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Test Instruments Part 4
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See all articles from Popular Electronics. Here is Part 5. The Vacuum-Tube Voltmeter - D.C. Ranges By Larry Klein Technical Editor  The
last three installments of this series were devoted to the volt-ohm-milliammeter. Now let's take a look at its chief
competition - the vacuum-tube voltmeter (or VTVM).Why put tubes in a voltmeter? To answer this question, it is necessary to understand the exact meaning of "sensitivity." As we found when checking out the VOM, the accuracy of any
For example, with two equal-value resistors connected across a 100-volt source, the voltage across each resistor is 50 volts. (See Fig 1). However, a voltmeter connected across either resistor will read less than 50 volts. Why? Well, if the VOM in Fig. 1 is a 1000-ohms/volt job, it has an internal resistance of 100,000 ohms (on the 100-volt range). Therefore, when the meter is connected, the total resistance between points Band C falls to 50,000 ohms. Since this is only half as great as the resistance between points A and B, the voltage divides unevenly and less voltage now appears between points B and C.
The higher the internal resistance of a voltmeter, the less current it will drain from the circuit under test and the more accurate the reading will be. This is the advantage of the VTVM. Because of its very high input impedance, the VTVM provides a reading practically identical to the voltage existing before the test leads were connected.
Tube V1 is usually a 6SN7 or 12AU7 dual triode, and the indicating meter (M1) is connected from plate to plate or from cathode to cathode of the triodes. In both circuits, R1 adjusts for the normal differences between the operating currents of the two triodes and appears on the front panel of the VTVM as the Zero Adjust control. Calibration control R2 is usually mounted inside the VTVM cabinet and adjusts for the small changes in total tube current over long periods of use. If you're wondering why the VTVM circuits is referred to as a "bridge," as you can see Fig. 2 (B) can be easily redrawn to the standard bridge configuration (Fig. 3) and may even be more easily understood that way. The theory of the bridge is quite simple. Triode V1b is the "reference" triode - in that its grid is grounded and the amount of current flowing through the tube is determined by the bias developed across cathode resistor R4 and the plate voltage of about 100 volts d.c. Triode V1a is operating under the same conditions except that its input grid is connected to a 5- or 6-resistor voltage divider which for the present discussion we will treat as a single resistor and call R5. Zero Adjust control R1 varies the plate voltages to tube V1 in Fig. 2 (A) and varies the cathode bias in the circuit of Fig. 2 (B). By adjustment of R1, the voltage differences between the two plates or two cathodes can be reduced to zero - and the meter therefore will also read zero. We have a balanced condition.
Let's take a look at R5. In a practical VTVM, R5 appears as the voltage divider shown in Fig. 4. The total voltage to be measured is always applied across the complete string of resistors in the input circuit. For higher voltage ranges, the input grid of V1 is tapped farther down on the voltage divider circuit. Note that the input resistance remains constant and equals the total resistance of the voltage divider plus the isolating resistor in the d.c. probe. Adding up the resistances in the divider, you can see that the sensitivity of the VTVM is quite high. The usual VTVM d.c. input resistance is 11 megohms and can be as high as 25 megohms. This is most important on the low voltage ranges of the VTVM which are often used for measurements in high impedance circuits. Note that, unlike the VOM, the input impedance of the VTVM remains constant regardless of which voltage range is used. Next month we will continue our investigations and see what makes the VTVM able to respond to a million cycles a.c. and read up to a billion ohms resistance. Posted December 27, 2013 |
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