Test-Measurement (Page 6)

This is a 20 Tera Ohm Insulation Tester I made around 20 Years Back. It had even Polarization Index for Transformer Insulation. It generated 1000V DC Regulated Voltage and the pA leakage Current was Measured. See more here Insulation tester or Tera ohm meter.

Insulation Tester

A regulated, low ripple, high voltage of very low current capability is applied by instrument on a DUT (device under test). The leakage current in nA or pA (picoamps, nanoamps) is measured and readout, The Innovation i did was to measure the current in Vref and not Vin of ICL7107, this made the 1/x computation.

Also using the ICL7650 Chopper Stabilized amplifier must be done as shown in data sheets with grounded guard rings. pA will leak on the PCB itself if layout is not done keeping this in mind.

Measurement Of Temperature – When power transistors are used, they may tend to over heat. Likewise resistors may also overheat in the event of faults or short-circuits. The knowledge of their temperatures may be advantageous. In addition, measurement of temperature constitutes a basic necessity in day-to-day life.

Measuring the temperature of a body, depends upon the establishment of thermo-dynamic equilibrium between the body and the device used to sense the temperature. In practice, this condition is rarely attained since it is difficult to establish complete instantaneous equilibrium. Hence great care must be exercised in choosing a method suited to the problem so that satisfactory conditions for temperature measurements are obtained. Temperature sensors possess thermal characteristics dependent largely on their size and shape and the materials from which they are made. These characteristics affect precise measurements. The introduction of a temperature sensor into a body tends to modify the temperature conditions at that point. In most cases the sensor is connected to a recording instrument by means of an intermediate system, along which the signal is carried. The intermediate system and the recorder may be subject to temperature and other changes. Hence compensating devices become a necessity to reduce or eliminate errors.

Diode Thermometer

The measurement of temperature in our instrument depends on the fact that the forward voltage drop of a silicon diode changes by about – 2 millivolts per degree centigrade. Thus, by measuring the change in forward voltage of silicon diode kept in a temperature probe, the voltage drop can be converted into temperature.

Since this involves the measurement of millivolt level accurately a precision voltage source is needed. This can be conveniently obtained from the 3 pin + 5v voltage regulator. This voltage is tapped using a preset VR6 whose output is used for adjusting the ice bath temperature reading to zero degree. This tapped voltage is fed to the diode in the temperature probe and the other end of the diode is returned to a negative supply of -8v. The negative supply uses a (-8v regulated output from IC 7808 voltage regulator) which has the least variation with temperature. Now, the voltage at the probe point is connected to the input of DPM via function selector switch ST.

The temperature probe can be made by a length of shielded audio cable connected to any type of mini plug and fitted onto the front panel socket SSG/T. The free end of the cable is soldered to the diode. The diode is kept just at the tip of the cable. A miniature glass diode like 1N4148 is preferred. The soldering makes a good fixture at the end of the cable. The meter can thus measure temperatures from 0°C to 150°C continuously and upto 200°C momentarily since above that the cable starts melting.

Epoxy Resin and a used Metal Pen Refill can be used to make a sensor to insulate the cable. The diode must be thermally and electrically isulated from metal tube.

(above text may have ocr and concept errors)

Extra Reading –

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.
VR3-220KE

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