Test-Measurement (Page 4)

This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.

U2A opamp LF353 is used here as a constant current source, R10-D13-D14 is for protection in case you measure voltage in the ohms range. U1 4052 helps digitally select four different currents, the currents pass thru the unknown resistor to be measured and an voltage developed across the resistor is measured.

U2B is a buffer which passes on the voltage measured to U5A for inversion of polarity as current source is a negative current (current sink). U5B amplifies to the level required for the LM3914 display circuit. U7 555 is used as a de-bouncing for switch SW1 so as to advance counter 4029 to change the range of resistance measurement.

Build a DMM - LM3914 Analog display

Use any FET input dual opamp, TLO72 will work but LM358 will cause error as it is transistor input..

D1, D12 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/-3.5V only, with some FET it needs to be tweaked. LF353 is a Wide Bandwidth Dual JFET Input Operational Amplifier.

Build a DMM – LM3914 Analog display

U1A LF353 opamp turns on a buzzer when the voltage at Test + falls very low indicating a short between Test + and Test – or a resistor less than 5 ohms. The R9-R14 divider determines the minimum voltage that can be at Test + for buzzer to just turn on.

The Opamp here is a comparator and R10 10 Meg gives a very small hysteresis so that there is no oscillation at threshold levels, that is when both inputs at same levels.The Zener is for protection and R6 to limit current.

U2A and U1B opamps form a precision rectifier, note that this is not a true RMS rectifier circuit for that see some devices from Analog Devices. When you need to rectify a 200mV AC signal you cannot use a diode like 1N4148 as the diode turns on at 700mV so this circuit is used.

AC conversion and continuity buzzer test

The AC signal measurement is best to do with a True RMS convertor. This Type is able to quantify the Heating power of a AC voltage, includes all harmonics and Noise too.

With these circuits you can make an insulation tester going upto two tera ohm, hence currents will be in pico amps, great care required in design. The rotary switches for voltages and ganged interlocked range switches must not be phenolic but industrial epoxy based molded switches.

The parts list which is not in the circuit, is listed below, the circuit is 15 years old. but you may get some idea on high resistance measurement. Use 1% MFR for all Resistors and low leakage plastic caps for low values. These instruments i used to calibrate with Victoreen Resitors and a Electrometer from Princeton Research, I faintly remember.

Theory of Operation.

The Device Under Test DUT say a transformer is placed in a Metal tray connected to the Guard SK3 terminal. The metal tray has a 3mm glass sheet on which DUT is kept. For 2 Tera ohm make sure that Humidity and Dust do not affect measurements.

Now 1000V is applied on a DUT terminal from the high voltage supply, ( in reference to Guard SK3 at earth and 0V). Then the point where the leakage is to be measured is connected to SK4 via a BNC short, scope, shielded cable. The current goes thru a shunt selected by S5 and voltage across the shunt is measured by ICL7650 a Chopper stabilized amplifier, with ultra low offset and bias. The amplified output is fed to Vref of ICL7107 which displays the Insulation Resistance. The full range is not valid in this circuit and for low values, change to a range where the reading is more number of counts. The above circuit does not include some upgrades and changes i did later using ICL7135 etc.. If i find it i will add it later.

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With these circuits you can make an insulation tester going upto one tera ohm (2 Tera ohm max). hence currents will be in pico amps, great care required in design. Also 1000 Volts DC is generated which can cause injury. take great care. the above circuit is for the advanced instrumentation hobbyist only, do not try it at home.

The parts list which is not in the circuit is listed below, the circuit is 15 years old. but you may get some idea on high resistance measurement.

IC7, IC9 – NE555 – Timer IC
IC8 – LM723 – Voltage Regulator

The above list is from my memory, hence it may be wrong, i have forgotten this circuit, this circuit was scanned by a hp photosmart and resized and optimized by irfanview. also note the technology may be obsolete, but basic idea is still the same today.

Insulation Tester power supply

Theory of Operation.

IC7 555 as an Astable chops the DC with T2 NPN transistor. TR2 was a Russian U Core High-Freq Transformer. The secondary was insulated with mylar, layer to layer and impregnated in Mica-Lacquer or varnish. The 723 Chip along with T1 is The closed loop regulator which changes the DC which is chopped.

The high voltage is attenuated and that is the feedback to 723 chip which by comparing to a reference, controls the output. The diodes and caps are in series to withstand 1000V and above.

Insulation Tester or Teraohm Meter with Polarization Index

This was designed, keeping in mind, that sometimes we have few components available and you could not carry your DMM somewhere, but you have to make some instrument for a quick need with what is available.

Analog Dial AVO Amps-Volts-Ohms meter

IC3 LM555 is to generate -5V from 9V. That reminds me of a story, In 1986 while i was studying BE electronics i got a chance to work part-time in a company that sold-serviced imported instruments, it was called trans-marketing. They were the agents for Racal-Dana, Genrad, Data I/O and so many more. Here i came across a Book called CMOS Hot Ideas of Intersil. I had a chance to observe, build and learn some things here. The ICL7107 circuit was the most fascinating, I also read the books of National Semiconductor here.

Analog Dial AVO Amps-Volts-Ohms meter

In those days small firms used to make DPM’s digital panel meters with 7107. In 1987 when i got a oppurtunity to design a DPM for a firm, I put the 555 clock in place CD4009 clock shown in intersil, to derive the -5.

This CircuitĀ  is a DPM or digital panel meter. It has a analog bar graph display and a 3-1/2 digit digital display. ICL7107 is used in the 200mV configuration.

U4A opamp LF353 amplifies the 200mV Full scale input to the level required for the LM3914 display circuit. D13-D14 are clamping protection diodes. Adjust P1 trimpot for a reading of 1000 counts when a 100.0 mV signal is fed at Vin. Adjust R8 trimpot to get the 5th ledĀ  to just turn on at 100mV input.

Analog and Digital Voltmeter using ICL7107

A combination of digital and analog display is helpful for quick decision making. Analog indicates even from a distance the process dimension.

Human Brain understands analog better. The digital is required to note down and record values for determining a setpoint or performance of a system.

Sometimes a analog recorder with a ink-pen plot against time is a very good way of process analysis. Many systems are better studied using graphs not tables and lists of numbers. A Computer based data-logger gives greater power to this methods. You can measure and plot graphs of various types and at different points and for much longer periods.

This is a part of my Build a DMM or Digital Multi Meter

Here is a easy to read ‘Analog’ Millivoltmeter. Just like the Moving Coil Voltmeter, but does not have that resolution. This gives a easy indication of process progress or parameter magnitude from a large distance. A bargraph is easy on decision making too, compared to a digital readout.

How this Works ? – The analog input in mV – millivolts is fed to R18, RC reduces Noise and the Zener Clamps protect. The LF353 FET Opamp offers High Impedance as a Non-Inverting Amplifier, which nullifies measurement burden, Remember the Moving coil voltmeter loads the measured circuit, causing sizable errors. Then they invented the Vacuum Tube Voltmeter to solve this issue. An FET voltmeter is near ideal, they ought to have inventing this first.

The Zeners and C3 Plastic cap should not leak, even if they do it should be in Pico Amps. Get quality stuff and do a neat job putting them together. To master this leakage and other aspects, try building an Electrometer with CA3140. Another Measurement challenge is uV Microvolt measurements, you will be faced with new glitches in connectors and PCB due to thermocouple effects and contact resistance. A soldered joint near a hot resistor will set up enough thermal gradients and create many thermocouples all over the board. Try to measure 1 Microohm with a 10 Amp pulse or 1 A DC. You will learn many things. Connectors have a craze for the Precious Metal, they act funny if they do not have enough Gold on em.

Once i observed, very low voltages or circuits with nominal voltages but very low currents, cannot break a near invisible layer between the plates of a good connector. A sub-micron coat of corrosion, dust or even some organic deposit, was forming a dielectric layer which was impervious to uV and pA. A good cleaning with a volatile organic solvent solved the problem but messed up other plastics nearby.

Millivolt Meter using a LM3914 LED Dot Display

Millivolt Meter using a LM3914 LED Dot Display. – This circuit is a part of my Build a DMM or Digital Multi Meter