Analog Multimeter Training

Why Learn Analog Meters?

In a world of digital displays, why learn analog meters? Because analog meters offer unique advantages. The moving needle shows trends and fluctuations that digital meters miss. When you are adjusting a voltage regulator, watching the needle rise and fall gives instant feedback. When monitoring a varying signal, the needle swing shows peak values. And for voltage and current measurements, an analog meter needs no battery at all.

Digital meters are precise. Analog meters are intuitive. Master both, and you are a more versatile technician.

The D'Arsonval Movement

Inside every analog meter is a delicate mechanism called the D'Arsonval movement. A coil of wire sits in a magnetic field. When current flows through the coil, it creates a magnetic field that interacts with the permanent magnet, causing the needle to deflect. More current equals greater deflection. A spring returns the needle to zero when current stops.

Advantages of Analog Meters

• Visual trends: See changes happening in real time
• Peak detection: Needle swing shows maximum values
• Zero-center: Easily see positive and negative swings
• No battery for volts and amps: The circuit under test powers the movement; only the ohmmeter function needs the internal battery
• Robust: Less sensitive to electrical noise
• Intuitive: Natural understanding of "more" vs "less"

Scale Types

Most analog meters have multiple scales printed on the face:

• Linear scales: Evenly spaced (voltage, current)
• Resistance scale: Non-linear, compressed logarithmically
• dB scale: For audio measurements (not covered here)

Reading Linear Scales

When measuring voltage or current, you will use linear scales. The process:

1. Note which range you have selected (e.g., 50V)
2. See where the needle points on the scale
3. Count major and minor divisions
4. Multiply the reading by any multiplier for your range

Avoiding Parallax Error

Parallax error happens when you view the needle from an angle instead of straight on. The solution: use the mirror scale.

Most analog meters have a mirror stripe behind the needle. Position your eye so the needle and its reflection align — the needle covers its own reflection. This ensures you are viewing perpendicular to the scale, eliminating parallax error.

The Resistance Scale Challenge

The resistance scale is different. It reads backwards — zero ohms is on the RIGHT, infinity is on the LEFT. It is non-linear, compressed logarithmically, with divisions getting smaller toward high resistance.

Read the number on the scale, then multiply by the range setting. R×100 means multiply by 100. Most accurate readings are in the center third of the scale, so switch ranges to keep the needle in this sweet spot.

DC Voltage Measurement

Measuring DC voltage is the most common analog meter use. The meter connects in parallel with (across) the component or source. Always start with the highest range to prevent overload.

1. Set function switch to DCV (DC Volts)
2. Select highest voltage range
3. Connect red probe to positive, black probe to negative or ground
4. Observe needle movement
5. If needle tries to move backward, reverse probe connections
6. Switch to lower range for better resolution once you know approximate voltage

AC Voltage Measurement

AC voltage measurement is similar but uses a rectifier circuit inside the meter to convert AC to DC for the movement.

• Set function to ACV (AC Volts)
• Select appropriate range
• Polarity does not matter for AC
• The AC scale is calibrated to show the RMS (root mean square) value of a sine wave; non-sinusoidal waveforms will read less accurately
• Needle may flutter slightly due to AC frequency

Practical Example: 12V Power Supply

Testing a 12V regulated power supply:

1. Set to DCV, 50V range
2. Disconnect load from supply
3. Connect probes to output terminals (red to +, black to −)
4. Should read 12 to 13.8 volts
5. Reconnect load while watching needle
6. Voltage should drop slightly but remain stable
7. If voltage drops significantly or needle fluctuates, supply is weak or failing

Critical Ohmmeter Rules

Zeroing the Ohmmeter

Zero the ohmmeter before each measurement and whenever you change ranges. Battery voltage changes with use, so the zero point drifts.

1. Select resistance function and range
2. Short the test leads together (touch them)
3. Adjust zero knob until needle points exactly to 0 on resistance scale
4. If you cannot reach zero, replace ohmmeter battery

Measuring Resistance

1. Verify circuit is OFF and discharged
2. Zero the ohmmeter
3. Connect probes to component
4. Read scale and multiply by range multiplier
5. For best accuracy, keep reading in middle third of scale

Cable Continuity Testing

Testing if a cable is intact:

1. Set to ohms, R×1 range
2. Zero the meter
3. At one end, short conductors together
4. At other end, touch probes to same conductors
5. Reading near 0 ohms = good continuity (cable intact)
6. Reading infinity = cable is broken

Testing for shorts between conductors:

• Leave conductors separated at one end
• Test between pairs at other end
• Infinity is good (no short)
• Low resistance is bad (conductors touching)

The Meter Goes In Series

Voltage is measured across a component. Current is different: the circuit's current has to flow through the meter. That means you must open the circuit and insert the meter in series, so it becomes part of the current path.

Measuring DC Current

1. Turn off power to the circuit
2. Set the function switch to DC current (DCmA or DCA) and select the highest current range
3. If your meter has a separate current jack, move the red lead to it
4. Open the circuit at the point you want to measure (for example, lift the positive wire from a terminal)
5. Connect the meter across the gap: red lead toward the positive supply side, black lead toward the load
6. Restore power and read the scale; step down the range for better resolution
7. Turn power off, remove the meter, and reconnect the circuit

How the Meter Handles Larger Currents

The D'Arsonval movement itself reaches full scale with a very small current, often 50 µA. To measure larger currents, the meter switches in internal shunt resistors that carry most of the current around the movement, so only a known, small fraction flows through the coil. Each current range selects a different shunt.

The same full-scale current sets the meter's sensitivity, rated in ohms per volt. A 50 µA movement gives 1 ÷ 0.00005 = 20,000 Ω/V. Higher sensitivity means the meter draws less current from the circuit when measuring voltage, so it disturbs the circuit less.

Practical Example: Door Strike Current Draw

A 12V door strike will not release reliably. Measure its current draw:

1. Power off and lift the positive lead from the strike terminal
2. Set to the highest DC current range, red lead to the supply wire, black lead to the strike
3. Power on and energize the strike
4. Compare the reading with the strike's rated current on its datasheet
5. Add up the draw of every device on the same supply and confirm it is within the supply's rated output

Congratulations on completing all training sections! Now it's time to demonstrate your knowledge with a final assessment.