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Measurement

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MEASUREMENT

Circuit measurement is used to monitor the operation of an electrical or electronic device or to determine the reason a device is not operating properly.

THE BASIC METER MOVEMENT

The meter movement is the part of a meter that moves giving an indication of some value. Meter movements convert electrical energy into mechanical energy.

The earliest type of meter movement is the analog meter movement. The amount of pointer deflection across a scale is an analog, or similar, to the magnitude of the electrical property being measured.

A fixed permanent magnet is used. A moving coil of wire is used. When current passes throught he coil it establishes a magnetic field that reacts to the magnetic field of the permanent magnet. This causes the coil to rotate.

Hairsprings are attached to each end of the coil to cause it to return to its original position when the electrical current is removed.

A pointer is attached to the coil and extended out to a scale. As the coil moves, the pointer moves across the face of the scale.

To increase the accuracy an iron core is placed inside the coil to concentrate the magnetic fields. Curved pole pieces are attached to the magnet to ensure that the turning force on the coil increases steadily.

Rectifier for AC Measurement

Alternating current can be measured with an analog meter movement by first passing it through a rectifier.

Damping

Damping is the process of smoothing out the oscillation of the pointer. The coil itself provides damping by inducing an opposing current as the coil moves. A vane may also be attached to provide friction against the nearby air molecules.

OTHER METER MOVEMENTS

Electrodynamic Meter Movement

The same principle as the D’Arsonval movement is used. Fixed coils are used instead of a permanent magnet. The moving coil is attached in series to these coils. A turning forces is created that does not change direction, even if the current is reversed. The most important application of this is the wattmeter, which is used to measure power.

Moving-Vane Meter Movement

The moving-vane meter operates on a principle of magnetic repulsion between like poles. The current to be measured flows through a coil, productin a magnetic field that is proportional to the strength of the cureent. Suspended in this field are two iron vanes. One is in a fiex position;p the other, attached to the meter pointer, is movable. The magnetic field magnetizes these iron vanes with the same polarity regardless of the direction of current flow in the coil. Because like poles repel, the movable vane pulls away from the fixed vane, moving the meter pointer to indicate a value of current.

Hot-Wire and Thermocouple Meter Movements

Hot-wire and thrmocouple meter movements both use the heating effect of current flowing throua resistance to cause meter deflection.

The hot-wire meter movement depends on the expansion of a high-resistance wire caused by the heating effect of the wire itself as current flows through it. A resistance wire is tretched taut between the two meter terminals, with a thread attached at a right angle to the center of the wire. Current flow heats the wire causeing it to expand. The motion is transferred to the meter pointer thought the thread and a pivot.

A thermocouple meter consists of a resistance wire across the meter terminals, which heats in proportion to the amount of current. Attached to this wire is a small themocouple junction of two unlike metal wires that connect across a very sensitive DC meter movement. The current being measured flows through the resistance wire onle, not thourgh the meter movement itself. The pointer turns in proportion to the amount of heat generated by the resistance wire.

AMMETERS

An ammeter is a device which measures current. The analog DC ammeter is almost always a Weson-type instrument. The moving coil typically carries currents not larger than 30 uA. To handle larger currents a shunt resistor (parallel connected) is placed across the moving coil leads. Current going thourhg the meter divides with part of the current going thrugh the coil.

I sh/ I m = R m / R sh

R sh shunt resistance

R m meter resistance

I sh shunt current

I m meter current

Ammeter Placement

The ammeter is always connected in series with the curcuit path you with to test.

Ammeter Sensitivity

Ammeter sensitivity is the amount of current I m necessary to cause full-scale deflection (FSD) of the ammeter pointer.  Typical FSD values are from 10 uA to 30 mA. The resistance is from 1 ohm to 2000 ohm. The smaller the FSD current, the more sensitive the ammeter

Multirange Ammeter

Several values of shunt resistors and a rotary switch are used to select the desired range of current to measure.

A make-before break rotary switch insures that there is a shunt resistor connected at all times.

An Aryton shunt uses a combination of resistors.

Measuring Current

 

  1. The curcuit path must be opened and the ammeter inserted in series.
  2. Always set the range to the highest scale and then reduce as needed.
  3. Observe polarity.
  4. Reading should be as close to full scale as possible
  5. Better quality analog meters include a mirror along the scale. This eliminates parallax error.

VOLTMETERS

A multiplier resistor is connected in series with the meter movement to allow measuring voltage.

R mult = V full-scale / I m – R m

V full-scale = full scale voltage at extended range

I m = full-scale meter current

R m = meter-movement resistance

Multirange Voltmeters

Through the use of a range-selector switch, a multirange voltmeter can be made that switches the appropriate individual multiplier resistance in series with the meter movement.

R mult = V full-scale / I m – (Rm + all previous R mult values)

Voltmeter Sensitivity

Voltmeter sensitivity is expressed in ohms/volt. A voltmeter is considered more sensitive if it draws less current from the circuit. The sensitivity of a voltmeter varies inversely with the current required for full-scale deflection.

Sensitivity = 1 V / I fsd

Where I fsd = current required for FSD of the meter movement

A 50 uA meter movement is 1 / 50 uA or 20000 ohm/volt

Lab quality voltmeters should have a min sensitivity of 20k ohm/volt.

Voltmeter Resistance

The multiplier resistance is usually a high value in order to limit the current flow throught the meter movement. On a 100V range a 20k ohm/V voltmeter has a resistance of 2000 k ohm or 2 M ohm. The sensitivity is constant and remains the same for all ranges. The voltmeter resistance is different for each range.

Since the voltmeter is placed in parallel, the higher the resistance the better. This ensures that the circuit performance is not hindered by the meter.

Voltmeter Loading Effects

When the voltmeter resistance is not high enough, connecting it across a circuit component can change circuit resistance, chich changes circuit current and voltage. The measured voltage decreases compared to the voltage without the voltmeter. This effect is called voltmeter loading, because additional current is drawn by the voltmeter. An ideal voltmeter would have infinite resistance and no loading effects.

  1. Always set the range to the highest voltage and reduce as needed
  2. Observe polarity
  3. The voltmeter must be connected across or in parallel with the component voltage to be measured.
  4. Reading should be made as close to full scale as possible
  5. Prevent parallax error

 

 

OHMMETERS

The basic meter movement can also be used to measure resistance. The resulting circuit is called an ohmmeter. In its basic form, the ohmmeter is nothing more than a meter movement, a battery, and a series resistance.

An ohmmeter forces current to flow through an unknown resistance and then measures the resulting current. For a given voltage, the current is determined by the unknown resistance.

When using an ohmmeter it is first necessary to zero it.

The ohmmeter scale is nonlinear. Voltmeter and ammeter scales are linear.

Because zero is on the right side of an ohmmeter it is referred to as a back-off scale.

  1. Zero before using or changing ranges.
  2. Ensure that power is removed from the circuit being measured
  3. Connect the leads across the component and read the resistance multiplying by 1, 10, etc. depending on the range.
  4. If the component under test has a parallel connection of another component, an invalid reading may be obtained due to the parallel combination of resistances. To overcome this isolate the component.

THE ANALOG MULTIMETER (VOM)

The analog multimeter combines the ammeter, voltmeter, and ohmmeter into one unit. The name multimeter comes from the term multiple meter. It is also commonly called a VOM (volt-ohm-meter). The VOM is a DC ammeter, an AC ammeter, a DC voltmeter, an AC voltmeter, and an ohmmeter all in one package.

Most multimeters use the D’Aronval metermovement and have a built-in rectifier for AC measurement.

 THE DIGITAL MULTIMETER (DMM)

A digital multimeter displays a digital value of the measurement.

An analog to digital converter (A/D) is used to convert the analog values at the input into a digital (binary) form. An LCD or LED display shows the value.

The input resistance is typically 10 M ohm on all ranges. For AC measurements a rectifier is used.

The frequency range for AC measurements is limited to 45 to 1000 Hz.

In most DMMs the left or most significant digit is known as a hlf digit because it can display only a 0 or 1. a DMM with four digits is often called a 3 ˝ digit DMM. A 3 ˝ digit DMM can display 19.99 V but 29.99 V is displayed as 30.0 V.

A drawback is sample rate. 2.5 – 4 times per second. Some DMMs have a bar-graph that updates 25-40 times per second.

Many DMMs include additional types of measurements such as frequency, capacitance, inductance, and transistor testing. Many DMMs also have a diode test feature to measure the junction voltages.

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