Process-Control

This is a Low cost controller, Analog Dial Temperature Controller. It is also called Blind Controller. This essentially means Open Loop, just control the fuel or energy input to the system to regulate heat. This is not a Blind Controller that way, it only cannot display the temperature value, that could be another reason it is called blind.

Blind Dial Proportional Temperature Controller

Dial cyclic timers were used to control heat, these were purely mechanical clockwork devices. They could regulate well, when the material flow (liquid) is constant and mains power is regulated. But when the job to be heated, varies in quantity, control temperature is close to ambient or when a precise control is required; closed loop controllers are used. Even a thermostat is like closed loop, as the bimetallic sensor is temperature dependent. But not good enough.

Blind Temperature Controller

This controller is closed loop, precision controller, only the digital display of temperature is absent. Fine one deg variations may not be easy in this.

Blind Temperature Controller

PCB Boards for Blind Controller –

Discussions –

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.

80C39 and MCS48 based Process Controller is the main circuit that has the LED 7 segment display for output and push keys for input. The old form of Human Machine Interface – HMI.

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.

BCD Thumbwheel Switch is used to input-set data in digital form, this can be read by digital circuits, uC and uP systems and PLC-SCADA Interfaces.

Temperature Measurement and Control

In the early transition of analog to digital, before uP became acceptable, Digital systems without uP were made, it even had printers, RAM and displays. The uP systems were coming in, uC had not yet come and uP systems had to still win the confidence of the Prudent Industrial Design Engineer.
The drawbacks of uP based systems used in Computers, in those days were.

  • Power Consumption was very high, needed SMPS.
  • Many chips, a CPU had a Retinue of many chips.
  • Large Board, Double or Multi Sided due to Bus.
  • Fussy, Hangs on minor Power Glitches or Resets.
  • Needs Firmware Development and Tight Testing.
  • Investment in all these areas, Tools and Manpower.

These made Industrial Automation with uP a challenge. CMOS digital and mixed devices and custom application devices were more easy to implement and affordable.

Analog Level by BCD Thumbwheel Switch

The coming of Low power CMOS uC changed everything and embedded systems became smaller and robust. These were packable in DIN standard and DIN Rail Mounting enclosures.

Coming back to inputting digital data. CMOS uC and Ni-Cd Battery backed up RAM with keyboards made thumb-wheels and other methods less attractive for digital data inputs. Then the Li-Ion Battery, Flash Memory in Combination with Application Specific uC and SOC have made inputting, retaining digital data very easy and affordable.

Read More →

The 0-1V to 4-20 mA Converter published earlier is a current sink, Here is a circuit that is a voltage to current converter but with a current source.

Voltage to Current Source 4-20 mA

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.

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.

This two opamp circuit converts analog voltage signals to current (sink) signals in a proportional manner. Current signals are more immune to noise and cross talk, hence long wires can be used. Voltage Signal to Current Signal Analog Converter.

Ensure +5/-5 dual supply for chip TL062 IC3. Gnd is common ps ground, let grounds radiate from ground plane in one side of PCB. R3-R8 is an attenuator that may need to be designed or modified.

0-1V to 4-20 mA Converter

In output R23 is for protection from shorting of +5V supply, R23 can also go to an unregulated or external. supply upto 24V DC which is referenced to this circuits gnd. More voltage more distance.

Q2 is the current control device, and R22 50E is the shunt for taking a sample of current. 4-20mA in the output (provided suitable load is connected) means 200mV- 1000mV across 50E shunt. This is fed to close loop control system of IC3a inverting pin.

An opamp on this type of feedback tries to drive the output in such a way, so as to maintain both the inputs at same level.

Mini RTD Pt-100 Three Wire Transmitter

If there is 1V at pin 3 and no current is flowing pin 2 is at 0V so output goes positive and drives Q2. this results in a flow of current till a 1V builds across shunt, if it exceeds then output of opamp falls This reduces drive to transistor and hence current reduces. That is the part of V to I conversion with open collector output.

Now we need 200mV to 1000mV to get 4-20mA 4mA is good for 0 as low level measurements are more noise prone. that is the reason 4mA and not 0mA.

Now we need to convert 0-2 V to 0.2 – 1.0 V using IC3B. R14 is a representation of that 200mV offset set by R16 pot. the opamp IC3B adds both the input and this offset to get 200mV to 1000mV. for that the opamp IC3B is an analog computer, summer, subtracter. Try to now calculate the values for that.

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.

Timer Circuit

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

7217 Timer Counter

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.

SSR or Solid State Relay or Electronic Relay Connection Method. This is a Thyristor Based Normally Open 230V Switch that can be turned on / off at a fast rate. This switches at Zero Crossover .Built In Snubber reduces EMI and protects the SSR. USE SAME PHASE FOR I/P & O/P.

AC-AC-SSR Solid State Relay

AC-AC SSR Wiring

  • No moving parts hence no wear and tear.
  • Dissipates Heat when in On Condition.
  • Use adequate Heat Sink or SSR will fail. ( 10g / Amp)
  • Dissipation is 1.5 Watts per Amp thru Load.
  • Input to Output is optically isolated 2kV.
  • This one is AC 230V Control AC 230V N.O. Output.
  • Off state snubber Leakage is around 5 mA.

Notes –

The Termination’s of High Current Lines going to Load must be very tight and crimped. Loose contacts will Spark and cause Fire. Fuse Rating of HRC Fuse – High Rupture Capacity (HRC) Fuse is Safe and Reliable. 5kW Heater at 230V is 5,000 / 230 in Amps of Fuse Rating.