Analogue multimeters

An analogue meter moves a needle along a scale. Switched range analogue multimeters are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it!

Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better because its resistance is much higher, 1 M or 10 M, compared to 200 for a analogue multimeter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an anlaogue display.

Used as an ammeter, an analogue multimeter has a very low resistance and is very sensitive, with scales down to 50 µA. More expensive digital multimeters can equal or better this performance.

Most modern multimeters are digital and traditional analogue types are destined to become obsolete.

Description

Versatile meter capable of measuring direct and alternating voltage on scales from 0.1 – 1000 V; direct and alternating current on scales from 1 uA – 3 A; resistance on 4 scales with midpoints 20 Ohms – 2 Meg-Ohms

Specifications

 

Voltage
Scales 0.1, 0.3, 1.0, 3.0, 10, 30, 100, 300, 1000 V.
Resistance
(low 4 scales) 100 k-Ohms 316 k-Ohms, 1 M-Ohms, 3.16 M-Ohms
(higher scales) 10 M-Ohms.
Accuracy ±3% of full scale for dc and ac 10-500 Hz.
  ± 5% for ac 0.5-25 kHz.
 
Current
Scales 1, 3, 10, 30, 100, 300 uA, 3, 30, 300 mA, 3 A.
Voltage drop is 0.1 V at full scale deflection
  130, 300 mV on 0.3, 3 A ranges, respectively.
 
Resistance
Scales (midpoints) 20 Ohms, 2 k-Ohms, 200 kOhms, 2 M-Ohms.
Accuracy ±5% of reading, near mid scale.
Voltage 3 V dc appears on open probes (upper probe +).
Current 150, 1.5 mA, 15, 1.5 uA passes through shorted probes.
 
Op. Position Upright or horizontal
Op. Temperature 10 – 40 °C.

 

Operating Instructions

  1. The desired meter function is selected by positioning the function selector switch and plugging the test probes into the appropriate receptacles. (Put black probe in 0 receptacle for voltage/current measurement).
  2. The meter is turned on by rotating the dial on the right-hand edge. Note the different `on’ positions for voltage/current measurement and for resistance measurement.
  3. For resistance measurement, the probes must first be shorted and the `ohms zero adjustment dial’ (left-hand edge) positioned for zero reading on the ohms scale. (Repeat when changing range).
  4. Connect probes, read meter on the appropriate scale, multiply reading by the appropriate decade factor, determine polarity on the polarity meter. The red probe is at a positive potential with respect to the black probe when the polarity is indicated as +.

Hints

Performance can often be checked roughly by reading a given voltage/current/resistance on two different ranges. Beware of “hay wire” circuits when measuring small currents or alternating voltages. Appreciable current may be induced in the leads from the ac power lines. Check this by verifying zero readings when the power is off in the circuit under test.

Principle

The basic meter is d’Arsonval galvanometer which measures direct current. Its sensitivity is increased by a battery operated transistor amplifier so that a current of 1 uA and a voltage of 0.1V will produce full scale deflection. Reduced voltage sensitivity is obtained by inserting different resistance in series with the basic unit (multiplier). Reduced current sensitivity is obtained by putting different resistances in parallel with it (shunts).

Resistance is determined by using the unit as a current meter in series with a batter and a resistance which is adjustable such that the closed circuit produces a full scale deflection. Inclusion of the unknown resistance in the circuit (in series) reduces the current by an amount dependent on the resistance value. The meter scale can then be calibrated directly in ohms.

Alternating current is converted to direct current by a rectifier circuit in the transistor amplifier. Likewise direct current of either polarity is made to flow through the meter in a single direction. A separate small meter is therefore included to show the polarity of dc and to differentiate ac from dc. It does this by indicating the average direction of flow of the current.

Caution

  • Set meter for correct function before connecting to circuit.
  • Beware of improper probe insertion, especially of attempting voltage/current measurement with probes in the ohms receptacles.
  • When switching to and from resistance measuring, remember to re-position the on-off switch.
  • Remember to turn off after using, but not between readings.
  • Check batteries and calibration if poor performance is suspected. See Instrument Instruction Card and your demonstrator for procedure.

Do not attempt resistance measurements in active circuits (power on).

 

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