Analog Multimeters (Mod.3 Sect.1 Part 3)

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External View of an Analog Multimeter

Rubrica: : A typical analog VOM is shown here.

 In the top half of the meter’s face, note the printed meter scales and the pointer needle used to read circuit measurements. The inner workings of an analog VOM are fairly complicated. If you were to open up an analog VOM, you would see a circuit board and a very complicated range selection switch. Let’s look at these components a little more closely. The meter movement of a typical analog VOM is basically a current meter. This means that no matter what circuit quantity you’re measuring (voltage, resistance, or current) the multimeter actually only measures current. The multimeter then “converts” its current reading to the units you select and displays your requested reading on the meter face. The range selection switch is used to select the range of units you want your measurements displayed in on the meter face.

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Internal View of an Analog Multimeter

Rubrica: : Shown here is an internal view of an analog meter movement.

 As you can see in this figure, the pointer or needle is connected to a coil of wire that’s suspended on jeweled bearings. The coil of wire connected to the pointer is placed between two permanent magnets. When a current of the proper polarity is placed on the wire coil, the coil turns an amount of distance that’s proportional to the amount of current in the coil. In the lower middle of the analog VOM shown, you can see the range selector switch. The range selector switch is used to select appropriate meter ranges and units for the measurements you’re making. So, for example, if you’re measuring voltage and the range selector switch is set to the 10 V setting, you would read your measurements on the 0 V to 10 V scale printed on the meter face. If the range selector switch is turned to the 50 V setting, you would read your measurements on the 0 V to 50 V scale on the meter. In contrast, if you’re measuring resistance and the range selector switch is set to R × 1, the resistance measurement shown on the meter face is the actual resistance. However, if the range selector switch is set to the R × 100 setting, you would need to multiply the resistance reading on the meter face by 100 to get the actual measurement.

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Analog Meter's Range Selector Switch

Rubrica: : These are the typical positions of an analog meter’s range selector switch.

The DC and AC voltage scales have ranges from 2.5 V through 1,000 V, which is the maximum voltage capability of most meters. The current scales offer readings of DC current from 0 A through 10 A. The maximum current value is the amount of current required to move the pointer all the way to the right side of the meter face. As such, it’s called full-scale current. This range selector switch has resistance range selections of R × 1 (a direct reading), R × 100, and R × 10,000. These ranges allow you to read resistance values from about 0.25 Ω through about 20 MΩ with reasonable accuracy. The range selector switch is used to connect precision resistors called shunts in series or in parallel with the meter’s coil. Since the delicate internal mechanism of a meter could be destroyed by a high circuit current, a shunt resistor is used to bypass a large quantity of the circuit current around the meter. Thus, instead of the circuit current flowing through the meter movement or digital circuit, the majority of the current passes through a shunt resistor. Shunt resistors are mounted either on the range selector switch itself or on a circuit board within the meter.

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Shunts in an Analog Multimeter

Rubrica: : This figure shows how three different resistors can be used as shunts across the series-connected meter and 60 kΩ resistor.

The shunt resistors allow the meter to read at ranges of R × 1, R × 10, and R × 100. Note that in the figure, the range selector knob is turned to the R × 1 setting, and the switch inside the meter is contacting the 60 Ω, resistor. Internally, the meter measures voltage and current ranges in basically the same way as resistance. However, when AC voltage and current scales are selected, the incoming voltage must be rectified before it’s placed on the meter movement.

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Rubrica: : The internal resistance measurement circuit for an analog VOM is shown here.

Zero Ohms Adjustment Knob

Two other knobs are located on the front of the meter. The knob on the right-hand side is the zero ohms adjustment knob. This knob is actually a variable resistor that’s used to adjust the meter’s pointer. This figure shows how this variable resistor fits into a meter’s internal resistance measurement circuit. An analog multimeter measures resistance by measuring the current flow from it’s internal cell through the meter movement, the variable resistor, and the component or circuit being measured. When the zero ohms adjustment knob is turned, the current flow through the meter movement is adjusted. Each time you use the meter to measure resistance, you must short the probes together and turn the zero ohms knob until the pointer rests at zero ohms on the meter scale. This process is sometimes called “zeroing the meter.” (Note that it’s a good practice to zero the meter each time you change the resistance scale with the range selector switch.) When the zero ohms adjustment is completed, the meter pointer is at zero ohms and full-scale current flows through the meter.

Reading the Analog Multimeter's Scales

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Rubrica: : A close-up view of the analog meter’s scales is shown in this illustration.

The top curve of the scale is the resistance scale. The second curve down from the top is the voltage scale, and the third scale down from the top is the current scale. Each scale is marked with divisions that are calibrated according to the range selector switch position. Note that the resistance scale is a nonlinear scale, while the rest of the scales are linear. (That is, not all of the small divisions on the resistance scale are equal in size. The divisions get smaller as you read from right to left.) Let’s look at how you would read these scales. First, look at the position of the needle against the resistance scale (the top curve). You read the resistance scale on this meter from right to left, so you can see that a resistance reading of about 15.3 is displayed on the meter. Typically, the most accurate readings are taken as close to the center of the scale as possible. For example, a resistance of 150 Ω could be read on the R × 1 scale on the rather crowded left-hand side of the scale. However, if the range selector switch was changed to the R × 10 scale, you could read the resistance measurement much more accurately. The scale directly below the resistance scale (the second curve from the top) is used to measure voltage and current, both DC and AC. The switch located on the left-hand side of the front of the meter would be used to select what quantity you would be measuring at any given time.

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Rubrica: : Shown here is one application of an analog VOM. The VOM’s pointer will move rapidly to the left when the switch reaches the notch in the cam.

Measuring Resistance w/ an Analog VOM

As mentioned, analog VOMs are ideal for measuring the resistance or continuity of slowly changing circuits. For example, Figure 27 shows a switch that’s actuated by a rotating cam. The cam rotates at a speed of 30 RPM (revolutions per minute). The switch can easily be tested by using an analog VOM. The test leads of the meter are placed directly across the switch’s wires. The wires connected to the switch must be removed so that no electricity is present on the switch during this test! As the cam rotates, the switch’s actuator will fall into the gap on the cam, opening the switch’s contacts for a brief instant. Here, you’ll see a quick flick of the meter’s pointer to the left as the actuator falls into the gap. With a digital VOM, this type of rapidly changing signal can be missed by the slower scan time of the digital meter’s electronics circuits.