Way back at the beginning of the book, I promised to show you how to test the cables you create. To do this, we ’ d typically use a gadget called a
‘digital volt-ohm-milliammeter’. Since saying that whole name repeatedly quickly becomes tedious, we use the acronym DVOM. It is also called a DMM, for digital multi-meter. Both names are good, describing the unit accurately. The older version of this meter, with the swinging needle, was called a VOM, as it ’s not digital.
I ’ ll show you one of these shortly, and also give a detailed description of some of its typical functions. But before I get so specifi c, I ’ d like to explain why you might want to use a DVOM to help you in your work.
All DVOMs measure electricity in at least three ways. First, they measure voltage: DC (direct current) voltage and AC (alternating current) voltage. As you may have guessed, voltage is measured in volts, multiples of volts and fractions of volts.
Second, they measure DC resistance, which uses the ohm as a basic unit of measurement. The same principle applies here: we can have fractions of an ohm and megohms (millions of ohms).
If you ’ re curious, AC resistance is called ‘impedance’, and is too complex to deal with here, as it is frequency dependent. Nor is it something you need to know about to do good wiring.
Third, DVOMs measure current – which is the amount of electricity passing a certain point, as opposed to voltage, which is the force with which the electricity is passing a point.
Think of the difference between a water hose with a nozzle that focuses the spray to a tight stream, and the same hose without the nozzle, with the water just gushing out. The hose without the nozzle may pass more water (current), but it will not have the force (voltage) of the tight stream from the nozzle.
2.2
The basic unit of measurement for current is the ampere, or amp for short.
But an amp of current is actually quite a lot, so we often deal in amounts as small as thousandths of an amp – or milliamps. From this comes the name
‘milliammeter’, which is a meter for measuring very small amounts of current.
Now you know what all the letters in the acronym DVOM stand for. Kind of makes you feel glad all over, doesn ’t it?
Fancier DVOMs add more functions, signal frequency, temperature meas-urement, audible tones and lights. But all will offer the basic functions of measuring volts, ohms and milliamps.
What this means is that we can check our soldering with the ohms function, check AC and DC voltage and, if the need arises, we can check the current fl owing in a circuit.
For our needs, I ’ m going to skip current measurement, as you can solder thousands of cables and never use that function. Google on ‘ using a DVOM ’ or ‘ using a DMM ’ if you want to learn more.
A typical DVOM is shown in Figure 2.2.1 . Before going any further, I ’ d like to quote from the owner ’s manual for this DVOM:
‘ Improper use of this meter can cause damage, shock, injury or death. Read and understand this user manual before operating the meter. ’ Nor does the manual exaggerate;
you really can kill yourself if you don ’t use a DVOM in a safe manner.
So my overview here is no substitute for reading the instructions that came with your DVOM. As the acronym says, ‘ RTFM ’ – which stands for ‘ Read The ******* Manual ’ ! Nor is my short explanation any substitute for common sense. Keep your hands away from electrical contacts and wear rubber-soled shoes. Have someone nearby with an insulated tool to knock you free of high voltage/current contacts if you ’ re doing such measurement.
Above all, think, plan and act carefully.
LCD display
Now that I ’ ve (hopefully) scared you into using it carefully, let ’s have a closer look at our typical DVOM. This unit costs about $100 and comes with lots of nice features I won ’t explain, because I want us to concentrate only on those features needed to wire cables and test them.
Our example is an ExTech model 22-816 that I got at Radio Shack when my old DVOM died. It ’s an RMS unit, which in a nutshell means that it measures voltage by averaging, and may have a built-in error value.
Peak reading DVOMs are more costly and not needed for testing cables.
Don ’t feel that you ’ re obliged to buy this, or any specifi c model of a DVOM, but you will need one of some kind to test your work. Shop around, and fi nd a model that suits your needs and budget.
Since the LCD panel is exactly that, let ’s start by examining the controls below it. Then we ’ ll go into using the three types of measurement useful for wiring: DC resistance, DC voltage and AC voltage.
On this model there are four option buttons ( Figure 2.2.2 ). From left to right in the fi gure these are:
● Hz/duty cycle button – for wiring, we can ignore this button. With the DVOM in Hz/% mode, you can select Hz for frequency or % for ‘ duty cycle ’ – which can be loosely defi ned as the amount of time a signal is present vs. the total time being measured.
● Range button – this DVOM is auto-ranging, but you can select a specifi c measuring range by toggling this button, e.g.
ohms or megohms. This is useful mainly for precision measurements.
● Hold button – this will ‘ freeze ’ a measurement on the LCD screen. Press once to freeze, once again to release.
● Relative (REL) button – this acts like the ‘ tare ’ function on a scale. When you press it, any residual voltage or test lead resistance is zeroed out, leaving the meter displaying all zeros, and ready to measure. This is analogous to the tare on a scale, which is set to zero out the weight of a bag holding the items being
weighed. Figure 2.2.2 Option/Mode/Light buttons.
The two other buttons in this section perform different functions. They are:
● Mode button – used in combination with the ‘ diode ’ function on the rotary function selector dial. ‘ Diode ’ is symbolized by a right-pointing triangle combined with a sideways cross (Christian style). In Figures 2.2.2 and 2.2.3 , the function selector is pointing to the diode function. When diode is selected, toggling the Mode button switches between diode check and a DC resistance measurement that beeps when the resistance is low. Diode check is not useful for wiring, but a beeping ohms range is very useful, as you don ’t need to look at the DVOM to know that you have a good signal path.
Figure 2.2.3 Function selector switch.
● Light button – turns on a backlight for the LCD display. On this model, you have to hold down the Light button for a second or two to activate the light. This feature helps to keep the light from being turned on by an accidental bump.
Next up, or down, if you realize that I ’ m describing the DVOM in descending order of the controls, is the rotary function selector dial.
One might say that this is the ‘ heart ’ of the DVOM, since it ’s by spinning the rotary function selector that we can adjust how and what the DVOM measures.
For our purposes, only the four functions on the left need attention. They are VDC, VAC, ohms and diode check/beeping ohms. Oh, and ‘ off ’ too, so we save the charge of the 9 V battery inside the DVOM. Make that fi ve functions.
For all your work with a DVOM, the red test lead goes in the right-hand (red rimmed) jack and the black test lead goes in the center (black rimmed) jack ( Figure 2.2.4 ). The only exception is for high amp tests, when the red lead moves to the leftmost jack.
Figure 2.2.4 Test lead jacks.
Figure 2.2.5 Test leads inserted.
Since such measurements can be lethal if done incorrectly, I ’ d like to say that you should not try high amp measurements, and thus 100% of your work with a DVOM will be with the test leads in the position shown in Figure 2.2.5 .
Make sure your test leads are fi rmly inserted, and we ’ re ready to work with the DVOM!
Our next series of shots describes the use of the four test functions above.
I ’ m totally confi dent that you can use the ‘off’ function without any further instruction!
The fi rst exercise is to measure the DC resistance of different types of wire, so I ’ ll set the rotary function selector to ohms or ( Figure 2.2.6 ). I could also use the diode/beep function just to the right of the ), but not all DVOMs have it. I ’ m also going to call the rotary function selector an RFS, because I ’ m tired of typing it!
Figure 2.2.6 Set to ohms function. Figure 2.2.7 First reading in function.
So I cranked the RFS over to and the cryptic message in Figure 2.2.7 appeared on the LCD panel. The O.L refers to ‘ open load ’ – which is the DVOM’s way of saying, ‘ Dude, I have nothing to measure! ’ . Electro-tech speak often refers to a ‘ load ’ or under-test. Since there is no object-under-test yet, the load is missing, or ‘ open ’ . The DVOM is kind enough (and smart enough) to tell us this.
There are three more bits of info here that are easy to miss:
1. In the upper left corner of the LCD is ‘ auto ’ , which shows that the meter is in auto-ranging mode.
2. Under the O.L is ‘ M ‘ , which indicates that the DVOM is currently in the megohms range of DC resistance.
3. Between the O and L is the decimal point, or ‘ . ’ – very easy to miss, as the decimal point will change its position according to what you ’ re measuring. So keep a sharp lookout for that little dot!
These types of information will always be displayed at the same location on the LCD. Or, all but the decimal will, and it moves in a logical manner. Learn to make looking for them part of how you see/read the meter.
But I want to actually measure something! Let ’s see what happens to that O.L when I touch the test leads together ( Figure 2.2.8 ).
Figure 2.2.8 Touch test leads.
Figure 2.2.10 Use of the REL button.
Figure 2.2.9 Resistance of leads.
I ’ m touching the test leads fi rmly together; they are dry and clean. The LCD display fl ickered and quickly settled to the reading seen in Figure 2.2.9 . Almost everything ’s changed now on the LCD. We ’ re still in auto, but now we ’ re measuring in the range, not M . The O.L is gone, and we see 000.4 on the display. That means the test leads themselves have an internal DC resistance of four-tenths of an ohm – pretty low, but we want to be really accurate. How to adjust for the test leads? Oh, the REL button ( Figure 2.2.10 ), that ’s right!
It ’s what you don’t see here that ’s critical to understand – I ’ m still holding the test leads fi rmly together with my left hand! If I didn ’t do that, I ’ d go back to that boring O.L. So while holding the
aforementioned leads together, I pressed the REL (relative) button and, lo, and behold all zeros.
Now the test leads and the DVOM are a calibrated unit, ready to measure accurately. Let ’s go fi nd some wire!
A good subject for our fi rst test is a short length of two-conductor shielded audio wire ( Figure 2.2.11 ). It ’s typical of internal studio wire.
Figure 2.2.11 Short audio wire.
With the wire resting so I can press down on the strands with the test leads ( Figure 2.2.12 ), I get the reading shown in Figure 2.2.13 .
Figure 2.2.14 Video wire test.
Figure 2.2.15 Video wire resistance.
Figure 2.2.12 Test short wire.
Figure 2.2.13 Short wire resistance.
The coax cable has a higher resistance – 0.8 . That ’s still pretty low. Let ’s go measure something longer!
We can see there ’s not much DC resistance – only 0.3 . This makes sense, because it ’s such a short piece of wire.
Let ’s try some different kinds and lengths of wire, to see if there are changes in what we measure.
In Figure 2.2.14 I ’ m measuring an equally short length of coaxial video cable. It ’s the kind of wire that would go to a TV cable box. I get the result shown in Figure 2.2.15 .
Hiding out of the shot in Figure 2.2.16 is the rest of the 20-foot guitar cord that I ’ m testing. The result is seen in Figure 2.2.17 . Our invisible 20-foot guitar cord measures out to 1.5 – a nice, healthy reading.
In general, your resistance readings should be 2 or less, unless you ’ re measuring a very long wire run. Higher readings may indicate cold solder joints, defective wire or tarnished connectors.
But this has all been much too safe and tame. Let ’s measure something a little more lethal!
Just to be sure you remember those acronyms – I ’ ll say it the long and boring way: ‘ Set rotary function selector to volts AC ’ ( Figure 2.2.18 ). We ’ re going to measure some plain vanilla 120 VAC from my wall outlet.
Figure 2.2.17 Twenty-foot cable resistance.
Figure 2.2.16 Test 20-foot cable.
Figure 2.2.18 Set the RFS to VAC.
OK, I lied – we ’ re measuring the same 120 VAC ( Figure 2.2.19 ), but I found it a lot easier to bring an extension cord and a couple of female three-way AC splitters up to the camera table than it would be to show where the whole mess plugs into the wall outlet. Same voltage, same current – it ’ ll still kill you if you mess up.
Notice that the test leads are fully inserted, and my hands are well away from any possible contact points.
The actual reading of my nominal 120 VAC is shown in Figure 2.2.20 . It ’s a tad high, at 121.9, but well within spec. Typical US house AC readings will vary from as low as 105 VAC up to 130 VAC. Electrical power supply companies are not known for delivering ultra precise power – but for us in the US, it ’s there all the time, like the air we breathe. Could you imagine the quality of life in a country where that is not a given?
So now you have some idea of measuring resistance to check your wiring.
And you also (I hope) know how to measure 120 VAC without hurting yourself.
You can check to see if there ’s electrical power for lights and soldering irons, maybe even some fans to blow the rosin smoke away.
Figure 2.2.19 Measure 120 VAC.
Figure 2.2.20 The 120 VAC reading.
Figure 2.2.21 Set the RFS to VDC.
Figure 2.2.22 A 1.5 VDC source.
Figure 2.2.23 Test AA battery. Figure 2.2.24 AA battery voltage.
There ’s one more test I want to show you – DC voltage. To do this, we have to invoke the RFS again. That ’s ‘ Set rotary function selector to volts DC ’ ( Figure 2.2.21 ). I just had to say that to make sure.
Now we need some DC electricity – what can we use for a source? How about an AA size fl ashlight battery, a great source for VDC ( Figure 2.2.22 ).
Just open up your Maglite and fl ip one out.
In Figure 2.2.23 I ’ m testing the AA battery, but wait – I ’ ve made a mistake and reversed the test leads. Normally the red lead would go to the positive terminal. What will happen because of this? Look at the reading in
Figure 2.2.24 . Aha! See the minus sign? Also the ‘ Auto ’ , ‘ DC ’ and ‘ V ’ ? So my measurement is accurate, but inverted because I fl ipped the test leads – another thing to look out for.
There ’s also a caveat that since a DVOM does not put a real-world ‘ load ’ on the battery, as a light bulb would, the DVOM will give a falsely high reading.
Real battery meters have built-in loads. Test a bunch of dead/semi dead batteries and you ’ ll soon get an idea of what your particular DVOM can show you. At the very least you can sort out the totally dead batteries with a DVOM.
This concludes my incomplete introduction to the world of DVOMs, how to use them, and why to use them. There ’s a lot more to say about them, but that info is already out there – you just have to go look for it.
If I ’ ve given you a taster and made you want to learn more, I ’ ve done my job. As I said at the beginning, Google on ‘ using a DVOM ’ or ‘ using a DMM ’ if you want to learn more. ‘ The truth is out there ’ , but I don ’t have room here for all of it – just enough to whet your appetite.
2.2 audio and AC power
After much skull scratching and soul searching, I decided to combine several concepts into one section, because they are so intimately interconnected. No, not that intimately, they ’ re just good friends.
So in this section I ’ ll talk about unbalanced and balanced audio, unbalanced and balanced AC power, and the best ways to wire and clean up the sound (and picture) of your studio/disco/home theater/whatever.