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.
Precision Op-Amp Current Source
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.

Resistance Measurement Analog LED Meter

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.
RS232 with Opto-Isolation

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.

Circuits by Application

Analog Circuits

  1. Battery Level Indicator
  2. Simple Sample and Hold
  3. Sample and Hold Standby
  4. Voltmeter Attenuator
  5. Precision Current Source
  6. Opamp Supply Virtual Ground

SCR and Triac

  1. Solid State Relay
  2. Normally Closed AC SSR
  3. AC-AC-SSR
  4. DC-DC SSR
  5. 2N2646 based Pulser
  6. Drive SCR thyristor

Mains Power

  1. Flashing Neon Lamp
  2. Dimmer power control
  3. Edison Bulb Life Extend
  4. Mains Current LED
  5. Mains Voltage LED

Digital Circuits

  1. Simple Digital Counter
  2. Running Lights
  3. Frequency Divider
  4. Crystal Oscillator
  5. Simple High speed switch
  6. Differential TTL converter

Measureall DMM

  1. Ohmmeter Measure Resistance
  2. Precision Digital Attenuator
  3. Precision Amplifier

Mixed Circuits

  1. Monostable Multivibrator
  2. Digital to Analog
  3. LM311 Oscillator
  4. PLL using 4046
  5. VCO with LM331
  6. BCD Thumbwheel to Analog
  7. V to F Converter ICL8038
555 Circuits
  1. OR gate with two 555
  2. fixed frequency duty cycle
  3. Pulse width modulation
  4. Astable Multivibrator
  5. uC Reset Generator
  6. LM555 Voltage Doubler
  7. 555 Power Oscillator
Discrete Circuits
  1. Isolated dual supply
  2. Sound to light converter
  3. Water operated relay
  4. Telephone Indicator
  5. Passive volume control
  6. RS232 Opto-Isolation
  7. Voltage Level Indicator
  8. Relay Driver
  9. Constant Current LED
  10. Voltage Doubler
  11. FET Current Source
Opamp Circuits
  1. Three Opamp Differential
  2. Two Opamp Differential
  3. Buffer Opamps
  4. Differential Op-Amp
  5. Inverting Opamp
  6. Non Inverting Opamp
  7. Digital gain control
  8. Square Triangle Oscillator
  9. Dual Polarity Output Amps
  10. Ammeter Precision Rectifier
  11. Voltage / Current 4-20 mA
  12. Current Source for RTD
Power Electronics
  1. Dual Power Supply
  2. Single Power Supply
  3. Battery Backup Supply
  4. 5V 1A Supply LM2575
  5. 5V Power Supply L296
  6. Dual Power Supply
  7. Tubelight Electronic Choke
  8. Voltage Doublers Multipliers
  9. White LED Lamp on Ni-Cd

uC and uP

  1. PC RS232 with MAX232A
  2. Battery Backup SRAM
  3. watchdog uC uP systems
Instrumentation Circuits
  1. Mains monitor LM3914
  2. Simple Mains monitor
  3. single digit voltmeter
  4. High Resistance Meter
  5. Diode Thermometer
  6. Function Generator
  7. Diode Leakage Tester
  8. Analog LED Ohm Meter
  9. Millivolt Source Current Loop
Process Control
  1. AD590 - temperature
  2. Thermocouple Amplifier
  3. Linearizing Thermocouple
  4. Thermocouple Amplifier
  5. 0-1V to 4-20 mA
  6. 1-5V to 4-20 mA
  7. InfraRed - Optical Switch
  8. InfraRed Detector

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