Apr 1, 2002 12:00 PM,
▪ WITH ALL OF THE NEW AND COMPLICATED INSTRUments and measuring devices around these days, we often forget the tools that were the most important instruments we had, namely instruments to measure voltage, current, resistance and temperature. These instruments mainly fall into three types: volt ohm milliammeters (VOM), vacuum tube voltmeters (VTVM) and digital multimeters (DMM).
These meters are invaluable for servicing equipment, checking continuity, and measuring power lines and equipment current draw. However, each instrument does not perform every measurement equally well, because they vary in sensitivity, frequency response, response time and input impedance.
VOLT OHM MILLIAMMETERS
the oldest type of these meters is the VOM. It does not require external power and normally uses an internal battery for resistance measurements only. The input impedance is usually fairly low, often 20,000W per volt for DC readings (20 kW/VDC) and 5000W per volt for AC readings (5 KW/VAC). If we are measuring 1 VDC on the 1 VDC scale, the input impedance would be 20,000W. If we measure 10 VDC on the 10 VDC scale, the input impedance would be 200,000W. This can lead to misinterpretation of some measurements, because the VOM will place a different load on the circuit being tested, depending on the range selected on the VOM.
For example, if you measure a 1 VDC input on the 1V scale, the VOM would place a 20,000W load on the circuit. However, if you measure the same 1 VDC on the 3 VDC range, the VOM would place a 60,000W load on the circuit. Depending on the circuit being tested, that could create havoc. If you measure a power supply with an impedance of 1W or less, the difference in loading between 20,000W and 60,000W would be insignificant in the reading. However, if you measure the voltage in the emitter or base circuit of a transistor with an impedance of 2,000W, using the 1 VDC scale, the 20,000W source impedance would introduce a – 10 percent error into the measurement. Setting the meter to the 3 VDC range would only produce a – 3.3 percent error (60,000W versus 2000W).
Also, the loading effect of the meter on the circuit being tested will change. The 3 VDC range will load the circuit less than the 1 VDC range. In some cases that makes a significant difference. The circuit may tolerate a 60,000W load and keep working, but a 20,000W load may shift the operating parameters of the circuit outside of its normal range, with catastrophic results. This can happen in transistorized audio power amps.
Taking a measurement with a meter with too low of an input impedance at a node in the circuit that has relatively high impedance causes the amplifier to shift its operating condition dramatically, often causing self-destruction of the amplifier and any load connected to it. Lesson: know your test equipment and the circuitry you’re testing with it.
A similar problem (differences in readings due to changes in input impedance) can occur when using digital meters, too. Although most digital meters have a 10 megohm (MW) impedance on most of the ranges, it is not unusual for the lowest DC range (often 200 mV) to have a much higher impedance (greater than 1,000 MW). This is not done to provide some measurement advantage to the user; it’s an artifact of a cheap input circuit configuration that DMM makers have be using for years. When making measurements in the vicinity of 100 to 300 mV, be aware that the meter could be either loading the circuit with 10 MW or more than 1000 MW (virtually no load).
A VOM’s low input impedance can be an advantage in some instances, because it doesn’t pick up stray voltages that can produce false readings. For example, if we connect a digital multimeter to an unpowered AC power line (with no load on it) or a long 70.7V loudspeaker line with no loads on it, we will probably read a voltage of several 10s of volts. A VOM connected to the same line will read about zero. This happens because there is a “floating” voltage caused by electrostatic coupling, through the air, to 60Hz power lines. A DMM loads it somewhat, and it measures several 10s of volts. A VOM loads it significantly, so it measures almost zero. Electricians often prefer to rely on the VOM’s reading, but several manufacturers that recognize the problem make digital meters for electricians that are purposely low impedance, so they won’t pick up stray voltages. The readings on these DMMs generally agree with the trusty old VOM.
▪ Vacuum Tube Voltmeters. The vacuum tube voltmeter was introduced when measurements of high-impedance circuits were required. The original meters used vacuum tubes to increase the input impedance to 11 MW, regardless of the voltage setting. Probes could be connected to the input to increase the impedance to 100 MW. The input circuit was often measured with respect to the case rather than floating. The original VTVMs were powered from the 120 VAC power line, but with the advent of field effect transistors (FETs) and high impedance ICs, VTVMs are now powered by batteries, eliminating the necessity of being connected to a power source. We still call them VTVMs, though, or sometimes analog multimeters. Transistorized VTVMs were sometimes called transistorized volt meters (TVMs) or FET VOMs. There were even combination VOM/VTVM units. The 1979 Triplett 603 included an auto-polarity feature that, if activated, would always read upscale.
all digital multimeters are high-input impedance (10 MW) devices and are battery operated. The output is digital, displayed on a LCD. Most DMMs display 32 or 42 digits. Many DMMs are auto-ranging so that you only have to switch the meter to the type of measurement you want to make. That reduces the number of meters a user needs, but it can make the readings slow. However, many DMMs let you set the range if you know about what you are measuring, which can alleviate some of the slowness.
▪ Bells and Whistles. Digital multi-meters tend to offer a lot more features than the other two instruments. For instance, they normally work whether a voltage or current is positive or negative: A DMM just places a + or — sign in front of the reading. Using a VOM or VTVM with a negative voltage or current causes the meter to wrap itself counterclockwise around the pivot point — which is not the most pleasant sight. DMMs often can measure frequency, capacitance and temperature. Temperature is measured by connecting a thermal couple to the input. (A few VOMs can also measure temperature.) DMMs often measure diodes and include a continuity check. Often a clamp-on ammeter can be connected to the DMM to read up to 10 A.
Many meters measure apparent voltage rather than RMS voltage. This is fine for measuring a sine wave, but a music or voice wave requires a true RMS reading meter. These meters measure effective voltage, which, when used to measure power by squaring the voltage reading and dividing it by resistance (V2/R), will produce the true power dissipated in the circuit. A meter that measures apparent voltage to determine the wattage of a resistor can make the resistor look like a used firecracker. Be aware that the RMS circuitry used in most DMMs has a crest factor rating of about five or seven to one. That handles most common waveforms, including noise. If the signal is a pulse, the RMS circuitry usually will not measure the magnitude correctly and will read too low. Consequently, when dealing with pulses, you might still fry the resistors if you believe the meter reading.
DMMs often can measure true RMS voltage and have a fairly wide bandwidth, anywhere from 40 kHz to 100 kHz. VOMs often go to several kHz on the lower ranges and only to 5 kHz on the 600 VAC range.
DMMs may also include one of many types of hold or data hold circuits that freeze the display reading at the press of a button, which is helpful for recording readings. An Auto Hold feature eliminates the need to press the button. That feature works because the meter recognizes that a stable reading is being made, at which point the meter freezes the displayed reading. Peak hold and valley hold (or min/max hold) record and display the minimum or maximum quantities. That is beneficial when you’re searching for transients, but remember that the capture times of meters varies considerably, from several seconds to several microseconds.
Some DMMs also include an analog bar graph to simulate an analog meter. This can be useful when monitoring a changing signal such as a slow beat between two signals.
Usually digital multimeters are more accurate than their analog counterparts. It’s also easier to read three decimal points on a LCD than to try to eyeball it on a 3-inch meter face. Of course, I always thought my DMM was totally accurate until I had two DMMs which never quite matched, but that is splitting hairs.
▪ AC/DC Clamp-on Meter. One other meter worth mentioning is the AC/DC clamp-on meter. AC/DC clamp-on meters are simple digital voltmeters with a frequency response to 1 kHz and resistance measurements to 400W. These meters also measure frequency to 4 MHz, plus diodes and continuity.
The great thing about AC/DC clamp-on meters is that they give you the ability to measure current without disconnecting the circuit and inserting the meter into it. Instead, these meters simply clamp around the wire and measure current to as much as 1000
USING METERS IN THE REAL WORLD
what can you measure in the audio world with all of these wonderful instruments? The first thing you can use the meters for is troubleshooting circuits. Remember the VOM has a low-input impedance and can affect the reading and the bias or voltage on the circuit being measured. The VOM and VTVM are easier to read if the voltage or current is varying, and the DMM is easier to read if the measurement is steady state. Auto-ranging is handy for measuring many voltages of varying levels but is slow and aggravating when measuring voltages at about the same level. It is better to turn the auto-ranging off for those measurements.
▪ Impedance. Any of the meters can measure impedance equally well, but I suppose the digital multimeter is easier to read as it does not require interpolation. However, they read only DC resistance, so they cannot be used to measure AC impedance, which is required when measuring loudspeaker impedances or for any reading where there is a transformer. DMMs are useful for measuring the DC resistance of a loudspeaker line that soaks up power so that the power at the loudspeaker is not the same as the power at the amplifier output. For instance, if you have a 4W loudspeaker and you measure the DC impedance of the line with one end shorted as 1W, you will lose 36 percent of the power to the line in the form of heat. Furthermore, if the amplifier is rated for a particular wattage at 4W, the impedance of the loudspeaker and line loss totaling 5W will draw less power from the amplifier, a loss-loss situation.
▪ Current. Clamp-on meters are good for measuring the current a sound system takes from an AC source. Just clamp the meter on one of the 120 VAC input cables (preferably the black or hot wire in a single-phase 120 VAC circuit) and read the current flow. An accessory clamp-on probe can easily measure AC amperes with VOMs and DMMs.
If you need to measure the DC current running through a transistor, you can measure the voltage drop across a series resistor in the circuit or insert a VOM or DMM in the circuit. (This will require breaking the circuit first.) Some meters possess accessory DC ampere clamp-on probes that will clamp on a wire in the desired circuit for a simple and fast measurement.
▪ Continuity. Another useful test is the continuity test. This is especially helpful when making many repetitive continuity measurements or trying to sort through a spaghetti of loudspeaker and microphone lines. It is important to know at what impedance the meter considers the circuit to be closed. The threshold can be between 2 and 1000W, but most meters are from 20 to 100W.
▪ Frequency. Many DMMs can measure frequency. This ability could be useful to determine the frequency of feedback, but don’t drive the system too hard or too long or you will destroy a component. They are also useful when you need to measure the frequency of signaling devices installed in the system. You also might be able to measure high-frequency oscillation in the system.
It is important to know the amplitude sensitivity and resolution of the meter; higher sensitivity is not necessarily better. A meter with a 10 mV sensitivity, for example, cannot measure power line frequency. There is usually several volts of noise on an AC power line, and the meter will read the noise, not the 60 Hz. For power-line measurements, a sensitivity of several volts is appropriate. Some meters let you select different sensitivities, but most meters have a single fixed sensitivity. Be sure to know the meter’s frequency resolution.
▪ Temperature. A temperature probe can measure the ambient temperature in an enclosure, the case temperature of a transistor and even body temperature.
As you can see, the old VOMs and VTVMs and the new DMMs are all useful tools to have in your toolbox. The key is knowing how to use them.
Glen Ballou owns Innovative Communications and is author of the first, second and third editions of The Handbook for Sound Engineers. Thanks to Mike Hahn of Triplett Corporation for his explanations of meter impedance.
Here are some of the specifications you can expect from the various meters, but be prepared for a good deal of variation. No one meter will match all of these values.