Transcript Advanced Licence Course

Chelmsford Amateur Radio Society
Advanced Course
(10) Measurements
Part-1 - Meters
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
1
Types of Movement &
Constructional Details
• Meter movements for DC and AC measurements can be moving-coil,
moving-iron, hot-wire and thermocouple. The latter are good for
measuring RF amps.
• For DC and low frequency AC, we will only consider the moving coil type.
• For AC a diode rectifier is needed in the meter circuit
• The maximum movement possible is called Full Scale Deflection, FSD
N
Chelmsford Amateur Radio Society
Advanced Licence Course
S
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
2
Ammeter
• The basic meter movement is typically 1mA for
FSD and has internal resistance of 100 ohms.
Internal ‘r’
• Its range has to be extended to the required
range by a Shunt Resistor, which be calculated
by OHMS LAW.
Meter
Shunt R
Example for 1 Amp FSD scale:• Calculate the voltage drop across the meter from V = I.R
• We get V = 1*10-3 x 100 = 0.1 Volts.
• If 1mA flows through the meter then 0.999 amps must flow through shunt.
• There is 0.1 volts across the shunt.
• As R = V / I, then the shunt is 0.1 / 0.999 = 0.1001 Ohms.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
3
Voltmeter
Multiplier
Resistor
Meter
Internal r
• To measure voltages consider the basic meter movement with typical
1mA FSD and internal resistance of 100 Ohms. This is extended to the
required voltage range by a series resistor. Called a Multiplier Resistor.
• The volts drop across the meter is 0.1 Volts so to have a full scale of 1V
it is required to drop (1 - 0.1) Volts across the series resistor
• Apply OHMS LAW: R = V / I, with V = 1 - 0.1 = 0.9 Volts and I = 1 mA
• Therefore the multiplier resistor is V / I = 0.9 / .001 = 900 Ohms.
- and the meter is a total of 1000 Ohms input resistance
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
4
Ohmmeter
+
Meter
Calibration R
Internal r
Black
Red
• To set for ZERO OHMS the two probe leads red and black are joined and
the calibration R adjusted for FSD.
• Measuring a resistance introduces another series resistor into the circuit
which reduces the current through the meter.
• Thus the scale can be calibrated in Ohms.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
5
Meter Loading
• Potential Divider opposite should have a
voltage at the midpoint of 0.5 Volts
• Total resistance of the meter is 1000 Ohms;
100 Ohms internal resistance plus
900 Ohms multiplier resistance.
470K
1V
470K
Internal
Resistance &
Multiplier
• The lower 470k is shunted by 1000 Ohms so the bottom pair becomes;
470k x 1000 Ohms / (470 k + 1000) Ohms = 997.79
• The volts measured will be 997.79 / 470997.79 x 1 = 0.002 volts. Oops!
• Care must be taking when measuring voltages across large resistors.
• Use a high impedance digital voltmeter or a very high ohms per volt
analogue voltmeter. NB: DVMs may be >10MOhm impedance
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 21: v1.0, 27-Sept-2004
(10) Measurements-1 - Meters
6