Test Equipment Scene: VOM's, VTVM's and TVM's
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For most needs to measure voltage, current, and resistance, modern users of test equipment do not need to give much thought to the electrical characteristics of the instrument being used. Other than setting the function switch to the proper position (ohms, volts, amps, milliamps, etc.) and not exceeding the safe input limits, measurement accuracy can usually be assumed to be good to within ±2 to ±5 of the least significant displayed digit. I.e., if the digital display shows 10.000, then the actual value is likely in the range of 9.995 to 10.005. Autoranging even removes the need to manually determine the proper range setting. For critical measurements, of course, you need to actually read the documentation to get actual accuracy ranges. One critical electrical parameter of a meter is its input impedance, since its value affects the voltage / current division at the measurement point. An extreme example is where you want to measure the voltage of a unit under test (UUT) that has a 100 kΩ impedance and an open circuit voltage of 1 volt, and the meter also has in input impedance of 100 kΩ, the reported voltage will be 0.5 V (50% error). Increase the meter input impedance to 100 MΩ and the reported voltage will be 0.999 V (0.1% error). The need for accuracy when measuring high impedance circuits motivated the design of vacuum tube voltmeters (VTVMs) that placed a tube in series with the input to achieve megohm levels as opposed to mere kilohms for standard meters. Test Equipment Scene: VOM's, VTVM's and TVM's By Leslie Solomon, Technical Editor
The VOM has been around for many years and was one of the first widely usable test instruments. Now being manufactured by a number of companies (Eico, RCA, B&K, Simpson, Triplett, Sencore, Heath, and others), the VOM is readily available from most electronic parts distributors. The modern VOM has many features and uses not possessed by its predecessors. The VOM of the past had relatively low input impedance, somewhat limited ranges, and a small meter size. It thus became less and less useful as circuit impedances increased and the levels of measurement for voltages and resistances decreased. Thus, the older VOM is now largely relegated to appliance servicing, where exact measurements are not necessary. As the need for more precise, non-loading measurements emerged, VOM's underwent some drastic changes. It was necessary to have input impedances of 50,000 ohms per volt or more; accuracies of 1% were necessary and voltages under 1 volt had to be measured. There were also important mechanical changes to be made. Not only did the physical size of the meter increase (to permit better interpolation of readings), taut band meters, which are inherently rugged and reliable, were incorporated. The low friction involved in the taut-band movement also provided highly repeatable measurements. Another physical problem, the high mortality of VOM's due to case breakage, was attacked by a number of manufacturers (Heath and Weston, for example) by the introduction of high-impact plastic cases that resist breakage. Some modern VOM's even come with circuit breakers and diode meter protectors to eliminate accidental burnout. Even the rotary function switch has given way, in many cases, to multiple pushbutton selection of ranges and functions. One manufacturer (Sencore) has even come out with a meter with 112 ranges! Enter the VTM. Updated VOM's are good, but there arose a need for an even more precise, multirange instrument that would load the circuit even less and could cope with more difficult measurement problems. Thus the vacuum tube voltmeter (VTVM) was born. The VTVM differs from the VOM in that the meter is driven by a vacuum tube circuit. In this way, a very high input impedance (11 megohms in typical) is obtained on almost all ranges, resulting in negligible circuit loading. Since the tube circuit acts as a buffer between the meter and the circuit being tested, the VTVM has a built-in meter protector. And, since the tube circuit has gain, a more sensitive measuring instrument can be designed. Another advantage of the VTVM - especially on the ac ranges - is that a tube circuit can be tailored to have a very wide frequency response. For example, a modern VTVM not only spans the audio range, but can reach high r-f when the proper probes are used. There are even special-purpose ac VTVM's - such as Eico's combination broadband ac VTVM and amplifier - to serve dual functions. The biggest asset of most VTVM's is in working with semiconductor circuits. Because of the low voltages involved in this type of gear, most VTVM's have a low-end range of 0.5 volt or less. With the very high input impedance of the VTVM, this makes it easy to check low-level transistor voltages. At the high end, most VTVM's can read 1 kilovolt (5 kV or more with a probe) making them doubly useful for servicing vacuum-tube circuits. Of course, VTVM's run somewhat higher in price than standard VOM's. Then Came the TVM. It's only natural that semiconductors are replacing vacuum tubes in test equipment - as they have in many other applications. Especially significant has been the introduction of the field effect transistor (FET) with its high input impedance - an ideal characteristic for measurement. Thus the transistor voltmeter (TVM) has come into being. Essentially a solid-state version of the VTVM, the TVM usually incorporates all the good features of its predecessor. Since the semiconductor elements are small and require little power, TVM's can be made highly portable (using battery power supplies in many cases). Making use of all the latest advances in circuit design, TVM's have increased ranges, excellent sensitivity, and high input impedance. However, when selecting an instrument for your own use, there are a few things that should be kept in mind. Sensitivity. The first thing to look for in selecting a VOM, VTVM, or TVM is the number of ohms-per-volt specified for the ac and dc ranges. Assume, for example, that a VOM is rated for 1000 ohms per volt. This means that the loading resistance of this instrument is 1000 ohms times the scale indication. Thus, with 10 volts input on the 10-volt range, the VOM resistance is 10,000 ohms. That's pretty high but stop and consider that, when indicating 10 volts, the meter takes 1 milliampere from the circuit under test. That may be OK for testing a power supply but the effect is quite different when the meter is connected across a load of a half a megohm in a grid or base biasing circuit. In that case, will the meter indicate the true voltage value? Will the loading seriously affect the performance of the circuit? Remember that from an electrical viewpoint, the meter looks like a 10,000-ohm resistor. Think what a measurement of a very low voltage across such a low resistance looks like and you will see why the VTVM and TVM with their 11-megohm input resistances became popular. Why are so many test meters specified at a much lower ohms-per-volt rating for ac measurements? Simply because they have dc meter movements and the quantity being measured must be rectified. And that means higher loading. With ac measurement, you must also consider the relationships between rms, peak, peak-to-peak, and average values. Most ac measuring instruments, unless otherwise specified, use rms (root mean square) values as the basis for sine wave measurements. If you have a need to convert from one value to another, remember the following relationships: peak value = 1.414 X rms value = peak-to-peak/2 rms value = 0.707 X peak value peak-to-peak value = 2.83 X nns value = 2 X peak value average value = 0.637 X peak value Rectifier-type ac meters do not indicate true rms except for sine wave inputs. Actually, they respond to average rectified values. For half-wave rectification, the average is 0.637 times the peak value, while rms is 0.707 times peak. In most cases, the meter scale has been calibrated to indicate about 10% higher than average so as to indicate rms values. Accuracy. This refers to the meter's ability to indicate true voltage, current, or resistance. Accuracy is normally specified as some percentage of full-scale deflection. For example, consider a 3% meter measuring 100 volts on the 150-volt scale. The accuracy would be within 3% of 150 volts, or 4.5 volts (maximum) at any point on the scale. Thus, in measuring 100 volts, you may have a reading as low as 95.5 or as high as 104.5. That's not too bad, but suppose you were measuring 10 volts on the 150-volt scale. You could hit 5.5 volts on the low end or 14.5 on the high end - an error possibility of about 50%. That is why you should always try to make all measurements as near as possible to the high end of the scale. Ranges. In the days when the vacuum tube was the predominant active element in most circuits, most voltages to be measured, even those on grids and cathodes, were over one volt; and this fact was reflected in the use of 2.5 or 3 volts for the lowest range on most test instruments. Now, in the solid-state age, many voltages under one volt must be measured. A glance at the schematic for any semiconductor device will show what low levels have to be measured. This new low level of measurement is reflected in the 0.5-volt or less full-scale ranges on a modern VTVM or TVM. These instruments also have ranges up to 1 kilovolt or so for use in testing vacuum tube circuits. Resistance Measurements. Resistance is measured by impressing a voltage across the unknown resistance and measuring the voltage drop produced by the current flow. In most cases, the greater the voltage sensitivity of the instrument, the higher the measurable resistance values. Most VOM's, VTVM's and TVM's are perfectly capable of measuring the usual spread of resistance values found in electronic equipment. However, there is one point to be remembered: the usual ohmmeter uses 1.5 volts or more to make a resistance measurement; and this voltage, if applied to a resistor in a semiconductor circuit, may be high enough to forward bias the associated semiconductor junction. This makes any resistance measurement invalid and also may lead to the accidental burning out of the junction. Many modern instruments use very low voltages to make in-circuit resistance measurements to avoid the forward bias effect. Remember that a silicon junction will switch on at about 0.6 volt forward bias, while a germanium junction requires only about 0.3 volt. Keep this in mind when using an ohmmeter to make in-circuit resistance measurements. This also applies when using an ohmmeter to "test" transistors. It is possible to deliver, unknowingly, enough current through a forward biased junction to completely destroy it through the thermal effect. The solution is to use an intermediate resistance range so that neither current nor applied voltage is excessive. The use of special low-power ohmmeter circuits will solve the problem. Uses and Abuses. There is no reason why a VOM, VTVM, or TVM should not provide good service for many years if it is properly handled. Just don't forget the basic rules: always make sure that you are in the correct function (connecting an ohmmeter or current meter across a voltage source can be disastrous) , and always start on the highest range, working your way down until the meter indication is as far upscale as possible. If you have a meter with color-coded banana jack inputs, check that the leads are properly connected. Black is usually ground, and red is the "hot" lead. With no power on the meter, make sure that the needle rests at zero. There is usually an adjustment screw on the front of the meter for zeroing. There is also a small thing called static charge that can accumulate on a plastic meter face (especially on large ones) that can cause erratic meter deflections. Most units are treated to remove this effect; but if you do run into trouble, there are several anti-static compounds that can be used.
Posted March 4, 2024 |
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The "bread and butter" instrument used by more
hobbyists and service technicians than any other single piece of electronic gear
is the ubiquitous multirange tester known as the voltohmmeter, or VOM.