A Newbie Modder's Guide to the Multimeter Introduction Ahh yes, the Multimeter. One of the most useful parts of the modders toolbox. Can be used to check your wires, check for shorts, check voltages, check resistance, all manner of things! But, of course, you need to know what all those funky symbols mean… and why on EARTH are there more than one socket to plug the probes in?? All will be revealed! This is my meter, happens to be quite a good one too, which can be used for mains testing as well. It’s a Fluke 77II Multimeter, if you can’t quite see in that pic. Hard as nails, and accurate all the way. This guide will go over the main parts of the multimeter, and how they can be used in reference to modding. I decided to put this in electronics rather than Mod guides, as its less of a mod than a quick reference. Table of Contents Introduction The probes The ports Symbols Part 2 Voltage Resistance and Continuity Current Conclusion The Probes If you’re wondering how I got that funky effect round the probes, its actually because I used my scanner to get the picture and used extreme levels of Unsharp mask and contrast! So, the probes. They are the way that you connect the meter into the circuit. Simply touch them to the connections you want to measure over, and read off the display. Obviously, this depends on how you’ve got the meter set up, and what you’re measuring over. Fluke (the people who make my meter) give you a selection of different probes, depending on the current/voltage you’re going to be handling. The higher the current, the more the wires between the probes and the meter will heat up, so the higher rated cables (CAT II or CAT III) the more current they can take (they are thicker, therefore offer less resistance, and the insulation is tougher and has a higher melting point). I’ve got CAT III cables, so I can use the meter in any situation (if I want to test my mains AC voltage, for example…). The Ports Now this part usually confuses most newbies. Why have only two probes, yet several sockets to plug them into? Here’s a close-up for the ports on my meter (which might not be the same as yours) Lets define what these do… 1. The Common port. Generally, the black probe goes in here (negative) and as the name suggests, it’s the common element to all of the testing circuits. Think of it as the ground rail. 2. Voltage, resistance and continuity port This is the one that I use the most. Connect the red (positive) probe to this port when using any voltage readings, resistance readings or when checking wire continuity. If you don’t know what this means, don’t worry, I’ll explain below. 3. Current up to 300mA This port is used for “counting electrons” in a circuit, and thus their rate of flow (current being the flow of electrons). You’ll notice that this side is “fused”, so that you don’t end up melting the meter’s circuits. 4. Current up to 10A Same as above, except it can take more current, as the name suggests. NOTE!!!! – My meter is certified to run on high voltage mains lines and power supplies, but yours PROBABLY ISN’T so don’t go plugging it into your mains socket and blaming me when it explodes in your face! I take no responsibility for what you decide to do with your meter! Symbols This part should help you enough to get you going if you know anything about electronics, and should give you an idea of how the key works. It should also help you if your meter has more settings than mine (which is quite likely with the cheaper ones). This is my multimeter dial. Its got all of the basic settings on it. As you can see, its got several symbols on it, which are common. AC This symbol means AC current. Use this when you want to test something that has AC current running through it. Typically you’d want to test the voltage of an inverter (for cold cathodes or neon’s) or something like that. DC This means Direct Current. This is what you get off a PSU. On a molex connector, the black wires are common ground (both go to the negative terminal) and the red is +5V and the yellow is +12V. Voltage This means voltage (well BUH) or Potential Difference. This measures the potential difference between the two probes. To measure voltage, you need to connect the positive probe to a port that is marked “V” or Voltage. Current Technically, this isn’t right. It should be “I” but since current is measured in Amps and the readout value is in amps, the symbol makes sense. This measures the current that is flowing through the part of the circuit between the two probes (the meter itself). Typically, you need to plug the positive terminal into a port marked “A” or Current. You should put the meter IN SERIES in the circuit to use this feature correctly. Resistance This symbol means resistance, and is measured in Ohms. You can use this setting to measure the resistance between two points, for example over a piece of wire, or a resistor (to check its value). If you don’t have a continuity check, then this can be used to check for shorts. Any value below 0.05 Ohms constitutes as a short, meaning whatever that the probes are attached to is connected electrically. A lot of fun can be had by holding the two electrodes and seeing how resistant the path over your skin is, and see what the resistance of you and all your friends are If you are into ESP, you could always attach the electrodes to your head and see if you can change the value just by thinking about it! But I digress… Continuity This is a weird one, but very handy function. Basically, what it does is put a current through the two terminals (the same as the Ohm-meter function) and if the resulting value is within the “contact” range, it will beep at you. VERY handy for checking shorts, as you don’t need to take your eyes off the work to find a short. Other meters may have a light that turns on for a short. This should help you work out what position you need to turn your dial to do measure what you want to measure. The next few sections are here if you still don’t know what to do, and goes into the details of some specific functions of the meter. If this is put in parallel with a capacitor, you’ll hear a short beep as it discharges.
Part 2 Voltage This part of the meter deals with voltage, or potential difference, between the two probes. For a definition of voltage, go here http://iroi.seu.edu.cn/books/ee_dic/whatis/voltage.htm When using this function, you need to put the meter in parallel. The V with a wiggly line over the top signifies voltage over AC (Alternating Current), and this measures the potential difference of an alternating power supply. It does this by measuring the amplification and the rarefactions of the wave generated by the power supply, thus calculating the voltage. It’s all a bit complicated actually. This function is useful for the modder if you want to check the voltage of an inverter that is powering another component, such as a cold cathode tube. The V with a straight line and a dotted line means Voltage over direct current. This is wholly more useful to the standard modder, especially for checking power rails (and checking how stable the voltages given off by the power supply are...) and also for checking that power is getting to your components (such as an LCD screen). “mV” means milli-volts… and does exactly the same as normal voltage readings except to a larger degree of accuracy. Probably not very useful to the modder (but might be, depends on what he/she is doing ) Resistance and Continuity Your multimeter is your friend for checking your connections and wires. If you don’t know what resistance is, I suggest you go off and learn Ohm’s law: http://www.grc.nasa.gov/WWW/K-12/Sample_Projects/Ohms_Law/ohmslaw.html (See, even spacemen need to know Ohms law, come on, what’s your excuse??) So, from that brief interlude, what can we conclude? In order to measure resistance, we need to know voltage and current. And you can’t measure current that isn’t there… so the meter provides voltage AND current (a very, very small amount). And so, it calculates the current flowing between the two probes, and the voltage between then, to calculate the resistance. The meter should be wired the same as with voltage – in parallel. But anyway! What can this be used for? Checking the continuity of wires. Simply touch one probe on one end of the stripped wire, and the other on the other, and if it beeps or shows a reading of less than 0.05 Ohms, or beeps, its connected cleanly. If its anything (significantly) above that, then there is a fault with the wire, in which case you should replace it. Most of the time it’s the connectors that are at fault in a cable, rather than the wire itself. Also, you can use it to check for shorts when soldering. E.g. on an LCD screen you have many pins close together. If you touch two adjacent pins, you can see if they are contacted – read the resistance, or listen for a beep on the continuity check. Another LCD related task… you can use this to check pin-outs, and work out how the colour coding works for a pre-made wire. Just touch one probe onto the pin you want to map, and check every wire that’s part of the cable. When you get to a wire that “beeps” or has a resistance of less than 0.05, you have found the correct coloured wire. Current This part of the meter measures the current. This can be useful for measuring current draw of components, but to be honest I’ve never used in while modding. Again, the wiggly line signifies AC and the straight line and dotted line means DC. This is measured in Amps. To use this function, the meter must be in series with the circuit you are measuring. If anyone wants to add something more useful than this, please reply and I’ll include it immediately Conclusion That about wraps up the multimeter guide. I hope this answers a few questions about how to use a multimeter, and what to use it for. Like any tool, you need to work out how to use your meter best, and work out how its labelled. The key shown here is just for my Fluke multimeter, but generally speaking they are all similar. If you think there is anything I’ve neglected here, or something that I’ve missed that’s on most multimeter but not mine, then reply and I’ll include it in the guide. Links http://forums.bit-tech.net/showthread.php?t=61317 - an excellent modders guide to electronics, which I hope this complements!
I kinda knew most of that but thanks for the guide, I'm sure it'll help someone out. I use my dad's Fluke for my metering work so I've gotta be careful... I think it's about $300 to replace But it's accurate to like 4 decimal places. His is some old-school one, only goes to 2A and has a really weird way that it does the voltage and resistance readings (the scaling is really confusing). As far as the AC checking goes, it's really only useful for checking the output voltage of a cold cathode inverter. Be *really* careful doing it though, it should put out just under 700vAC. I've done it myself and I wasn't making good contact so it was like a mini lightning bolt complete with a very annoying (and rather scary considering how close I was) buzzing noise. A LOUD one not that occasional hum some inverters give out. Of course I've also blown up an inverter before because the fool who made it somehow got the + and - inputs switched!
Yeah its basically a sorta newbies guide, because in the mod guides section somebody asked for it, as nobody seems to know how to use a multimeter any more? but anyway. Yeah.. this one can go up to 1000V AC with these probe cables, (the CAT III ones) but that doesnt mean you wouldn't get arcing if you wern't too careful. I was rather tempted to put that its only ever useful for measuring cold cathode invertors, but I'm sure there are some other things it could be used for, I've just not come across them yet.
Another thing to explan might be True RMS and non-RMS readings for AC. For example my multi-meter reads 85 some volts coming from my UPS when I am using the UPS for power instead of mains, since the UPS outputs modified sine instead of a true sine wave. But there is no apparent change in a light hooked up to the UPS, and the computer still runs, even though the minimum input is 90Vac. So I assumed it had to do with the meter. A little clarification on that would be helpful. Also should you measure pulsed DC as AC or DC, since it could be thought as AC with a DC bias?
Thats a good thing to bring up. I've no idea myself actually, in fact I'm not completley sure exactly how it gets AC readings anyway, so I better do some research and clarify it a bit Thanks cpemma Hopefully it'll be useful to the people in mod guides who were asking for it
Actually, many DMM's have a select button for AC or DC reading. Or and I not reading you right and your saying that you don't understand how it can 'understand' AC readings?
AC RMS values are alot different than just measuring the 'maximum' voltage. If you have an expensive multimeter, it may find the 'true' RMS value by performing the calculus equations required. However cheap multimeters usually output a 'pseudo-RMS' value which is found by finding the maximum part of the sine wave and multiplying it by 0.707. The problem with doing this is that it assumes your dealing with a perfect sine wave. If it's a nice sine wave, both methods will give the same value, if it's not, then you'll get very different values.
Ah okay... I see. What I was saying is I don't know how it calculates the RMS of the wave that is input.
