7. MEASURING RECTIFIER
7.1. Tasks of the measurement
7.1.1. Measure the dependence of the rectified mean value of the output current on the RMS
value of the input voltage for the full-wave (Graetz type) passive diode rectifier loaded
by resistor R 1 = 100 The rectified mean value shall be evaluated using voltage on
the resistor (connection according to Fig. 7.1). Measuring range of the DC voltmeter
V 2 is 200 mV. Observe the current waveform using oscilloscope.
7.1.2. Insert the decade resistor box in front of diode rectifier (see Fig. 7.2) and select its
value R D so that the whole combination creates AC voltmeter with measuring range
2V (keep the 200 mV measuring range of the DC voltmeter V 2 ). Measure the
dependence of DC output voltage U 2 on the RMS value of the input voltage. Observe
the voltage waveform on the load resistor R 1 using oscilloscope.
7.1.3. Create an AC voltmeter with the same measuring range as in the 7.1.2 using an
operational amplifier with rectifier circuit in the feedback path (i.e. operational
rectifier) - see Fig. 7.3a or 7.3b. Derive the formula and enumerate the value of the
resistor R D to achieve the correspondence of 1V of RMS input voltage to mean
voltage value U R1 = 100 mV across the R 1 resistor. Set the decade box to achieve the
required voltage range experimentally and explain the eventual difference between
calculated and set values.
7.1.4. Measure the dependence of output DC voltage on the RMS value of input voltage.
Observe both the output current waveform and voltage waveform on the OA output.
Explain the op amp function in this circuit as a current source for the rectifier.
Note: Measure in 7 points of each curve (for voltage values U 2 = 5; 10; 25; 50; 100; 150;
200 mV measured by voltmeter V 2 ). Plot all the three measured curves into the same
graph.
7.2. Schematic diagrams
R2
R
~ 6 V, 50 Hz
R1
R
V1
C
V2
OSC
+
Fig. 7.1 Volt-amper characteristic measurement on the the rectifier loaded by R 1 resistor
RD
R2
R
~ 6 V, 50 Hz
R1
R
V1
Fig. 7.2 Passive full-wave rectifier
+
C
OSC
V2
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a)
RD
R
~ 6 V,
50 Hz
R
R2
R1
+
V1
C
V2
OSC
+
R2
b)
R1
+
R
+
C
OSC
V2
~ 6 V,
50 Hz
V1
R
RD
Fig. 7.3 Active full-wave rectifiers
7.3. List of the equipment used
V1
- AC voltmeter (PMMC voltmeter with rectifier), accuracy class 1.5; voltage range
2.4 V;
V 2 - digital multimeter, type ..., function DC voltage, measuring range: 200 mV;
R
- adjustable resistor ... , ... A;
R D - resistor decade box, accuracy 0.2 %;
Full-wave rectifier with load resistor R 1 and RC filter (R 1 = 100 , R 2 = 10 k, C = 10 F);
Operational rectifier
Power supply for the OA ± 15 V (± 12 V);
OSC - analog oscilloscope, model ...
7.4. Theoretical background and hints for measurement
Full-wave diode rectifier with serial resistor is usually used for PMMC voltmeters as AC
voltage to DC current converter. The non-linear characteristics of semiconductor diode is used
for the rectification and, consequently, also the dependence of rectified mean value of the
output current on the RMS value of the input voltage is non-linear (the circuit on Fig. 7.1
allows to measure volt-ampere characteristic of the serial connection of the resistor R 1 and
rectifier). Influence of this non-linearity is compensated by non-linear scale in case of PMMC
devices (e.g. in case of V 1 voltmeter). Series resistor R D compensates partially the nonlinearity of the rectifier. The resulting non-linearity depends on serial resistor value, that
means on the measuring range used.
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The passive full-wave rectifier cannot be used in AC digital voltmeter (where the voltage
across the load resistor is measured by AD converter). Such a voltmeter (see Fig. 7.2) would
have non-linear dependency of the reading on the rectified mean value of the input voltage
(see the result of the measurement 7.1.2). Therefore, operational rectifiers are used in digital
multi-meters for AC current and voltage measurements.
The rectifiers based on OA according the Fig. 7.3 have a linear dependence of the rectified
current on the RMS value of the input voltage. In this case, the passive rectifier is supplied
from current source (Fig. 7.3a - inverting voltage-to-current converter, Fig. 7.3b – noninverting voltage-to-current converter). Considering ideal OA, for voltage-to-current
converter the following formula is valid (see [1], Chap. 3) i 2 = - i 1 = - u 1 /R D . From this
equation it follows for rectified mean value of the output current:
I 2RM 
where
U 2RM U1RMS

R1
RD
(7.1)
I 2RM is rectified mean current value at the OA output (A),
U 1RMS
RMS voltage value on the OA input (V),
U 2RM
rectified mean value of the voltage on the R 1 resistor (V),
RD
resistance of the resistive decade box set value (.
For the sinusoidal waveform there is U 1RMS = 1.11 U 1RM . This formula should be used in (7.1)
for finding value of R D .
Note:
The rectified mean value (RM) is the DC component of full-wave rectified voltage.
This value can be found at the output of the low-pass filter filtering off all the
harmonic components of the rectified signal. The passive integration circuit in
Fig. 7.2 plays the role of that filter. In the case of full-wave rectifier it should filter
off component of 100 Hz and higher harmonic components, so there should be
  R2 C  10 ms.
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