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 Circuit  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 led  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

This is a Voltage to pulse converter using opamps for measuring speed or rate from a Tacho Motor mechanical coupled to a Conveyor System.

See the complete circuit in my site to study other sections. The motor DC out is filtered and scaled. Then the clean safe dc is fed to this VCO based on a generic application note design.

The pulses can be used to advance digital counter or mechanical counter using a solenoid driver transistor circuit. The Voltage Vin can be further scaled and fed to a Digital Panel meter to read the rate. VCO - Voltage Controlled Oscillator using Opamps


VCO – Voltage Controlled Oscillator using Opamps. LM339 is a quad  Comparator.

 The complete PDF circuit and other such circuits at my Mixed Circuits Analog with Digital

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

This is dual 555 multivibrator like power oscillator. This is my contraption and conception but couple of decades old. It appears to have errors. It worked but the document fixing after design worked, not done. This can be used to make a small inverter, for powering 230V gadgets of low power. The transformer is used in step-up mode. It may make a sound if the laminations are not of good quality.

This circuit can be modified using 7555 and switching transistors like BD139 driving a Ferrite 100 kHz transformer to build a Fluorescent Lamp of 10W to 20W.

From Schematics of delabs

Fluorescent lamps can be fired and lit at high frequency at relatively lower voltages. Experiment both with frequency and pulse width to get nominal lighting at good efficiency. One of the reasons CFL lamps -230V and emergency lamps 12V fail, is failure of the switching devices. These have to be overrated thrice or more, the switching heat dissipation should not be underestimated, during low/high voltage and keep a aging allowance.

Designing a working circuit is no big deal. But making a batch of reliable and robust power electronics products, which are idiot proof and abuse resistant is a challenge for engineering. Then they will need it at an affordable price too… Checkmate !

This circuit is a Parallel interface between 89C52 of Atmel with 7135 of Intersil.  With This circuit you can read analog data of both polarities. You can change the range scale with extra circuits, you can store data on a EEPROM or send them to PC thru RS232 or Comm port.

Using Linux in Embedded Electronic Devices

The Circuit Is Shown for One Anode Drive and one Segment Drive for Display. In similar Fashion connect rest, all 5 anodes and 7 segments and one dp decimal point. The +5 V of 2N2907 and gnd of BC547 must be directly from regulator with a big cap or even a separate supply.

AT89C52 Parallel Interface to ICL7135

Parallel interface is faster that a serial interface.It is more “Real Time” for a given processor and clock speed. The code and accompanying hardware also can speed up the data acquisition.

AT89C52 Parallel Interface to ICL7135 – Complete Page

A single maxim chip can act as a good interface between the uC and Serial Port. You could also do this with Logic chips but correct levels and isolation cannot be achieved, Hence better to use use these interface chips.

These days USB and Wireless Interfaces are being used and Tablet Computers becoming Popular. The Interfacing of the future is “Device Networking” and Wireless may be common.

This is a SMPS Circuit application very close to the Application Note in the book SGS Motion Control Application Manual. This worked well. Read about SG2525 – SG3525 – PWM SMPS Regulator Chip.

Some Notes Related to the Project

  • In main circuit do not link different grounds.
  • Main circuit can be used both for 110 AC and 230 AC
  • Ferrites are partial conductors use proper insulation before winding.
  • For main transformer TRX2 use split bobbin for good/safe isolation.
  • Epoxy coated toroids have to be further insulated before winding.
  • Line of isolation between primary and secondary circuits should be explicit.
  • Primary components like Q1, Q2 tabs R5, R6 etc can give shock take caution.
  • Each main module is 500W and can be used in parallel for more current.
  • Q1 and Q2 should have appropriate isolated heatsinks TO220 type 50sq cm
  • Schottky diodes D5, D6 should have heatsinks TO220 type 100sq cm.
  • Diodes D5, D6 eg D83004 are TOP3 packages are used 2 in parallel.
  • For 200W and above R2=0.1e 5W in the main circuit fusible ceramic.
  • C10, C11… Add 4.7uF 100v more in parallel to reduce ripple.
  • C13, C14 can be 250V for better safety margin.
  • TRX1 SEC1 and SEC2 dot polarity is anti-phase if same phase danger!!

100kHz Half Bridge Convertor - SG3525
Zoom Image

Magnetics Design

All Transformers Isolation 1kV PRI To SEC / SEC To SEC / PRI,SEC To Core. Use Yellow Mylar Tape Insulation or better for all. Vacuum Impregnate all Magnetics in Epoxy or Varnish. All Power Tracks on PCB reinforce with Copper Braid.

TRX1 Mosfet Drive Transformer

T25.0 MGQ-5L Hitachi – Type Torroid – 100khz Signal

  • PRI1 20 Turns #22 Awg
  • SEC1 9 Turns #22 Awg
  • SEC2 9 Turns #22 Awg

486T250-3C8 Ferroxcube – SEC1 And SEC2 Antiphase

TRX2 Stepdown Invertor Transformer

EC52 Siemens/Hitachi – Can Be ETD/EER Cosmo Ferrites

  • PRI1 22 Turns 2 Layers (44 Turns) – 2* #16 Awg(18 Swg) In Parallel.
  • SEC1 4 Turns Ct Copper Strap 0.01″ * 0.8″ Copper Strap

Coper Strap/Ribbon used, cause Hi-Freq Skin Effects, PRI 2 Wires paralleled for same reason.
EC52-3C8 Ferroxcube Phillips

Trx3 Current Feedback Trx

T25.0 MGQ-5L Hitachi – Type Torroid – 100khz Signal

  • PRI1 1 Turn 4 Amps Max
  • SEC1 20 Turns #22 Awg CT.
  • Ceter Tap 10T-CT-10T

486T250-3C8 Ferroxcube

L1 Series 60A-80A Inductor

Type EC/ETD/EER EC42 Hitachi – Air Gap In Inductor Core Both Sides
100khz Power IF30-3C8 Ferroxcube – 6 Turns 4*#12 AWG In Parallel

4 Wires Of 12 AWG Twisted & Wound For 6 Turns (Use Less AWG For Less I)

TRX5 50hz Transformer Small

  • PRI1 & PRI2 115v
  • SEC1 24V 0.2A
  • JP2 2-3 Short 230V
  • PRI1 & PRI2 In Parallel For 110V
  • JP1 1-2 Short 230V 2-3 Short 110V


This is A 10mH Common Mode Filter

The Reference Application on ST Half Bridge Convertor – SG3525

PCB Layout

The PCB of module will be added later, if i locate it. It is small and be designed easily.