Measurement of Voltage : –

In testing electronic circuits, Measurement of voltages is important for diagnosing faults and making the circuits work. In circuit diagrams given in equipment manuals, voltages at various points in the circuit are usually marked. A deviation from these values indicates that some component has failed and eventually leads to clues for isolating the faulty areas.

Voltmeter Attenuator Rectifier

Specifications :-

D.C. Voltage
Ranges : +/- 200 mV, 2V, 20V, 200V, 2000V.
Input impedance: 10 mega ohms.
Circuit protection: + 2000V D.C. all ranges.
Over range: 100% to 1999.
Accuracy: +/- 0.5%.

A.C. Voltage
Note: Average responding Ranges calibrated for sine wave.
Ranges: 200 mV, 2V, 200V, 2000V
Input impedance 10 mega ohms.
Circuit protection : 750V r.m.s., all ranges.
Over range: 100% to 1999.

Description :-

As our DPM is capable of measuring only 200 mv full scale deflection, the input voltage in the case of exceeding the range needs scaling down. This is achieved by an attenuator chain.

D.C-voltage -measurement:

The circuit for the measurement of voltage (AC. and DC) from 0.2V to 2000V is as shown. In case of DC voltage measurement, A mode switch selects the input voltage and passes it via an attenuator chain. Resistors R6, R7, R8, R9 and R 10 comprise the attenuator chain. The attenuation chain is in fact the range selection network.

The voltage ranges are provided in 5 decades i.e. 200 mV, 2V, 20V, 200V, and 2000V. The input voltage after attenuation is fed, depending on the range selected by switch Rs, through switch Sad to the DPM input point. The reading on the DPM gives the value of DC voltage being measured.

A.C-voltage measurement:

Most D.C. measurements are made with AC. to DC. converters which produce a DC. proportional to the AC. input being measured and apply this DC. signal to the DPM. Converting the signal to DC at an early stage minimizes the serious errors which otherwise could result from frequency selective circuits.

When an AC voltage is to be measured, the switch Sad is to be operated. This switches enables the signal to pass through a buffer and precision rectifier and then to the DPM input while measuring AC. but passes it directly to the DPM input while measuring DC. So, now the signal after passing via the attenuator chain is fed to IC2. The buffered output of IC2 is fed through the capacitors C 10 and C 1 1 to IC3 (CA 3140-TL071) which is an FET input operational amplifier, acting as a precision rectifier. By means of diode D4 and resistor R24, rectification with gain is obtained for positive half cycles of the AC. signal while the negative half cycles are directly fed back by the diode D3. The half-wave rectified voltage is filtered by the resistor R25 and capacitor C12 combination.

The capacitors C6, C7, C8, C9 connected across resistors in the attenuator chain provide some frequency correction during AC input. The presence of offset voltage in IC3 is to be compensated using variable preset VR2. Preset VR3 is used to correct the reading so as to indicate the true a.c. value of the voltage. On passing the preset VR3, the signal enters the DPM. The reading on the panel gives the value of AC voltage being measured.

Parts List :-

1. Semiconductors
IC2 and IC3-CA3140 or TL071. D1 and D2-5V Zener 1W, D3 and D4-IN4148,

2. Resistors.
a. 1/2 W 1%,
R6-1M, R7-100KE, R8-IOKE, R9-1KE, R10-100E,
R18 and R19-10K, R23-15KE, R24-100 KE, R25-1 KE,

3. Presets.

4. Capacitors
C10 and C11 10MFD, C6-47PF, C7-1 KPF, C8-6.8KPF, C9-8KPF, C12-1MFD.

5. Miscellaneous
SSG/T-SOCKET, 51,52-DPDT, A,B,C,D-BNC,SKT, F2-100mA fuse, RS-8P2Wx5 INTERLOCKED. S-2p2wx7 Interlocked

Every electronic gadget primarily needs a D.C, power supply to energize it. It also forms the basic requirement for any constructional project. consequently there is a need to obtain multiple voltage values for cost reduction, convenience and compact arrangement for all the above applications

List Of Components For Power Supply.

1. Transformers

X1-6-0-6 (500 ma), X2-12-0-12 (500ma)

2. Semiconductors

IC6-7805, IC7-7808, IC8-7908, D1 to D10-IN4007, D11 and D12 – 12v, 1W, Zener

3. Resistors.

R1 and R2 – 100E 1/2 W CFR

4. Capacitors. C 40v

C5 and C8 – 1000 Mfd , C1 – 2200 Mfd, C5 and C7 – 0.1 Mfd, C9 to C12 – 100Mfd

5. Miscellaneous

F1-250ma, N1-Neon, 3-Pin Mains Chord.

Multi Output Instrument Power Supply


The required D.C. power supply is usually obtained by means of a transformer. It is also possible to have transformer­less power supplies. Though the elimination of the transformer makes the circuit compact, economical and simple, also facilitating quick assembly and built in short circuit protection, certain drawbacks creep in. These power supplies are useful only for low current applications. Special safety precautions ? are to be followed while using them. Physical contact should be strictly avoided, since the output terminals are not isolated from A.C. mains supply.

This obviously necessitates the use of a transformer. By suitable modification it is possible to obtain multiple/ fractional dual voltages from a transformer. Different not-so obvious voltage values can also be obtained from the transformer by rectification circuits. The output so obtained from a transformer secondary is unregulated. For good load regulation, the internal impedance of any power supply should be as low as possible. The regulation can be improved either by resistor zener method or series regulator method.

However, the three-terminal regulators greatly simplify the power regulation problem. These regulators need no external components. They employ internal current limiting and thermal shutdown which make them tough. For simplicity, compactness, convenience and accuracy the use of three­ terminal regulators is ideal. These IC voltage regulators are freely available in various ranges both positive and negative. A functional schematic of a three terminal regulator is shown in the datasheet. It can be seen that the device is a complete regulator, with built-in reference, error amplifier, series pass transistor and protection circuits. The protection circuits include current limiting, safe area protection to limit dissipation in the series pass transistor and thermal shut down to limit temperature.

Low power IC voltage regulators of the 78L series used in our measuring instrument are now so cheap that they represent an economic alternative to simple zener-npn stabilisers. In addition they offer the advantages of better regulation, current limiting/short circuit protection at 1000 mA and thermal shunt down in the event of excessive power dissipation. In fact, virtually the only way in which these regulators can be damaged is by incorrect polarity or by an excessive input voltage. Regulators in the 78L series upto the 8v type will withstand input voltages upto about 35v, whilst the 24v type will withstand 40v. Normally, of course, the regulators would not be operated with such a large input-output differential as this would lead to excess power dissipation. All the regulators in the 78L series will deliver a maximum current of 1000mA provided the input-­output voltage differential does not exceed 7v. Otherwise excessive power dissipation will result, causing thermal


Two transformers have been used to step down the voltage from 230-250v a.c. mains input. One of the transformers produces an output of 6-0-6v at the secondary terminals. This output is fed to a full wave rectifier and a capacitive filter. The filtered output is fed to IC6 which is a 3 pin voltage regulator which gives a regulated output of + 5v. This is used to activate the DPM circuit. It is also fed to the temperature network as a precision voltage reference source.

The other transformer produces an output of 12-0-12v at its secondary terminals. The centre tap is grounded like in the previous case. The other two terminals of the secondary are fed to a bridge rectifier constructed using diodes. The rectified output is filtered by using capacitor C5 and C6 fed to IC7 and IC. The IC7-8 which is are 3 pin voltage regulators gives an output of ±8v. These two voltages are fed to the signal generator. The -8v source output is fed to the temperature network, also as voltage reference. It is also necessary to produce a +12v and -12v supply for application to operational amplifiers. This can be conveniently done by means of 12v zener diodes. The output of the bridge rectifier is clamped to +12v and -12v respectively using two zener diodes. The zener output is fed to the operational amplifier supply terminals. Since the supply to

the operational amplifiers need not be very efficiently regulated to + 12v, the use of zener diodes proves economical.

For the testing of electronic components a voltage of above 50 V is required. This can be achieved by means of a voltage quadrupler circuit. It consists of four diodes and four electrolytic capacitors. The secondary ungrounded terminal of the 12-0-12v is connected to the quadrupler circuit. The output of the quadrupler circuit is 68v with respect to ground.

The two transformers can be controlled by the power supply switch PS 1 The switch also controls a neon lamp, which lights up once the transformer supply is on. The instrument is prevented against short circuits-excessive voltages by fuses. When the a.c. power supply exceeds beyond 250 volts resulting in any overload or damage, the fuse F1 blows out thus saving the rest of the circuit within the instrument.

The a.c. power is drawn from a 3 pin plug connected to a cable of 1000 mm to activate the instrument. The ground terminal in the 3 pin plug is earthed to the chassis, while the other two terminals are connected to the primaries of the two transformers.


D7 one end should be grounded.

Power supply part of the mesureall instrument i built 20 years back, ocr errors may be there, like I – 1 and 0 – O, other errors also uncorrected, reference textbooks, appn notes, datasheets

Reference Resources and Extra Reading