush wire to the other and slowly rotating the rotor, so that all pairs of commutators are connected in turn. Resistance should be low, but will vary a bit when the brushes are "between" commutator segments.

Knowing how the motor is wired is, of course, very helpful (or let's call it absolutely necessary) for interpreting what you find.

I haven't even touched on the voltage scales, have I? This is probably well covered in the videos. If not, just ask for clarification.
 
This is probably a good approach. Take each basic function of a meter (consider just a plain jane DMM) and one at a time, let's talk about everything you can think of that you can do with that basic function. It's going to get far too confusing to jump from measuring continuity (actually different than resistance with some meters), then move to voltage checks, breaking them into AC and DC, then on to current measurement, and so on. A little structure will make this much easier to understand. We who use meters quite a bit might follow along just fine, but for a basic instruction breakin, let's keep the topic limited to one at a time. If they overlap, let's allude to that and save it for the last when we can talk about special uses.
 
Hard to tell from the photograph of your meter but it does not look like it has a current measuring function, mA or A, on the dial. Do not attempt to measure current with this meter. One of the basic requirements to using a meter is to understand the difference between parallel connections and series connections. Voltage is always done with a parallel connection and current is done with a series connection. When measuring voltage you simply connect one lead to one test point and the other lead to a known common or ground thus placing the meter across or in parallel with the circuit or device being measured. To measure current you have to put an opening in the circuit and connect the meter between the two open points thus placing the meter in series with the circuit. Why this is important is that in a parallel connection, because of the high internal resistance of the meter, only a very small amount of current will flow through the meter itself so it is safe to handle. In a series connection all the current in the circuit will also flow through the meter. If there is a short in the circuit being tested then all the fault current will also try to flow through the meter resulting in a damaged meter and possibly an exploding bomb in your face. Better multimeters will have separate low current, usually marked mA (milliAmps), and high current, marked A (Amperes), inputs. These will usually both be separately fused inside the meter to provide protection. These fuses are usually a buss type fuse but with really oddball ratings. If you ever blow one of these fuses it is important that it only be replaced by the exact same type and size. These fuses can be hard to find and cost considerably more than conventional fuses so the temptation is to just stick a regular buss fuse in just to get the meter working again. If the incorrect fuse is installed you may not have any short circuit or over current protection. If you ever buy a used meter the very first thing to do is check that these fuses are of the proper size and type before ever trying to make a measurement with it.

I can't tell from the photo of your meter but does it have a CAT rating? All test meters manufactured today have a CAT, short for Category, rating of either I, II, III or IV. This is an indication of how much internal short circuit protection is built into the design and construction of the meter itself. Or, in other words, how much short circuit current it can handle without exploding. The higher the number, the greater the amount of protection. You said in your post that you wanted to measure 220 volt circuits. The fault currents in 220 or 240 volt circuits can be much higher than conventional 120 volt circuits. For this type of use you need a minimum of a CAT III rated meter and leads. An unrated, old, meter or a CAT I meter should only be used for making measurements inside 120 volt equipment that is plugged into a wall outlet. If you want to measure three phase circuits than only a CAT IV meter should be used. If you want to see why this is important do a search on youtube for "exploding meters". Yes, a CAT III meter will cost a few dollars more than an elcheapo chinese import but not really that much more. And they can usually be found in most any big box home center store.
 
Back
Top