MultiMeter Basics

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****Note: I am not an expert, nor do I have any official training. I just have some basic knowledge and plenty of experience doing what I cover in this article. Following the steps outlined below should not pose any danger though nonetheless, all precautions should be followed. If you have any doubt, do not do it. Consult an expert or experienced person in the matter if possible. The author, Overclockers.com, nor any other affiliated entity will not responsible for any damages, injuries or consequential damages that may occur from the information contained herein****

“What the? Why won’t my computer turn on? Crap! Did the power supply die?”

Pretty much anyone who has owned a computer has been in this situation. But sometimes there just isn’t that handy spare power supply laying around to double check your suspicion. Thus it is always good knowledge knowing how to test a power supply. But how do you test a power supply? This seemingly magical act can actually be performed multiple ways.

One way it so simply buy a power supply tester online (as most computer gear stores don’t carry them, at least in my neck of the woods). Most of them are very simple devices – plug in your main 20/24 pin power supply connector, make sure all the lights come up green and you’re good to go. Coolmax has recently (to me anyway) released a very cool upgraded power supply tester – you plug in your main power connector as usual and it displays the voltages on an LCD screen for you.

You can check it out on Newegg.

But that involves getting online, buying the item and waiting for it to ship – not always convenient. Thus, a good backup plan is knowing how to test a power supply the alternate way. Best of all, you can use this knowledge to test many other things that give off power! We will cover some these other common items at the end of the article.

A Multimeter!

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There are also old fashion ones with a needle. What is more recommended these days (at least from the people that I have talked to) is a digital multimeter. They give a very clear reading from the LCD so there is no room for ambiguities, unless of course the reading is rapidly fluctuating – but that’s beside the point.

A multimeter is a simple electronic device that allows you to test the electrical characteristics of Voltage, Amperage, and Resistance (Ohms). All that I will be covering is the Voltage aspect, as it is the most easily identifiable piece of info that you can read from an electric piece of equipment.

MultiMeters are pretty cheap tools luckily, and they have many applications beyond computer usage. If you shop around your local hardware stores, you can usually find one in the 10-15 dollar range that will be more then adequate for our needs. The digital one pictured above cost me something about 12 bucks at Menards (regional home improvement store).

Alright, first a few simple tidbits on electrical symbols:

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This is the Greek symbol Omega – it stands for Ohms, or resistance. This is something that we do not cover in this article, so do not worry about it.

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The capital A is usually used to denote Amps. This is a very important number to be aware of as anyone who shops around for a good power supply knows; however, again, it is outside the scope of this article.

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This is the symbol for Alternating Current, AC. This is the kind of power coming out of a wall socket. In America its 120v at 60 Hz (cycles per second, thus the squiggly line, as the current flows up and down, so to speak). In Europe it is 220v at 50 Hz. Thus American and European equipment either needs to be designed to auto-detect the incoming voltage and compensate automatically, or there must be a toggle switch (that little 120, 220 red switch on the back of your power supply) to manually select the range; lacking these optiosn, you must use an adapter to convert one standard to another.

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This is the symbol for Direct Current, DC. This is going to be the most common mode we use.

For more information on all these terms and what they mean, see one of my favorite places to go that can help give fundamental knowledge on almost any topic one can think of is Howstuffworks.com.
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When testing anything with a multimeter, you must determine what range you are going to be dealing with. In the case of a power supply, we are going to be testing Direct Current Voltage. A power supply has several different voltage ‘rails’ – 12v, 5v, 3.3v, -5v, -12v.

Thus, we should never greatly exceed 12v of power (at least, under normal circumstances). In that case we simply set our multimeter to the next step above 12v; in this case for my particular meter, it would be 20v. With it set to 20v, the meter will be able to safely handle any voltage below 20v and give a pretty accurate reading (all meters have a margin of error, but it’s usually pretty small).

There are two different ways we can test the voltages that a power supply is putting out: The first way is called ‘back probing.’ You simply pop open the side of your case, jam your black probe into any black wire you see and leave it there, and then take your red probe, and touch it to any wire you want to touch, like so:

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You should see a number appear on your meter. If it’s fluctuating wildly, make sure you are making good contact with both probes, and if it persists, that should be a hint about the condition of the power unit.

Otherwise, it should be a fairly solid number. But what do you know is an acceptable number to be reading? Why, we just turn to our trusty wire guide so kindly illustrated HERE for 20 pin power connectors and HERE for 24 pin power supplies. They are very similar but be sure to be familiar with the differences in both.

Some wires return funny numbers, like the green wire. This is normal. However from my experience, sometimes a -5v line doesn’t seem to be there. To be quite honest, I do not understand this. Sometimes when the -5v line isn’t working, neither will the power supply, but on certain power units, especially microATX power supplies, the -5v line is sometimes missing all together but everything operates just fine. I have not been able to find any literature on this circumstance, but if anyone can point me in the right direction, I would more then love to update this article with more accurate info.

You can take your red probe and probe all of the main power connectors to verify their voltage. You can also check the readings on your molex connectors as well. Simply take your red probe and stick it inside either the 5v (red wire) or the 12v (yellow wire) and see what you get.

This method is really only preferred when you think your power supply is flaky, not failed. If your computer is not booting and you think your power unit is to blame, you typically do not want to use it on any motherboard until you verify that it is OK to reduce the risk of damage.

With the power unit turned off and unplugged, stick one end of a paper clip into the green wire and the other end into any convenient black wire. When a paper clip is in this position, it forces the power supply to turn on as soon as it is plugged back in. This is perfectly safe to do for a limited time; no more then 10 minutes should be a limit. Staying away from the paper clip (touching it shouldn’t really hurt you as the amount of electricity flowing through it should be fairly low, but no one likes a jolt nonetheless) you can do just as you did before – jam your black probe into a black wire and test the remaining wires with your red probe.

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The chances of your 5v rail reading at exactly 5v is really pretty slim. The common rule of thumb is +/- 10% of the voltage you are testing is an acceptable range. Thus a 5v rail should test somewhere between 4.5v and 5.5v. (Though to be honest, this seems like too large of a range for me.) I like to go with +/- 5% on the voltages. Thus a range of 4.75 and 5.25 is more comfortable for me. Anything far outside this range is not good. Anything that is flirting with the edge of this range is also a possible indicator of something not working right.

And voila! You have just tested your power supply with a multimeter!

This is a very practical skill, and as such, can be applied to other circumstances:
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Laptop Adapters

Ever had a laptop that seemed to have issues charging the batteries and you would have to fiddle with the AC cord in order for it to work right? There are two possible issues for this circumstance:

The first is a no fun prospect of a damaged DC power jack. This is a little box inside of the computer that your AC Adapter plugs into in the back or side of the laptop. If this is damaged, you are pretty much stuck to taking it to either an electronics repair shop or a local mom and pop joint that hopefully has the experience and knowledge for this kind of repair.

For people with some electrical knowledge and good soldering skills, it’s a fairly straight forward repair although dissasembly is NOT!). I know in my town of 200,000 people there are only 3 places that I know of for sure that can effectively do this repair (luckily, I work at one and I personally know the owner of another one.)

However, if you’re lucky it could just be a flaky AC adapter! Well, sometimes not so lucky, as a new adapter can be a hard to find or rather pricey to replace. Though this is a very simple thing to test with a multimeter. A close inspection of the brick part of the adapter should yield some numbers to you.

It will have an Input Voltage. This should always in the range or 110v 2A, as this is what it is drawing from the wall socket itself. What is more important is the Output Voltage. This is the voltage that the brick is supplying to the laptop. A typical number would be something like 19V @ 3.13A. A quick bit of math for you:

Wattage = Volts * Amps

Thus, you know that if this adapter were being used at full load, it would put out a max of 62.7 watts. Good info to be able to figure if you ever wanted to estimate how much of your electric bill is coming from your laptop.

Moving on, you should also see a little diagram somewhere on the power brick’s case that looks something like this:

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This is a diagram of the polarity of your AC Adapter plug. To make this a little more clear, let’s take a look at this picture:

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The inside of the adapter (where you should see a little metal ‘post’) is positive, and the outside metal part is negative. Thus, I would stick my positive probe (red) inside the plug and my negative probe (black) on the outside of the plug while making sure that my multimeter is set to the next highest voltage above what my rated output is.

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And that is all it takes to figure out if your AC adapter is putting out the proper amount of power. Again, the same rule applies with AC adapter voltage as with the power supply – a range of +/- 5% is good. If your number comes out negative, you just got your probes mixed around; no worries, it doesn’t hurt anything. Just switch the probes around and you should get a positive readout.

It is always a good idea to play with the wire some. Bend it, twist it and just generally be rough with it, all while holding the probes in place (a second person is handy for this). If there is wild fluctuation in your voltage, then either you didn’t keep good contact with the probes or you have a short in the wire.

A short would be bad as it would cause your adapter to only work when you fiddle with it. Sometimes if you are lucky, the short is simply a break in the wire somewhere. If you can locate that break, you can just chop up the wire and then solder it or in some, but safe, fashion, patch the wire so life is good again.

This technique isn’t just for laptop adapters; it’s for any adapter that exists. However with other adapters sometimes the plug is too small to get a probe inside the tip. Don’t worry as there is a backup plan. Simply to take a paper clip, jam that inside the plug and place your probe against that instead. It does throw off your reading some, but usually not enough to make any real difference.

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Miscellaneous DC measures

Alkaline Batteries

We have all been here before – you need two good batteries for something and you have four batteries. You play the swapping game hoping to get two that work. Well, now that you know how to wield a multimeter and test voltages, by god, use that skill. Simply take your battery and look for the polarity markings and voltage output (typically 1.5v for the A series batteries).

And test your battery like so:

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If your battery is way below what it’s rated at, it’s a goner. If it’s real close to its rated voltage, you know you have a good battery.

Car Batteries

Car won’t start? Hmm… wonder if it’s really your battery before you go through the trouble of jumping it? Just test it like so, and know for sure if it’s your battery:

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Testing AC Power

*Note: The following can potentially be dangerous. If you are unsure about anything, DO NOT ATTEMPT! Seek professional or experienced help*

Got that wall socket or surge strip plug that doesn’t seem to power things that are plugging into it, right? This is an item that is very easily tested with a multimeter. If you live in America, set your meter to the next step above 120v AC. Remember, this is a wall socket, so now you are measuring AC power not DC, so you will have to adjust your meter to a completely different range to ensure nothing bad happens to the meter, the plug, or more importantly – you!

Simply take each probe and shove it into any of the slits like so (polarity should not matter on a wall socket) with your meter set appropriately:

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NOTE: DO NOT TOUCH the metal part of the probes!!

And there you go – you should see the voltage coming out of your wall socket. Wall sockets tend to put out a fairly large range of voltages. If you are between 105v and 130v, you should be sitting pretty good. Anything else way out of whack from that, you either need to replace that surge strip or call an electrician to come and check the socket for you.

Knowing how to use a multimeter is a great life skill to have. You can use it in many different circumstances and save yourself all kinds of trouble.

Charlie

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