Two Op-Amp Differential Amplifier

The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.

Dual Differential Amp – Interactive Simulation

The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating Outputs may be random or Oscillating, it is good to have a bias network of 10M resistors to a potential even zero or COM this enables Vout when input floats.

Two Op-Amp Differential Amplifier

Vout = (Vp – Vn) * (Rf+Ri)/Ri

Related Reading

Precision Instrumentation Amplifiers

Presettable Up-Down Counter Timer

When i had put the near Obsolete digital circuits online in the late nineties. One person who works in a public institution in the usa, wanted a modification of one of my existing circuits. He had those parts the CD40 Series Logic Chips. He wanted to use only those that he had in his Stock.

I made some modifications and sent it to him, that helped him with his task. These things can be done very easily using the Arduino. One could make a programmable Arduino Timer/Counter with a matching Configuring Software without coding, for such people. Easy and Affordable.

Digital Circuits 2 from delabs

Circuit 1 – Digital Timer Clock With Preset using Thumbwheel switch.

A Thumbwheel Switch has to be used in place of DIP switch shown, just know that 1-2-4-8 nibble (4 bit) should be generated by Thumbwheel switch at preset or jam inputs of 4029.

Use CD4511 if 4513 is not available, but circuit has to be changed a bit around 4511

Circuit 2 – 1 Hz or 1 pps crystal clock using CD4060 and 32768 Hz Crystal.

They have not been tested much… The 4513 control pins 8-4-5-3 connections verify, as i did not get the datasheet.

The circuits will work as the concepts are right, but some tweaks in R C values may be required.
the R C values can only be corrected if you have problem in making it work.

The main problem in the R C values may be related to “the reset at 6 for the tens of seconds and the tens of minutes”.

Digital to Analog Converter with uC Watchdog

This is the continuation of the earlier post. Part of 80C39 based Process Controller. In this schematic you can see the Watchdog and D/A Converter.

Digital to Analog Converter with uC Watchdog

My first observation of a very complex watchdog in action was an Agilent(hp) Benchtop Multimeter based on this 8048 family of 1st generation microcontrollers that did not even have a UART among many things.

At that time CMOS was just making an entry and FLASH memory was unheard of. The UV Eprom was the way firmware was set on these systems. These consumed a lot of power. 80C39 was the CMOS one.

The  4040 counter derives a slow clock from the 7555 timer. The counter has to be reset by firmware by periodically sending a reset pulse on port pin P2.7 to say “Alls Well”.

If the firmware or uC “hangs” or due to EMI or Spikes the uC gets into an endless loop. Then the “Alls Well” pulses stop coming. The 4040 keeps counting till Q10 output goes high and resets the uC or can we say Wakes it up rudely.

The D/A converter was used to get the 1-5 V to obtain 4-20 mA control Signal to operate the Actuators like a Motor Drive or Heaters in a Industrial Process control System.

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.

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.

Industrial Process Control Circuits

Flow Measurement and Control

There are three Controls to be Adjusted to make a Proportional Flow Controller Perform Properly. This method has to be practiced and experience gained from it can be used to get very good and stable Control of the Flow or Velocity of a Fluid.

1. Set Point. (SP)

This is the Flow Rate at which you require the Fluid to be controlled at. Adjust the rate at which the fluid flow is expected to be controlled .

2. Process Value. (PV)

This is the Actual Flow Rate of the fluid in the flow sensor or its path. It is very important that the Flow Sensor is placed at a position in the fluid circuit in such a way to avoid cushions which may lead to oscillations around the Setpoint.

3. Proportional Band or Dead band. (PB)

Dead band or H % or Hysterisis are terms used in on / off Controllers in proportional controller we use the term proportional band.

4-20mA Control Signal

The Flow rate zone in which the Output is 4.1mA to 19.9mA which in turn Drives the AC-Drive >> Motor from 0% to 100% is the proportional Band. It is Given in % e.g. 50% PB of 200 lph SP is 100 lph. Band 150 lph – 250 lph

eg : The Motor is at 100% Drive ‘on’ till 150 lph and ‘off’ above 250 lph. ` Between 150 to 250 is the PB. A little above 150 the Motor reduces power gradually till at 250 it turns off. When SP=PV the output is 12mA (ideal) here the motor runs at 50% of the total power.

Selection of Motor / Drive and circuit Capacity:

The Motor / Drive Combination must at 12mA Control signal give a flow Rate at which the system is used for most of the time this gives good stability. The max flow rate setting required by system must be achievable at 80% of Motor Time Axis Power this is to make allowance for load and line regulations. The Flow Circuit should have normal resistance to flow to reduce Oscillations.

STC1000PFC – P = Proportional Control, F = Flow, C = Current Output.

Zoom Image

Flow Measurement and Control

Tuning or Adjusting a Proportional Flow Controller

Step # 1

Ensure Flow Sensor Output 4-20mA is properly connected to the Flow Controller polarity reversal will show reducing reading in the Display as Flow rises. The Motor / Drive used and Power selected must be able to bring the Flow more than the maximum required Flow rate directly without control (open loop).So when in doubt connect motor/drive and run at max power and observe maximum flow rate e. g. if max. flow obtained with Motor at 100% Power (direct) is
300 lph the STC1000PFC can control flow rates upto 260 lph.

Step # 2

Keep PB in middle position and power on system e.g. set Flow rate to 200 lph. Now Observe maximum overshoot. and adjust proportional band as in table. (PB Control is a 300 lph Single Turn Control with Ends.) SP 200 lph, PV (Process Value).

PV overshootProportional Band
10 % 220 lph or moreNear Maximum fully clockwise till end.
5 % 210 lph to 220lphMiddle of the PB Control or towards max.
2% 204 lph to 210 lphLittle above present setting.
Less than 200 lph Droop e. g. 190PB is Critically set Do not Change.

After each change turn on system again to see response till 2 % or less variation or overshoot or oscillations are obtained.Flow Control Curve

Thumb Rule ! –

  • Increment PB to Decrease Overshoot.
  • Increment PB to Decrease Oscillations.
  • Stop adjustment when PV droops around SP with no oscillations.
  • Adjust EC to match SP = PV after PV is stable at a point less than SP.

If proportional band setting is maximum, fully clockwise turn till end (single turn pot) the motor will slowly ramp up to full flow speed and ramp down slowly to reduce flow.

If proportional band is at the minimum, the motor will go full speed till it comes close to the set flow rate and turn off abruptly almost like an on / off controller the motor may pulsate on & off near setpoint.

Step # 3

There is an additional control called Reset or Error Cal EC ( Integral) which is factory set for SP=PV 50% Power Output i.e. Output Control = 12mA. In certain cases after stable reading is obtained after adjusting or tuning PB the Flow may stabilize say at 195 lph for a set point of 200 lph the process is stable but a ten lphrees process error is present. this can be Corrected with EC or RESET POT at the rear panel (this is a endless 10 Turn Pot).

Adjust Error Cal provided in the back panel to increase Flow rate to 200 lph from 190 lph. when this is done give some time for system to respond after every 1/2 a turn 180 lph of the RESET (EC) control pot .

The RESET control is a Ten turn potentiometer like the SP potentiometer after 10 turns the direction of turning must change. Clockwise Increases Flow rate Anticlockwise decreases Flow rate. (at min. PB setting EC pot sets the On / Off or Operating Point).

Multiplexed Presettable Timer with ICM7217

This was a attempt to make a Sequential Timer with ICM7217 of Intersil, even though it worked well, it was not developed beyond the prototype stage or first iteration. Only when a product is made in some numbers, the documentation and designs become clear, streamlined and seasoned.

The PCB and Circuit are not complete. It may give ideas. During this time, as far as i can remember, these were the only large CMOS devices. 8080-85 and Z80 devices consumed lot of power and needed big boards and supplies.

ICM7217 4-Digit, Presettable, LED Up/Down Counter Maxim

Study this, if you are not good at firmware or you need a simple solution, this is still a versatile chip. It is better you make your own PCB. This board can be used for prototyping only.

Timer Circuit pdf

Clock Circuit pdf

The MM5369AA and 3.579545 MHz

I have converted from DOS Orcad to Windows, corrected some mistakes, use with care. Orcad on DOS had a very user friendly interface, it had a near windows like GUI on DOS, when windows did not even exist, It had right mouse button controls too.

Analog Mux for Data Acquisition Systems

Here is a 4-20 mA In/Out Analog Mux with Cascade option. This is a simple circuit i designed to make a Automation System within a budget.

Analog Mux using 4051

This takes 4-20mA from many Transmitters and gives out just one 4-20 mA output. The Mux is done with a digital byte or word. This is a slow scanner as process is slow, that way many analog inputs can be multiplexed and sent into one analog input of a D/A. In near real time systems a faster mux could be used or mux totally avoided. This was made in some numbers, so the pcb is better than others.

4-20mA Multiplexer Circuit – pdf