EVERYONE Needs a Multi-Meter

A multi-meter can save the average person a lot of money over a few years, even though ladies who choose to buy one may not carry it in their purse when going out for the evening. You will often be able to solve many problems yourself in less time than it would take to get a technician to come to your home. This Instructable will show how to do that simply and easily.

I often meet handy people, both men and women, who do not go near to anything electrical because they are frightened of electricity. It is good to have a strong respect for electricity. It is also good to know how to make basic electrical measurements around the home. A multi-meter is the tool every home ought have. They eliminate the guess work from so many things and can save a lot of money. 

A suitable multi-meter for occasional use around the home and automobile can be had for less than $ 5. This link is to a very inexpensive meter that includes some functions found only on meters costing twenty times more just a few years ago. More expensive meters do not necessarily have more features. They give greater accuracy for professional work, and they are more rugged in their construction and durability.

Step 1 Will I electrocute myself?

No, that will not happen if you follow a few simple precautions. Most home uses of a multi-meter will be done with the power disconnected, or with very harmless low voltages. Regardless, you will always handle the probes by their well-insulated, completely safe plastic "handles." Usually, you will hold one in each hand.  

The black probe is normally associated with the ground or negative terminal. The red probe is normally associated with the "hot" or positive terminal. For household uses illustrated in this Instructable you can ignore all of that and use either probe on any terminal. 

Comments

[Taerzik]

Thank-you for putting this together. I agree, everyone needs a multimeter (and the knowledge to use it of course.) I only got one a couple years ago and I'm still not fully proficient with it but it's been a really handy tool. I'd say most folks should know how to do basic soldering as well - it's pretty common to hear about common electronics that fail due to a faulty component, often a capacitor and how DIY'ers figured out how to fix it. Just the other day I re-soldered a loose capacitor in a faulty power supply - saved myself somewhere between $10 -30.

[Phil B]

Thank you for looking at this. Soldering is a very useful skill. One of the most helpful things I learned about soldering was to heat the parts of the joint so that they, rather than the soldering iron, make the solder flow. That is still a good principle, but now many of the semi-conductor parts are very heat sensitive and care is needed to avoid ruining a component. I yielded and added a low-wattage soldering iron to my old soldering gun. Congratulations on saving your piece of equipment and saving some money.

[nowuknowjack]

A couple of safety tips.
If you have to open a breaker panel/bus box, BEFORE you open it, touch the door with the back of your hand. An electric shock will cause you muscles to contract so if, by chance there is a problem and the panel is HOT, the electric current will force your muscles to draw your hand AWAY from the hot panel. You will get a shock, but will live to tell about it.
Next, in choosing a multi-meter, get the highest category you can afford. The higher CAT rating the better protection if something goes wrong. The higher protection will help keep the multi-meter from exploding due to a power surge. CAT IV is the best option if you can afford it.
Finally try to hold the leads in ONE HAND when testing higher voltage circuits such as 110 AC. If something goes wrong only your hand will be affected. If you happen to be holding the leads one in each hand and there is a short, the path of least resistance is from one hand to the other and your heart just happens to be in that path.

[nowuknowjack]

I see the one hand rule and CAT ratings were covered in earlier comments, sorry for the repetition. One other suggestion if opening a panel is to wear safety glasses or a face mask and cotton...NOT POLYESTER...long sleeve shirt. YES I am a safety freak, but my college instructor could not emphasize safety enough... "Gentlemen, when I was growing up in Argentina, they gave me a light bulb and two wires to test circuits. You must know how to do this safely and memorize this information so you do not get electrocuted!"
I have a couple meters and I ofter use my Simpson analog meter for certain situations where a digital meter is not fast enough to register an electrical pulse such as in a fuel injector system.

[Phil B]

Thank you for your comments and for your emphasis on safety.

[AntMan232]

Great instructable, I used a multimeter only this morning. I am 15, and a great fan of the AVO8 analogue meter, which I picked up for £2 at a car boot sale.

[Phil B]

Thank you and congratulations on getting a good bargain.

[80$man]

Excellent Instructable! 5 out of 5!
Everyone must have a multimeter, it is a very good learning tool!
I have got 'n poor/cheap one 'currently'... must find a replacement, any suggestions? 3-phase capabilities would be good...

[Phil B]

Thank you. I am not aware what makes and models are available in South Africa, although I am often surprised that the same things are often available in other nations that are available in the USA thanks to international marketing. In the USA the name Fluke is very well-respected for excellent meters of a professional grade. I wish I could be of more help.

[80$man]

I found a Fluke supplier in Cape Town, 35min away! Thank you for your reply!

[Phil B]

You are welcome. I hope it works out. Fluke meters are not inexpensive.

[ac-dc]

It is not true that corrosion could be a problem if it is not visible. For there to be enough corrosion to cause a problem it is progressed to the point it is very plainly visible and there is no need to check it with a multimeter as it is obviously in need of cleaning.

[askjerry]

Sorry ac-dc but I am a Senior CET with 30+ years of experience... and I have seen equipment fail because of a micron or so of corrosion build-up. The metal looked shiny, everything seemed normal... hitting the contacts with some emery cloth and a bit of denatured alcohol fixed the problem. (Or sometimes a rubber pencil eraser... keep one in your toolbox!)

Depending on the current going through a circuit... it may only take a tiny film to provide enough resistance to cause a problem... in some circuits the addition of as little as 1K ohm is enough to cause a failure.

Jerry
http://en.wikipedia.org/wiki/Electronics_Technicians_Association

[Phil B]

Last week I had a great example of the point you are making (i.e. corrosion not visible to the eye can cause a circuit to malfunction). My '99 Oldsmobile Alero has an electronic instrument cluster that sometimes refused to come to life recently when I turned the ignition key. On some GM cars that symptom is the beginning of problems that will worsen until the car will not start. I fully expected I might be forced to spend for a new car. In my case, a set of unsoldered metal on metal circuit connections had oxidized enough over 12 years to keep the instrument cluster from operating. I cleaned the terminals and bridged them with solder. You can view the resulting Instructable here.

[johnny3h]

Howdy askjerry.  Thanks for the comeback to ac-dc as his comment could send a novice on a lengthy time-wasting chase "around Robin Hood's barn" by ignoring connections that "look" good.

I have been down that road myself, and what you describe is definitely a commonly occurring problem.

[sschoemann]

OH almost forgot... they are getting almost impossible to find these days but I also carry a couple old fashioned typewriter erasers (ink eraser in a wooden pencil type stick) for cleaning contacts, they are abrasive enough to clean the metal contacts but not so that they remove metal, and since they are sharpened in a pencil sharpener they are pointed enough to use on some pretty small packages.

[Phil B]

Thank you for your comments. And, the typewriter eraser you describe is dielectric so you can use it on "hot" terminals. Your comment about lead battery contacts being phased out reminds us that if something works the federal government is always ready to ban it for our good until they make a mess out of everything.

[sschoemann]

Jerry You speak gospel! Prior to becoming a full fledged EE I to was a CET (ISCET). I discovered a fantasitc product worth it's weight in gold on any socketed (plug in) type device that I have used on car batteries since I always have it on hand despite it's cost... DEOXIT. I keep the pen dispenser in my kit and the bottle on my bench (I'm a Ham radio operator, not one of those idiot design engineers who forgot what it's like to work on equipment in the field). It works and thats all that it takes in many instances... I cant count the number of times I've pulled a prom, eeprom, or any number of chips or micro procs, cards or so on that were flaking out, applied the deoxit to the contacts, reinserted them and all wierdness was completely eliminated for evermore. Not only does resistance cause problems but a minute amount of oxidation can act like a diode and even though it will conduct, it will "rectify" a 'stray' radio signal or two adding enough noise to really screw things up at the most Murphy's Law moment.

[ac-dc]

The devil is in the details, I was writing within the context of the prior post. On that topic (battery and lights), you will not have sufficient corrosion to cause any problems if you cannot see it. The contact area is sufficiently large, the clamping force too, that even an invisible film is not enough resistance to be an issue with lights consuming only a single digit # of amps.

Of significance is also the metals used for the battery and clamp and the nature of the corrosion causing discoloration that would be visible. Further, if the film is so slight that it can't even be seen, the abrasive action of taking the clamped connector off and putting it back on alone would have cleaned it sufficiently for a light load like lights.

You wrote "1K Ohm" as being a small resistance, but between a battery terminal and the clamped connector where you cannot even see any corrosion, you will not find even three orders of magnitude lower resistance... less than 1 ohm.

As for dissimilar circuits and different connectors, I do agree in those circumstances a contact can be fouled by corrosion or other contaminants and cause a problem without it being visible, particularly logic circuits where you end up with a capacitive junction.

[sschoemann]

Actually I've measured considerably more resistance on fully functional battery clamp/post connections the worst I found was a full meg ohm. The residue from sulfuric acid is very resistive, often with the only conductive path being a single arc or two. Living in Wisconsin, the dead of winter proves these situations time and again, where a simple dirty post and clamp are the difference between the battery charging and going dead and freezing. This is going to become much more apparent as time goes on as autos become more Rohos compliant, as the lead battery clamp is now mandated for extinction and will be replaced with plated or tinned brass or copper clamps which will not hold up to the corrosion.

[ac-dc]

Car batteries are exempt from ROHS, the whole inside of them is LEAD plates. Other car electronics are also exempt. Note how ROHS was already implemented in the EU, changing global design materials where they are affected, but they are still lead.

With enough (a visible level) corrosion, you could measure 1 Mohm, but it would not be a "fully functional battery clamp/post" anymore. The key to the battery post and connector is that when you tighten it down, the areas where mechanical stress occur are the primary current path. If there are air gaps elsewhere, you'd have corrosion sooner in that area but should measure where the connector friction scrubs away the surface layer on the contact point.

[sschoemann]

Actually there has not been an automotive battery made with true lead plates in decades. They are currently manufactured with a lead grate, and less and less lead is used every year as larger and more powerful alternators are taking over the demand for continious electrical power, and the battery is no longer acting as a reserve capacitor while the vehicle is running. And the EPA is targeting the car automotive battery due to the health and environmental concerns involved in the recycling of them... Give the government an inch and they WILL take your testicles...

[static]

Wow nice misleading rant. No mention that the newer plate construction is a major reason that batteries now have more cranking capacity. in the same package. Auto starting batteries are a comprise, and where never, ever intended to handle any large part of a car's electrical load for anything but brief periods. Alternators have gotten larger because of an increase of the continuous electric load not because of less lead in the battery. Just as the hazards of manufacturing LA batteries have been been mitigated so have the hazards of recycling them been mitigated. You are creating a bogeyman about recycling LA batteries to lodge a complaint against the EPA. The stamped metal battery terminal clamps where being used on some imports before  RoHS.  Judging by the longevity of the brass battery clamps on old tractors I have had to use, brass would be an acceptable substitute for the lead clamps. The only advantage with the softer lead is the ease of restoring a clean contact surface.

[skrubol]

1k ohm? Do you mean 1m ohm?
With car batteries it is pretty rare for corrosion to cause problems without being plainly visible. There is a reason the terminals are typically made out of lead and not a better conductor like copper.

[askjerry]

Mo, I meant 1K ohm.

Depending on the circuit you have... 1K ohm can make a huge difference. Let's look at an LED running on 12 volts.

An LED uses 2.2 volts and takes 20mA... or 0.020 amps. (typical)
So you take 12v and subtract 2.2 volts... that leaves 9.8 volts that needs to be dissipated. Okay... 9.8 volts divided by 0.02 amps is 490 ohms. Now... add 1K of resistance from corrosion... 1,490 ohms total.

9.8volts / 1,490 = 0.0065 amps... or 6.5 mA. That is a WHOLE lot less current than the LED expects... so it barely glows if at all. Now imagine a microprocessor that is connected to an input via an optocoupler... and the LED above is the one for the optocoupler... see the issue?

Jerry

[skrubol]

1k ohm is huge for resistance (no such thing as a K ohm btw, lowercase is the proper terminology for kilo.) A car battery with a 1 k ohm terminal resistance would basically read near 0 volts from just the indicator lights in the car. Even 1 ohm would not let the starter budge.
I was referring to 1 milliohm, not 1 Megaohm.

[Phil B]

I am sure I remember reading that not all corrosion is visible. Before responding I did a search and found someone had problems with corrosion he could not see without removing the battery connectors. I have found the newer side post terminals can be quite corroded, but the corrosion remains covered and not visible from a quick glance under the hood.

[ac-dc]

I cannot speak for what anyone else meant, but when I think or write about visible corrosion I do mean visible with/after the connector (is) disconnected.

[NaturalCrafter]

You are right. I have watched my husband working on a small circuit board that would not work he took out the battery and used very fine sand paper on the contacts, suddenly the thing just stared working right again. This always amazes me. I have to say my first experience helping to fix something was my grandmothers toaster would not work. My dad took out his meter, tested good, then tried another plug and looked inside and scratched his head. The two copper leaf contacts were only very slightly brown like a dull penny. I offered maybe an eraser on the contacts. So he chuckled and let me try it. Then when we plugged it back in and it lit up, he was amazed...My dad was an electrical engineer...only time I ever got over on him as he was sure it was a broken internal element...lol

[ac-dc]

Apples and oranges, brass and copper especially degrade much more resistively and faster than lead, which is why lead is chosen, BUT we were speaking about visible corrosion too - brown like a dull penny is considered visible corrosion on copper.

[Phil B]

If you are familiar with electrolytic capacitors, those are made by allowing an oxide to form on one side of the aluminum foil. It serves as an insulator and has the advantage of being thinner than paper or mylar film. It is a powerful reminder that oxides look like conductive metal, but actually insulate. Your father's experience with the toaster shows we look for what we expect to find, and sometimes that blinds us to what is.

[79spitfire]

This is true, corrosion can "hide" in the contact areas between the terminal and post. What your doing is called a "voltage drop test". It's a quick and easy way to locate where a bad connection is raising havoc. There are several good videos showing how to use a voltage drop to solve real world problems, including cars that won't start with battery terminals that look clean!

[Phil B]

Thank you. I assume the videos are at YouTube. Is that correct?

[79spitfire]

Yes, and they scattered about the web as well. Several auto manufactures are teaching their factory techs how to use voltage drops to more effectively repair cars and especially computer circuits in cars.

[Phil B]

I first became aware of the voltage drop test for auto battery connections testing from an old Motor's Auto Manual. That particular manual was published in 1969. Thanks.

[pfred2]

It depends meters can also lead to even more expensive service calls as well contingent on the outcome.

20 years is about the time frame when Rat Shack made the corporate decision to put their weight behind meters as opposed to oscilloscopes. There was a time when The Shack sold scopes too. I often wonder how different the world might be had they chosen otherwise?

They're a poor excuse for a phone kiosk now so it serves them right.

/me hugs his TEK 2336 with the Y option ... Oh yeah!

[Phil B]

This Instructable is for the person who feels inadequate to do more than change a fuse. I have played with a virtual oscilloscope, but do not feel like I know what I am doing.

[johnny3h]

I'm an electrical/electronis hobbiest with less ability than you and do appreciate your efforts to "keep it simple."

From reading the comments transactions to this point it's becoming obviousl that there are some people who just refuse to stay with your goal, and instead have to "display" their expertice!!!!!  Such a shame.

In order to not miss something in the comments that would be helpful to me, I am forced to wade through all of these wise-acre comments, which as you alluded to from the beginning,  are extremely intimidating AND confusing.

It would be nice if these "butinskys" would just stay out of 'ibles intended for the novices.

Again, thanks for the efforts to keep it simple.

[static]

Respectfully I believe most hobbyists are capable of recognizing, and ignoring comments go beyond they want to learn. The problem with expecting the buttinsky to butt out, those hobbyists who desire to learn more would miss that opportunity if the those leaving the more experienced did butt out. You yourself indicate you read the comments in hopes to find something useful.

[pfred2]

You can lead a human to knowledge but you can't make them think.