## Saturday, November 19, 2016

### Current Source for Resistance Measurement

Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm's Law the Voltage across Resistor is proportional to Resistance value.

Resistance measurement with Current Source

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

Design of a Constant Current Source

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded - Interactive Java Resistors Tutorial.

## Friday, November 18, 2016

### LM317 based Regulated Power Supply

This is a Regulated Power Supply based on the LM317 IC. It will need a Boost Power Transistor and heatsink for higher currents. It is a versatile building block for stable instrumentation supplies. Consumer Electronic gadgets can use a SMPS chip. In case you wish to use a SMPS for a Precision Instrumentation Block, then take extreme care on Shielding and EMI-RFI.

This is a General Purpose Chip, Series Regulation. It can be varied or trimmed. There is an Internal temperature compensated reference. The minimum trim value is around 1.2. In case you want a Low value voltage like 0.5 with a good current, then use a good negative supply to offset the 1.2 V.

Power supply with battery backup for DMM

The transformer can be s Split Bobbin with Pri-Sec copper shield foil.  This can be earthed along with the metal enclosure. C4 sends any hi-freq components to earth.  It is better if you do not earth the ground but use such capacitors. A Supply should simulate a a battery with both ends floating wrt Earth. A option to eartth the ground is fine. This also helps the user to configure his own dual supplies.

Q1 and R5 form a Short Circuit OR current fold back OR constant current mechanism. TP1 can be used to vary the output voltage.  Better use something like a Bourns 10T trimpot. An open preset may introduce a noise due to dust and vibration. I don’t remember why i added a zener DZ1, a diode may suffice.

See more at my Power Supplies Section.

## Wednesday, November 16, 2016

### Millivolt Meter using a LM3914 LED Dot Display

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

## Friday, October 14, 2016

### Precision Op-Amp Current Source

In this circuit we tackle the error indicated in the earlier Current Source. The LM336-2.5V eliminates the tiny error of the regulated supply and resistors. Thereby increasing Precision to a higher degree.

The opamp mirrors the stable 2.5V across P3 + R13. With P3 Bourns 10 Turn Trimpot you can trim the current for calibration. Q1 BC557B having a Beta - hfe of 200 is used. But a higher gain or a FET here may reduce error further, that may be needed if you are going for 16 Bit or more resolution. Then even opamp needs to change.

Mini RTD Pt-100 Three Wire Transmitter

Suppose you build it with the best Opamp, FET etc., but place it close to a Warm transformer, Regulator chip or even a Cooling Fan, you will see the lower digits of a 5-1/2 DMM spinning fast.

## Tuesday, October 11, 2016

### Resistance Measurement Analog LED Meter

This is a LED Analog Meter, This can be used as a Resistance Meter and Low Impedance Voltmeter for Battery Levels.

Resistance measurement Current Source

To measure battery voltage, the R5-R12-R17 etc. part of the Reference Resistor Divider Network can be modified to suit. Shown here is for 4 LEDs, Use Three LM324 for 12 or More LEDs and Cascade as shown.

This cannot Measure Voltage levels from High Impedance Sources, will work for Battery Voltage Tests. To make it into a Continuity tester. R27 must be a short and R23 5 Ohms. The Black probe should have a Built in Resistance of 2 Ohms. If you want it to be a dedicated voltmeter, remove R3, The Probe has to be a 10X Attenuator with 10M Ohm and The Resistor Divider Steps in 100mV per Step. The R27, R23 etc. is 20K.

Simple Test Equipment

A Leakage Tester a Mains Voltage Monitor are other possibilities. Use LM3914 for a easier solution. A nice book for your Design Library - Measuring Circuits By Rudolf F. Graf

## Tuesday, October 04, 2016

### RS232 with Opto-Isolation

I had to once interface an high voltage circuit to PC, The uC had to communicate thru RS232--Comm port--Serial Port.

Part of the 80C31 8051 SBC

Even though i had isolation at the sensors and actuators to make doubly sure the PC also has been isolated. There are chips that are available for this purpose, The circuit above is built with discrete and passive components except for the opto 4N35. You can use MCT2E and CNY17-3 Optos too. For MCT2E some tweak may be needed as current transfer ratio is 20, for the other two CTR is 100 so above design will work.

The circuit derives power from PC but does not load the PC supply. Any voltage above 5V applied to the PC connectors may lead to damage of motherboard in PC. Old PCs were more vulnerable but PCs today maybe a bit rugged at the Ports. Due to internal current limits and clamping.

The VCC, VDD and Agnd are derived from PC no other power needs to be applied on PC side of opto. On uC side of opto the uC power supply lines +5 and gnd has to be used. There is no copper link between the two sides and depending on opto a 1KV isolation is possible if PCB is well designed. The PCB should show the visual isolation above and components should be laid on separate areas of PCB to prevent creepage.

The LEDs are to indicate the port activity Rx and Tx, they are not required once testing is over. The circuit can be simpler, but this worked for me and it is not tested at very-high buad rates.

The levels of RS232 are not TTL like 0-5 we have both polarities +10 and -10. The circuit has to change that to drive the Opto Leds.

RS232 software. Understanding RS232 Serial Port Communication.

## Wednesday, August 17, 2016

### Single Digit Voltmeter with LM311

I wanted to design a logic probe as a tutorial, but there were many good ones in the web so i have tried to design a single digit voltmeter. This circuit is a design, i am unable to test it now, later if i test it and find mistakes i will update this page. You can help me by pointing out the errors.

First bear it in mind that it is a single digit voltmeter which is 0-9 counts only on the positive side, that is it can measure +0 to +9V DC +/- 1V error. That may not be practical for the cost of the components above. It may be used as a toy logic probe. The reason for the circuit is not for usage, but to give design ideas. The methodology used is Gut Feel - Thumb Rule method.

First i explain the simple part, D1 a seven segment common cathode LED display is chosen as CD4511 is a sourcing driver. 4511 can be latched so it has been used here, it decodes binary 4 bit decade info to seven segment output. The four bits are derived from CD4029 up-down clock pulse counter. LM311 is a analog comparator with single supply capability which is the A-D interface.

To avoid resistors for each of the LEDs the LEDs are turned ON-OFF at 10KHz 50% duty cycle. The Nand Schmitt Trigger CD4093 is used as in IC4D as a 10KHz Clock which drives T1 transistor with a resistor R5. On turn on IC4D one input is high which is pin 12 pulled up to +5 and another Pin 13 is Low as C4 is in discharged condition in NAND gate both inputs high, gives a low output, the other combinations the output is high. So the output goes high, this starts charging the cap C4 which soon makes both inputs high, which in turn makes output low starting the discharge of C4. This is now evidently a endless loop, hence it is a oscillator. R*C = T .... 0.01uF * 10K = 100uS or 10KHz as F=1/T approx or better still multiply by 1.1 ?. I am not good at formulae but i manage with a calculator.

The supply and ground pins of CMOS chips have not been shown, see datasheet or earlier circuits.

IC4A is also a oscillator but slower which is good enough, it is slow so that a measurement can be made nearly every second. The IC4A slow clock is read by 4029 which produces a count-down binary nibble at Q1....Q4, This is converted to crude analog with R1...R4. The voltage generated is compared with the voltage you are measuring by LM311 which generates a Latch pulse to 4511 to freeze the reading where both voltages match.

Method of Operation :
IC2 is a Counter in Decade-Down mode and IC3 is a BCD to Seven Segment Decoder which Drives the Display D1. The Circuit is wired in such a way as to keep counting the Pulses from the Clock IC4A. The IC4A which is wired as Schmitt Nand Oscillator Clocks the Counter. Now to understand how this Counter and display works see this Interactive Tutorial Simple Digital Counter. For every pulse at Pin-15 of 4029 the Counter Counts down from 9-8-7-6.... and so on. But the Display is Latched by IC4C, So the Display is static even when counter is running. So while testing counter you can remove IC4C and keep LE Pin-5 of 4511 low to ground. For testing this Circuit you can use the manual clock with a pushbutton (single step) or a slow clock rate 1 Hz as in the Tutorial Simple Digital Counter.

The Transistor T1 and Oscillator IC4D is to chop the power to display at a fast rate, this avoids the adding of seven resistors. This is not required, but it saves power and reduces parts count. IC4B is is like a ON indicator, it is a spare gate.

The BCD value at output of 4029 Q1-Q4, four bits, a nibble, is converted to an analog mV value across R6. The resistors R1-R4 which are connected to Q1-Q4 have weighted resistor values for the BCD 1-2-4-8. By ohms law you can understand that the analog value across R6 is approximately proportional to the BCD value. This circuit is just a single digit A-D converter, not even as good as 4 bit converter. Which means approximate value of analog at R6 will do.

Now lastly LM311 is a Comparator, it compares the Analog BCD reference at Pin-3 and the Attenuated Input signal at Pin-2. Output Pin-7 goes high when Pin-3 voltage becomes less than Pin-2. This is made to a narrow latch pulse by C2-R12-IC4C. The latch pulse freezes the BCD data to display till the next latch pulse. R8-R9 attenuate the 0-9 V DC input to a 1/100 value. The zener Diode Z1 is for protection.

I guess the LM311 circuit should work off a single supply, but a dual supply may be required as voltage levels may be near zero. You must be able to see a staircase waveform or ramp across R6. Narrow Latch pulses at Pin-5 4511 on every ramp cycle.

## Monday, August 15, 2016

### LED Voltage Level Indicator

This circuit is derived from a Siemens Application Note 1974. This circuit uses common components of today.

The circuit is here as it is of high educational value. I have not tested it. You can 'simulate and test' or 'wire it up and try' and let me know how it worked. The Circuit is also a simple analog to digital converter. You can use optos in place of LEDs.

Battery Level Indicator

T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators.  When input voltage is increased T1 is turned on which leads to more base current for T3 which Lights LED1. When input voltage is less T2 turns on as it gets a better base current from P3 which turns on LED2 via T4. When both LEDs are off T5 gets biased as no drop across R5 which lights the LED3 thru T5 hopefully.

What you need to know is a small current Ib thru the base-emitter path in the direction of the emitter arrow will lead to a large Current Ic thru the emitter-collector path in direction of arrow. Ic = B * Ib where B - beta is the DC current gain, it could be 100-400

Fluid or Water Level with Reed Relays

Beta is different in each transistor you buy and varies with the test conditions and even with temperature and age. The LED1 and LED2 will indicate above or below Limits set by P2 and P1. The Limit Threshold itself is set at P3 i think. LED3 will light when Hi LED and Lo LED both are off.

The applications of this circuit are FM tuning indicator, Stereo Balance Indicator (Wire T2 like T1 then we get two channel inputs) and battery level indicator.

## Saturday, August 13, 2016

### 5V -1A Power Supply using LM2575

A Power Transistor which is having a drop of 4 Volts across it and passing 3 amps thru it, may dissipate around 12 Watts of Heat, This is the problem in Series Regulators. While a Saturated Transistor or Mosfet with 1 Volts across and 3 Amps Thru will be just 3 Watts. But then a fully on transistor or mosfet cannot be controlled or regulated, for that we turn it ON and OFF very fast so that the right amount of current or voltage is delivered.

Power Electronic Circuits

The way this is done is PWM - Pulse Width Modulation. In this the mosfet or transistor is switched ON-OFF at say 100 kHz, but the ON duration is varied to control the output. The longer the duration of ON time more energy or punch is transferred. Switching losses will be present depending on how fast the rise and fall times of the pulses are.

The Pulsed AC or Chopped DC can be smoothed to the Average with Inductors and Capacitors. The reactive pulses of the Inductor has to be absorbed by a Schottky Rectifier 1N5817 -- 20V-1A fast switching diode with low switching losses.

This circuit is derived from an application note of LM2575, It is a Power Switching Regulator from National Semiconductor The details are here LM2575

## Monday, July 11, 2016

### Interface ICL7135 Voltmeter to PC

A PC based Analog Voltmeter. The Printer Port or Parallel Port Interface is used to acquire high resolution analog data using ICL7135. This is a method of getting analog data into your PC. This is only of Educational Value. Applications include Data Loggers and PC based Process Monitors, chart recorders. (USB, Wireless Interfaces and Tablet Computers are in vogue today)

ICL7135 to Printer port Interface

The four Digit Drive outputs and four BCD outputs of the Multiplexed Display Driver of 7135, along with A-D status output which is also four, are routed to four input pins on printer port via three 74HCT373 Digital Switches. This is because we have less Input pins on Port and 4 are shared by making one 74HCT373 transparent and others High Impedance when PC takes a reading. So the three sets of four outputs of 7135 can be read sequentially.

74HCT139 U9A which is controlled by the PC program can select U3, U4 or U5 for sending data to 4 pins on printer port. Data sent on the eight output pins of printer port can be latched by U10B into U6. This eight outputs of U6 can drive relays or lamps.

The ICL7135 details can be seen here 41/2 A/D with BCD ICL7135 . This Mixed device was far ahead of its times, a product of Intersil.

## Friday, July 08, 2016

### AT89C52 Parallel Interface to ICL7135

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.

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.

## Thursday, July 07, 2016

### Mains Voltage monitor using LM3914

This Circuit helps in the monitoring of mains supply voltage. It does not use a isolation step down transformer. This has to be constructed only by skilled people with knowledge of safety requirements.

C1 limits the current and drops most of the voltage. The zener regulated supply is for the chip. C2 can be raised to 220uF or more if required. The bar mode display may consume more power.

Mains Voltage Power Transformers

R2-R3-R5-R6 form a voltage divider to get a sample of the input voltage, D11-C3 get the DC value.

Adjust R5 preset with a log Plastic tweaker to get the 5th led to just turn on when input voltage is at 138V AC. This has to be done after PCB is put in a sealed fire-retardant-plastic or epoxy box. drill a hole in box for plastic tweaker.

National LM3914 - Dot Bar Display Driver
This Circuit is Not a Tested Design. It is an Idea for study

Warning : This Circuit is Mains Operated without Isolation Transformer and will give lethal electric shock if touched when the circuit is turned on. Test circuit only with DC 9V Bench Power Supply to try it out. Do not use 230V AC.

## Monday, June 13, 2016

### Edison Bulb Life Extender

The Edsion Filament lamp may be less popular, but it is the most cost-effective light even today. It is very affordable, The light is soothing to the eyes and best of all gives a warm feeling.

Sometimes due to the mains being restored when bulb is on, or a rapid flick of switch, blows it. This may be due to the cold filament resistance which is low, allowing a huge inrush current.

LED Lighting - The Future is Here

The problem can be solved with a soft start based on thyristor or mosfet using PWM etc. The circuit here is a low cost solution which may help a bit in reducing the inrush cold current Protection from long duration high voltages is also required but will make circuit more complex.

Now why would anyone spend for a R-C network for a bulb, you could as well buy a new bulb. But I once got a back sprain while replacing a bulb in a hurry, So the price of the bulb alone is not the issue.

CFL Lamps and Simple Inverter

The two diodes 1A form a AC path with a switching delay of diodes, The R-C network is for soft start. The Varistor-Thermistor has a cold resistance 4.7E which reduces as it warms up. If switch is flicked rapidly or power fluctuates, then varistor cannot help as its response is slow. The 10E resistor ceramic fusible wirewound resistor and C1 does the job of absorbing .

This circuit has not been tested for long times at multiple places, so i am not sure of its performance.

## Saturday, June 11, 2016

### Microohm Meter with LED Analog Bar

This indicates like LM3914 in dot-mode. It is a drawing i made made to troubleshoot a gadget, around two decades ago. Strangely it had a echo of a design i had made into a 7107 dpm years before that. Now i am scanning all my drawing and notes, useful or not. Clean or with errors. Many Errors = 1 Blunder. Some projects i made have been expensive Blunders. So see them with a skeptic eye, fix them, try them. Thats all for now.

See the Circuit Full Size - Microohm Meter with LED Analog Bar

This has a 9V battery power. The 555 spins and a negative voltage for Opamp is created. This is a Low Offset amp of OP37 of Precision Monolithics, Inc PMI an early innovator. This diff-amp amplifies the uV of a 4 wire resistance measurement.

Now the current pump is the 2N2222 you see above the OP37. The FET and 555 do synchronous rectification. The LM324 is the Indicator and Analog to LED Dot-Bar Converter. The probes are Gold Plated, or use solid gold pins if you have them in plenty.

### Simple Mains Voltage monitor

This Circuit helps in the monitoring of mains supply voltage. It does not use a isolation step down transformer. This has to be constructed only by skilled people with knowledge of safety requirements.

C1 0.47uF can be brought down to 0.22uF for low LED currents, use high efficiency ultra bright LEDs.C1 should be 440V AC or 630V DC plastic axial yellow, polyester, polycarbonate, polypropylene, metalized film.

R3-R6-R9-R14-R18 resistor divider determines the LED turn on or threshold switch points, 10M for hysteresis.

Mains Voltage and Power Circuits

Adjust R16 preset with a log Plastic tweaker to get the led D2 to turn on when input voltage is at 220V AC. This has to be done after PCB is put in a sealed fire-retardant-plastic or epoxy box. drill a hole in box for plastic tweaker.

LM324 - Low Power Quad Operational Amplifier

This Circuit is Not a Tested Design. It is an Idea for study

Warning : This Circuit is Mains Operated without Isolation Transformer and will give lethal electric shock if touched when the circuit is turned on. Test circuit only with DC 9V Bench Power Supply to try it out. Do not use 230V AC.

## Thursday, May 12, 2016

### Analog PID control using OpAmps

The Measured Value and The Setpoint are two inputs to a Control System. The Measured Value is the Amplified input of a Transducer or Sensor for some Parameter that needs to be controlled. It could be Pressure or Temperature...etc.

The Setpoint is the User Defined Input using a Potentiometer, Thumbwheel, EPROM or Flash Value. This is the value at which the process has to be maintained for that parameter.

Industrial Process Control Circuits

The difference of these two is the Error, this is the input for this PID Analog Computation Stage. The three Opamps are configured as Proportional, Integrator and Differentiator Amps.  The Addition or Summation of these Values is the PID Control Output.(These days it is Math in the Firmware on a MCU, DSP or Software Application in SCADA)

This Analog PID Control Output can now be translated to a 4-20 mA Control Signal, that means 0-100% of power to the Actuator, which could be a Heater, Pump, Fan, Motor using AC/DC Drives. It could be a Steam Valve, Pneumatic or Hydraulic Motorized/Solenoids. The Actuator Size/Array must be right for the Process, a tiny fan cannot cool a Large Furnace, a small solenoid valve cannot fill a Big Tank. An effective Proportional or PID  control depends on choosing or designing the Sensor, Actuator and System Environment prudently.

The Auto Reset is needed to ensure the Integrator does not dampen the Process so much that it fails to even raise to the Process value fast enough (Diffrentiator). So in the Proportional Band the Integrator is Active.

If the Setpoint is 1000 deg C, the proportional band is 10%. The Raise of temperature till 950 deg is Undampended. After that Integrator is called in by the Window Comparator made of two opamps, the integrator prevents OverShoot, Undershoot, Ringing and Oscillations.

The PID control output can also be a Time Proportional Output like PWM. With a large cycle time of 20 or More seconds. Like 2 Seconds on and 18 Seconds off for 10% Control.Fast Cycle times may be needed for small systems with less inertia.

## Wednesday, May 11, 2016

### Diode Reverse Bias Leakage Tester

This is a Test arrangement for Leakage Testing of Diodes on Reverse Bias. The leakage current indicates the ability of the diode to withstand higher voltages. An AutoTransformer or Variac can be used to vary the test voltage. Even Plastic capacitors can be tested for leakage this way.

High Resistance Indicator - del50004

Safety Precautions -
Use a Isolation 1:1 transformer for safety. This circuit has to be enclosed in a insulated cabinet. A Jig or Acrylic Safety Plate with clamp can be used to connect the diode. The voltage is only applied as long as a Mains two way push switch is pressed. This adds to the safety.

Build a DMM or Digital Multi Meter

Caution Instruction -
Use this with Low voltages like 24V AC for Learning. Do not Use it with High Voltage AC. If you are learning, first work with other circuits using batteries or Low Voltage Mains Adapters.

## Tuesday, May 10, 2016

### Digital 5V Power Supply using L296

This circuit is derived from an application note of L296, It is a Power Switching Regulator from ST. The advantage of using a switching regulator is that there is not much Heat Dissipation in this circuit.

Switching Battery Charger with L296 - del20031

If you had to build the same with a series regulator, it would be very big due to external transistor and a huge heat sink. This circuit takes a small place on PCB, efficiency is high so power is saved and reliability of product improves, lastly the thermal gradients within the cabinet is avoided so that any form of drift or component specs variation can be avoided.

L296 and L296P are stepdown power switching regulators 4 A at a voltage variable from 5.1 V to 40 V.  External programmable limiting current. Soft start, remote inhibit, thermal protection, a reset output for microprocessors.

The Schottky rectifier BYW80 is used as it switches very fast 200V-20A-35nS. The Inductor and Capacitor is for the filter to get a ripple free DC from the Chopped DC output. There may be a small high frequency ripple riding on the DC signal of 5V in most SMPS circuits. So for very sensitive circuits use extra filters and shields.

The Current output is limited, and can be reduced further with a resistor from Pin 4 to ground. Also if the feedback to Pin 10 is thru a Voltage Divider then more voltage can be set at the output. See the datasheet and application notes for other design details and circuits.

## Thursday, April 14, 2016

### Voltage Doublers and Multipliers

Voltage multipliers are used to obtain low current high voltage, from an existing AC Source. Step up transformers can be used, but the insulation problems and care that has to be taken for HV above 1kV, make it difficult. After SMPS technologies and Ferrites were developed, HV for CRTs was generated by blocking oscillators and step up Transformers at High Frequency AC.

Mains Voltage and Power Circuits

Caution Instruction -

Use this with Low voltages like 24V AC for Learning. Do not Use it with High Voltage AC. If you are learning, first work with other circuits using batteries or Low Voltage Mains Adapters.
This circuit uses Diodes and Capacitors, just like the text book circuits.

Voltage Multipliers - The Creative Science Centre

## Sunday, April 10, 2016

### Voltage to Current Convertor using LM723

his Circuit converts a voltage control output from a Process Controller to be converted into a Current Control if the AC-Drive or Valve needs a Current Control Signal.

Significance of Current Loop 4 to 20 mA Standard

This is a three wire voltage to current loop converter. The 1-5 V DC is attenuated and fed to pin 5 LM723 opamp section which tries to maintain the same voltage at pin 10 across the 10 E, thereby producing a open collector constant current sink proportional to the 1-5V input. By trimming the attenuator you can scale-calibrate 1-5V input to 4-20mA output for looping many instruments in series, like a controller, recorder or PLC. With a supply voltage upto 24V, three instruments can be looped. The connection to pin 6 is required to convert 0-1 input to 4-20mA.

All the transmitter circuits can be seen here. Industrial Process Control Circuits

This circuit was designed by me in the eighties, the 555 was for negative supply, The whole thing went into the anodized cast aluminuim head of a sensor.

How 4-20mA Works

## Friday, April 08, 2016

### Analog Blind Dial On Delay Timer

It is called analog dial timer as it has no digital display, but the chip is digital it is CD4541. It is Mains operated, you could make on-delay or off-delay. By changing the caps and resistors or with even dip-switches you could get timing of few seconds to many hours. Circuit is Analog Timer 4541 .

Some timers energize the relay after a set delay on power start. Some start with relay energized and turn off after set time. Motors and big systems cannot take fast turn-on and turn-off, Some systems will require sections or modules to start one after another as they are interdependent. So Timers in automation were made to effect such delays, the early ones were mechanical clockwork.

The CD4541BC Programmable Timer is designed with a 16-stage binary counter, an integrated oscillator for use with an external capacitor and two resistors, output control logic, and a special power-on reset circuit. The special features of the power-on reset circuit are first, no additional static power consumption and second, the part functions across the full voltage range (3V¨C15V) whether power-on reset is enabled or disabled.
• Available division ratios 28, 210, 213, or 216.
• Increments on positive edge clock transitions.
• Built-in low power RC oscillator.
• Oscillator frequency range . DC to 100 kHz.
• Power on Auto reset initializes all counters.

## Sunday, March 20, 2016

### Square and Triangle Opamp Oscillator

Here is the circuit of a Op-Amp based Square Wave Generator. One of the main application of this is in a Simple PWM circuit and Triangle Opamp Oscillator. The Slope of the Triangle wave is compared with a DC Level to derive a pulse width or On-Time proportional to a Voltage.

Tutorials on Basics and Instrumentation Electronics

This On-Time proportional to voltage is for a fixed oscillation rate based on C2-C3 and R15. The C2-C3 form a Unpolarized cap, This type is also used in crossover networks in speaker boxes. This is because, a plastic 4.7 uF (2 * 10uF series) is big and costlier, but a plastic cap is very stable and closer to an ideal cap.

First you need to know that most opamps in such circuits can give a square wave, but very few can swing to the rails (+/-5). For designing take 75% of Vcc. CA3130 is one that can swing to rails like a CMOS gate.

Assuming the swing of output is around +/- 4V, the pin 10 + input will be at +/- 2V. When the cap is discharged at 0 V, let us take that the output is +, the cap charges to a little above +2, - input becomes dominant, so output swings negative. This discharges the cap to 0 and then charges it negatively upto -2. This in turn flips the output to + as pin 10 turns dominant. Hence this continues as a oscillation, with a nice triangle across the cap for the PWM comparator.

Dominant means more +, 0 is positive compared to -2V. Also -2.5 is negative compared to -2.2.

Related Reading - Op Amp Triangle-Wave Generator

## Saturday, March 12, 2016

### Tubelight Electronic Choke

This is an electronic choke for a Conventional Fluorescent Lamp. This was an application note of MJE13005 a High Voltage Switching NPN Power Transistor. I Might have modified it. This was very popular in the early days, The Coil Winding and Ferrite is very Critical design.

Emergency Light with Fluorescent Tubes

The diodes, npn transistor and caps are high voltage 350V-1KV, The tubelight is 40W-60W 5 feet long. This circuit replaced the choke and starter in an existing frame.

Low Cost Simple CFL lamp

## Tuesday, March 01, 2016

### Voltage to Frequency Converter AD Interface

This is the Analog front end of the 80C39 Process Controller. The analog input is protected by a Zener barrier, low leakage. You could use clamping diodes too.

The non-inverting low-offset amp offers high input impedance. After further amplification it reaches the VCO LM331. The pulse train from the VCO reaches the uC port and is gated and measured by the MCS48 firmware. The voltage is deduced from the Frequency or Pulse width.

You can get a resolution near to a 8-10 bit A/D converter. It does not work for negative voltages. It is a low cost Voltmeter or Process Display solution.

80C39 and MCS48 based Process Controller is the main circuit that has the LED 7 segment display for output and push keys for input.

## Monday, February 15, 2016

### Battery Backup for SRAM or Microcontroller

This circuit can be used as a low cost SRAM and Microcontroller-Microprocessor Battery Backup. All the diodes are 1N4148, The diodes prevent battery discharge back to power source. D8 gives a one way path to charge Battery thru R13 which limits current. D4 ensures a one way path of supply to chip when power is present. D5 is backup supply on power failure.

The chip a real time clock, RAM or Processor can be put to standby or sleep on power failure. If it is not a smart chip then make sure on power failure all outputs of chip are high impedance or floating. do not use any pullups or resistor dividers to Vbat, which is the supply to chip. There should be no leakage path from Vbat, decoupling cap of chip must be plastic.

Microcontroller in Process Control

If you want to use this circuit for short term retention or for CMOS logic chips then you can use a 4700uF Cap in place of battery. This works for many hours but the cap has big footprint on PCB. For long duration use more battery AH Ampere-Hour. Vcc is 5V DC regulated.

The Vbat and Vcc can be monitored with comparator like LM339, this circuit can generate the reset or low battery signals. The power on reset and power down reset can corrupt data on brown outs or black outs or even spikes and EMI. So back up data on flash. For Rapid writing and reading SRAM is better and if write-read cycles are high SRAM is best. But if you need to store values and refer to them like a look-up table flash is better.

Serial Interface a 80C31 to ICL7135

The power fluctuations can hang the chip, so a watchdog chip may be required. The conventional way was the to monitor the keyboard-display scan on a i/o port. If the pulses are coming at the rate you programmed the cpu is alive and kicking and doing its job. If the CPU is taking a nap, then the pulses stop coming and it needs to be reset.

## Friday, February 12, 2016

### Simple High speed data switch

This circuit is a small representation of a very low cost printer sharer. It has no Active Devices only diodes.

Pot the product in epoxy with a black dye, they serve the purpose. Output impedance of this circuit is high, sink is 220K source is 3.9K+ so use some buffers or drivers at Output. If Buffers are omitted, then fix this unit on the printer port connector of Computer.

when Enable A is at float-high impedance or low the output O1-O4 is not influenced by A1-A4 inputs. If Enable A is made logic high or 5V then A1-A4 is available at O1-O4.
By turning Enable A or Enable B high, you can route the data A1-A4 or B1-B4 to the output O1-O4, you can also mix data and you can expand to any number of input sets or data width. 1N4148 is fast, 4nS, that makes this data switch quite fast. This circuit cannot drive long printer cables without drivers. They will load the output.

## Tuesday, February 09, 2016

### 100kHz Half Bridge Convertor - SG3525

This is a SMPS Circuit improved over the Application Note in the book SGS Motion Control Application Manual. This worked well. It helps you learn more about High Frequency Power Supply Design.  More details and PCB's Here SG3525 SMPS Project.

This was the Module that contained the smps chip. It was done to prevent easy replication by other firms. Alos to Keep the Signals isolated from the Main board board currents and Corrosion Prevention by impregnating Module in Epoxy. Yet the main board suffered some corrosion even with some coating protection.

Industrial Systems that involve processes like Etching, Coating and Electroplating have to be well ventilated so that electronics around them have some life. Else find ways to package Electronics in Sealed Enclosures with a new plan for Heat Dissipation.

## Saturday, January 23, 2016

### Simple Thermocouple Amplifier

The OP07 is in a non inverting amplifier so as not load the mV of thermocouple, the zeners are to protect circuit if junction contacts heaters or the earth gets broken.

Thermocouple and Pt-100 RTD

The RC is to filter out 50Hz pick up in thermocouple wires if near heater wiring and also reduces reading jumps when high current three phase contacter operates.

The Pull-up 10M is when a Thermocouple breaks the output of circuit will be max. This is open sensor protection, in case Thermocouple breaks, Required only in industrial temperature controllers for protection. This means it will be 3.5V which should make you turn off the heater in software.

J and K Thermocouple with 4-20 mA

The other opamp is for further amplification as OP07 is set to around 30 gain and offset has to be adjusted with R9. If OP07 is kept in > 100 gain it may be difficult to adjust offset of 75uV. If you need very high gain in the first stage use some instrumentation amplifier or chopper stabilized amplifier. I am not very sure. This is the very basic Thermocouple Amplifier used as a front end signal conditioning in Process Control.

## Wednesday, January 20, 2016

### Simple Sample and Hold with CD4066

A sample and hold is like an analog memory. If The digital control A is low 4066 switch is open, and when A is high switch is closed. U2B is a buffer so as to ensure quick charging of C1 thru 4066 on resistance of 100E.

Mixed and Interface Circuits

U2A is a FET input opamp buffer which does not load or drain the cap C1. When A goes high the input analog sample is stored in C1. A has to be high for say 10*1uF*100E = 1mS, so that a proper stable sample is stored. When A is low C1 undergoes very slow discharge as opamp input resistance and 4066 off resistance is in giga ohms. The accuracy of reading Vout falls with respect to time due to leakage currents.

## Sunday, January 17, 2016

### Voltage to Current Source 4-20 mA

Here is a circuit that is a voltage to current converter but with a current source. That means it supplies a current from a positive terminal and can drive a remote shunt in a Measuring Instrument. The other end of the shunt returns to gnd or low.

More than one instrument can be connected in series if the the sum of the voltage drops in the instrument shunts are much less than the voltage 12V of the current source.

The 0-1V to 4-20 mA Converter published earlier is a current sink,

You can use a LM358 or LM324. The first opamp is a Voltage to Current with a sink output. That current creates a varying voltage w.r.t the 12V DC supply, this varying voltage is mirrored by the second opamp across the source output resistor. This way a constant current is obtained with a sourcing output. The control elements are small signal high gain transistors. Any suitable equivalent can be used. Even the opamp can be chosen by the precision and application you want.

Industrial Process Control Circuits

In this form of feedback. way to understand .... "Op-Amp drives the output to maintain both inputs at the same level" and also the "Output takes the polarity of the dominant input" and lastly "dominant means, more positive". +5 is more Dominant than +3 or 0 or -2. Then -3 is more dominant than -12. See which is more positive.

Long distance of current loop may need higher voltage and lower source resistor value. Then the output transistor needs to change, if you use 24V DC then that voltage should not reach opamp. Design needs to foresee all possibilities of I/O troubles, as these are wired by a customer, mistakes happen. Hence, Industrial Designs have to be rugged.

## Circuits by Application

 Analog Circuits SCR and Triac Mains Power Digital Circuits Measureall DMM Mixed Circuits 555 Circuits Discrete Circuits Opamp Circuits Power Electronics uC and uP Instrumentation Circuits Process Control