The Case for the Transistorized Multimeter, January 1968 Popular Electronics

Once upon a time, all non-passive electronic products and test equipment used vacuum tubes. Since tube diodes need a voltage bias, even something as simple as a rectifier circuit was "active." Even though modern day transistorized equipment has largely overcome most of the disadvantages of solid state versus vacuum tube, in the early days of silicon and germanium transistors and diodes issues like voltage and power handling and input impedance was a limiting factor to some applications. Until the advent of rugged and reliable field-effect transistor (FET) transistors, if you needed a very high input impedance for an oscilloscope or multimeter, a vacuum tube circuit was a necessity. A high impedance test instrument input is required with high impedance device under test (DUT) in order to avoid loading down the DUT with a voltage divider effect and changing not just the measured voltage level, but also possibly changing the operation of the DUT circuit. This article from a 1968 issue of Popular Electronics introduces readers to the relatively new phenomenon of transistorized multimeters.

The Case for the Transistorized Multimeter - Pro's and Con's of New Type of Test Equipment

The ever-increasing use of transistors and IC's in electronic circuitry has produced new problems for experimenters. Because the voltage levels for proper operation of solid-state circuits are usually very small - as a look at any solid-state circuit will show - any change, even slight, in these voltage values can produce improper circuit operation. The problems start when you try to measure these low-level voltages.

Using a conventional VOM (volt-ohm-milliammeter) is usually a poor way to make these measurements. Why? Take a close look at the electrical characteristics of some typical VOM's. In many cases, the input resistance (in ohms-per-volt usually found on the meter face in one of the corners) on the lowest voltage range is sufficiently low to cause serious changes in the measured voltage level.

The Bootstrap Circuit

The major reason that a bipolar transistor circuit is a low-impedance circuit is the fact that the input signal "sees" a parallel combination of the transistor base bias resistors, the input resistance of the transistor, and the leakage resistance of the transistor. When the resulting equivalent resistance is calculated, it will be found to be a low figure.

The input resistance of a transistor is determined by multiplying the emitter resistor value (if it has one) by the beta (Β) of the transistor. Therefore, with any reasonable value of resistor, and beta, the input resistance will be high. This is the reason why emitter-follower circuits are said to be high input resistance circuits.

With the introduction of improved manufacturing processes, the leakage current of a good transistor will be very low, thus making the leakage resistance a high value. The remaining resistance, the parallel combination of the base bias resistors, unfortunately remains with us, and it is this value that has the greatest effect on input resistance.

As shown in the schematic of a bootstrap circuit (above), R1 and R2 form the base bias voltage-divider network, while R3 is an isolating resistor connected between the base of the transistor and the R1-R2 junction. The signal input is fed to the base of the transistor and the output is taken across emitter resistor R4 and also coupled to R3 via capacitor C1.

When a signal appears at the top end of R3 and the base, it also appears at the emitter in the same phase - and for all practical purposes at the same amplitude. Thus, an identical signal voltage appears at both ends of R3 and no signal (a.c.) current flows in this resistor. Resistor R3 then represents an infinitely high resistance to signal (a.c.) current, thus effectively isolating the base bias resistors. Since R3 has no effect at d.c., however, the base bias is unchanged. The circuit literally lifts its impedance by its own "bootstraps," hence its name.

In practice, the signal voltage at the emitter is slightly less than on the base, thus limiting the effective value of R3. If, for example, the emitter follower voltage gain is 0.99, and the value of R3 is 100,000 ohms, the effective resistance of R3 is raised to 10 megohms, an increase in value by a factor of 100 times. Dependent upon the Β (beta) of the transistor in use, and the leakage value of that particular transistor, the input impedance of the circuit will be a value not too much less than 10 megohms.

What does this have to do with measuring voltage? If you recall Ohm's law, you will remember that when two resistors are connected in parallel, the resulting equivalent resistance is found by (R1 X R2)/(R1 + R2). For example, if you assume that a pair of 1000-ohm resistors are connected in parallel, the resulting equivalent resistance is 500 ohms. (We usually remember this equation when we parallel actual resistors to produce some desired lower value, but we seem to forget it when we connect a voltmeter into a circuit!)

Now, if you assume that one of the 1000-ohm resistors is a 1000-ohms-per-volt VOM on the one-volt range, and the other 1000-ohm resistor is in a circuit that should measure one volt, the resultant 500-ohm equivalent resistance produces a meter indication of only 0.5 volt - 50% off the circuit value required. In cases where the 1000-ohm resistor determines current flow in the circuit, reducing its value to 500 ohms may produce enough current flow to damage a semiconductor. This is why your VOM probably doesn't give you the voltage level indicated by the manufacturer, and it is also why some of your semiconductors may have been damaged for unknown reasons.

What about VTVM's? Don't they usually have input impedances measured in megohms, making them almost non-loading? True, they do have this characteristic - but they also have several small drawbacks. First, until very recently, most VTVM's had 1.5-volts full-scale as their lowest range. This meant that the very low voltages (below 0.25 volt) found in many solid-state circuits were indicated at the bottom end of the meter scale, where, in most cases, they were difficult to read and slight changes to interpolate were necessary. Second, they required connection to a.c. power, thus limiting their use to the bench. Third, most VTVM's use vacuum tubes (that's why they're called VTVM's), and vacuum-tube circuits often require recalibration as the tubes age.

VOM + VTVM = TVM. Recent VTVM's have overcome some of their disadvantages by utilizing pre-aged tubes, and incorporating 0.5-volt full-scale ranges. However, these changes still did not eliminate the need for another voltage measuring instrument having the total portability of the VOM, the non-loading of the VTVM, full-scale ranges of 0.5 volt or less, and requiring a minimum of recalibration.

Two developments helped bring such an instrument into being - the FET with its very high input resistance, and the bipolar transistor "bootstrap" circuit in which a novel approach makes an ordinary low-impedance transistor circuit look like a very high impedance circuit. The use of semiconductors meant that batteries could be employed as the power source, providing portability; and the fact that semiconductor devices require no "aging" removed the last electronic barrier. The creation of a low-voltage range is only a component change in the input voltage-divider circuit. Thus, the stage was set for the introduction of the transistor volt-ohm-milliammeter or TVM.

As new TVM's are appearing on the market with regularity, the four units discussed on these pages represent only a small sampling. However, there are sufficient differences among them to illustrate some trends in TVM's.

As one of the major reasons for the existence of the TVM is portability, most units are powered only by batteries. There are exceptions - the Heath IM-25, for example, is powered either by an internal battery supply or by the commercial power line, with selection made by a front-panel control. When the a.c. power cord is not in use, it is stored on the rear of the cabinet. TVM's having this feature can be employed both on the bench and in the field.

Types of batteries used by the various TVM's range from "AA," "C," and "D" cells, through conventional 9-volt transistor radio batteries. All units have several batteries, often in various combinations as required by the respective circuit, and, with all, battery replacement is easy. One unit (the Amphenol "Millivolt Commander ") has a provision on its function selector switch for testing its internal battery, and its meter scale is marked accordingly. The others have special, easy-to-perform test procedures included in their operating manuals to simplify battery testing.

As TVM's were designed with solid-state circuit voltage measurement in mind, all are provided with at least a 0.5-volt range, and most also incorporate a 0.15-volt range. The remainder of the voltage ranges are as found on VTVM's, ranging in 5 to 7 steps to about 1500 volts. Of course, all TVM's have switch provisions for measuring either positive or negative volts.

D.C. full-scale accuracy for all TVM's is between ±2 and 3%. There is a greater variation in input resistance, however. The Heath and Amphenol units have about 11 megohms input resistance on all ranges ; the Triplett Model 600 has 2.75 megohms on its 0.4-volt range, 5.5 megohms on its 0.8-volt range, and 11 megohms on all other ranges ; while the Aul TVM-4 has 500,000 ohms on its 0.15-volt range, 1.5 megohms on the 0.5-volt range, 5 megohms on the 1.5-volt range, 17 megohms on the 5-volt range, and 36 megohms on all other ranges.

TVM Grand-Daddy?

In August, 1963, Popular Electronics reported on the first commercial transistor voltmeter - the De Vry TRVM. Still available, it comes as a kit ($64.50), or wired unit ($89.50), and features a.c. ranges from 5 to 1000 volts, d.c. ranges from 1 to 1000 volts, and current measurements from 50 μA to 50 mA. External shunts permit current measurement from 500 mA to 5 amperes, and a conventional ohmmeter range is provided. Input impedance on a.c. is 650,000 ohms on the 5-volt range and approximately 2 megohms on the others. The d.c. input resistance is 10 megohms on all scales except the 1-volt range, where it is about 1 megohm. The device operates from three "D" cells and one "C" cell.

As it is seldom necessary to measure low-level a.c. voltages, many TVM's do not make provisions for such measurement below the usual 1.5 volts. However, there are exceptions - the Heath unit measures down to 0.15 volt, while the Amphenol unit goes down to 0.1 volt.

A.C. full-scale accuracy is not quite as good - ranging from 3 to 5%. Input impedance once again varies widely, ranging from 10 megohms for the Heath and Amphenol units, to 750,000 ohms for the Triplett, down to 250,000 ohms for the Aul TVM. The frequency response of the a.c. measurement circuit also shows wide variation. The Heath unit is flat from 10 Hz to 100 kHz, the Amphenol from 50 Hz to 50 kHz, and the Triplett from 15 Hz to 2 MHz. Voltage measurements outside these limits may be in error.

This seems to be an area of disagreement. While some manufacturers provide for d.c. measurement - in the case of Heath from 0.015 to 1.5 A (ampere), and Aul from 0.15 to 1.5 A - others do not include this measurement facility.

Since the TVM is a voltage-sensitive device, the inclusion of a series voltage-dropping resistor in its current measurement circuit may produce external circuit problems. For example, the insertion resistance of the Heath unit is 10,000 ohms for the 0.015-A range. When measuring current in a circuit, the user should be aware of the presence of this unseen series resistance, as in many cases it may curtail certain circuit operations.

Measurement of low-level a.c. current flow is seldom required in any service work, and only one unit discussed here (Heath) makes provision for it. In this case, the a.c. current range duplicates the d.c. range (0.015 to 1.5 A) , and the same problem of insertion resistance exists as discussed above.

The RCA TVM Entry

As this issue goes to press, we have learned that RCA has introduced the Model WV-500A solid-state Volt-Ohmyst. Resistance can be measured from 0.2 ohm to 1000 megohms; d.c. voltage measurement is from 0.2 to 1500 volts; and a.c. (r.m.s.) measurement is from 0.1 to 1500 volts, complex waveforms to 4200 volts. Input resistance on all d.c. ranges is 11 megohms. Price, $75.00.

As in VOM's and VTVM's, TVM's are provided with the usual ohmmeter ranges. Where Aul and Triplett are content to go to R X 100K as the upper end of their units, Heath and Amphenol provide an R x 1M setting.

Three of the representative units have the usual "Zero Adjust" and "Ohms Adjust" controls; Heath uses a "Zero" control which is common for all functions. Like conventional ohmmeters, the TVM's use "10" as the center scale indication.

Test leads have also undergone a design evolution during the past few years. Gone are the days of the unshielded length of wire supplying the "hot" meter input with a signal. Today, with very high input impedance VTVM's, and now TVM's with their very low full-scale voltage ranges, stray pickup on the test leads can lead to erroneous indications. The trend is toward a length of shielded wire terminated in a plastic probe having some form of fingertip switching between the d.c. and a.c./ohms functions.

While all probes are terminated with a reasonably sharp metal tip, many are also provided with a friction-fit alligator clip that can be slipped over the metal tip. Heath, on the other hand, uses a threaded metal tip so that the screw-on alligator clip forms an integral part of the tip.

The modern TVM has that "uncluttered" look. Meters are large, clearly printed, very easy to read, and range in width up to six inches. Although the familiar box-on-end packaging is still in vogue for VTVM's and VOM's, TVM's are starting new style trends.

Amphenol, for example, encloses its "Millivolt Commander" in a simulated leather case with carrying handle, with test lead storage space provided in the cover. A tilting "foot" at the rear of the unit permits standing it at any easy viewing angle. Another Amphenol novelty is the use of a rocker-type on/off switch so arranged that when the cover is installed and closed, a rubber bumper on the cover will automatically switch the unit off if the operator forgets to do so.

Heath is following its latest approach to clean packaging design with retractable handles mounted on the sides of the unit. The Heath unit, with its multiple functions and large meter, has gone to a horizontal design, making it the largest of the TVM's available at present. The IM-25 is the only unit so far that comes either in kit or factory-wired form.

Triplett features a clean, uncluttered front panel with a single large-size function selector knob and easy-to-read range markings, while the Aul unit is a business-like service instrument compact enough to fit in a tube caddy.

There also seems to be a wide variation in test lead input jack type. Amphenol uses a coaxial screw-on fitting, Heath has a telephone type jack, and the other two representative models use variations of the banana plug fitting.

TVM's are here to stay. At the approximate cost of a VTVM, you now can have a voltage-measuring device with the very high input impedance of the VTVM and the portability of the VOM. And, most important, you can now measure down to extremely low levels of voltage and current with excellent accuracy.

Which one to buy? Obviously, if you do a lot of bench work where line power is available, the Heath unit comes to the fore. This is also the most versatile of the TVM's, and can operate from batteries if desired. For greatest all-around portability, the Amphenol unit, contained within its own carrying case and having an automatic on/off power switch, will make a hit with most outside servicemen.

The Triplett unit is a very easy-to-use instrument having the simplest operating controls (only one knob) . It also features a combination handle/foot, for portability and viewing convenience, and a leather carrying case for protection. The Aul unit is the most compact of the TVM's covered.

All TVM's are good, and selection should be made based on your needs, present prices, measurement ranges to be used, and personal taste in instrument appearance and brand names.

The Case for the Transistorized Multimeter January 1968 Popular Electronics

The Case for the Transistorized Multimeter
January 1968 Popular Electronics