This is the pcb board details of a Two Set Point Controllers for any process, shown here for temperature. For new types of transducers or input types, module card has to be designed or modified. The other cards remain the same.

The cabinet of these process controllers were made of steel for shielding, but the display card would still pickup EMI in some cases. These were more in instances where the Instrument supply was derived from the motor 3-phase supply. Instrumentation Supplies 230V AC must come from a Lighting Circuit of another supply arm, this has to come after conditioning with EMI-RFI filters and Servo Stabilizers or UPS if possible. This way the load spikes-glitches due to turn-on and turn-off of Motors and Heaters. dont act as a feedback to instruments. If line-load regulation is bad and mains voltage unstable, more problems can be expected.

This front panel shielding was done with a semi farady cage, by having a ground plane on the front of PCB, facing operator. This is just the negative of solder mask, but is the copper layer in front, no pth processing, even though it is two layer pcb. The solution worked well.

Display Card - Shield

When you need a proportional control output, either 4-20mA or Time Proportional On-Off, This module is used. It does a slow PWM control, the cycle time for SSR or Thyristor Banks can be closer to Mains Frequency. The 4-20mA can be used to drive motors for turning valves for fuel or fluid heat control.

Schematics of Module

Board of Module

This input module converts J, K Thermocouple and 4-20 mA Inputs to 0-2V Full Scale. These can be used for any voltage/current inputs too. The RTD module can be modified more easily for Voltage inputs. The control output can be On-Off or 4-20 mA/Proportional with another card. The 4-20mA I/O STC1000I is not complete in documentation.

This is a Input Signal Conditioning Card for the Temperature controller. The voltage levels from sensors are either too low or need to be translated in level and span. Then for greater accuracy some linearization methods have to be used for a more precise reading. This also increases the cost. The circuits here do no cover the linearization see others in this and my related pages.

The step or segment linearization can be done by transistor, diode or CMOS switches to accomplish varying attenuation/gain for stages of the curve or voltage levels. In Microcontroller systems it can be done by lookup tables or math.

In some older digital systems without a MCU, the A to D drives the address of an Eprom Array to get a Digital Data for Display, as a linearized Reading. This Corrected Data was in turn made into analog using a D/A and then on to a Chart Recorder. This was a Logic only System of the early days. Microprocessor systems was expensive, power consuming and use to frighten people by getting lost in loops or a short nap.(they have fixed that, make sure you code properly).

Input Module - J and K Thermocouple with 4-20 mA

Input Module – J and K Thermocouple with 4-20 mA

PCB Boards of the Module

This contains the Main card with a Power Supply and Relay Control. On this card is connected the Display ICL7107 – Temperature controller.

The Thermocouple and Control Modules can be plugged into this card, these change the type of control and type of inputs. This way this can be made into any parameter controller with any type of input and output. But it is all set in production, not configurable at site.

So even if you make a 4-20mA output Flow Controller with this, the Main card and Display card remains the same. Only the Modules change. No Connectors are used, to make it vibration resistant.

The PCB Layout is here

Here is a Millivolt Source i built for Calibration in the early days. It uses only CMOS Digital and Mixed Chips from Intersil and CD40xx Series.

Millivolt Source

Later i tried a unit with 8748 part of the code in my uC section. This is with Ramp-up and Ramp-down using only two buttons. This works even now, The support below is an HRC Fuse Holder made of Phenolic or Epoxy Resin. The mV Terminations are on Top.

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

This is easy to rig millivolt source for field calibration or troubleshooting of 4-20 mA current loops. Here a Darlington pair is used for current amplification which reduces the Ib error as gain is very high.

Millivolt Source - Field Callibration Current Loop

A rotary switch selects, 4-12-20 mA Preset points. A Bourns multi-turn wirewound Pot can also be used with a digital dial. Enclose in a dust proof handheld box. Read more on process calibration.

When a Inverting Opamp Configuration is at a steady state, we say the Inverting Input is at a Virtual Ground. That means it is at 0V w.r.t to the dual power supply ground, but it cannot drive or draw any current. It is at a high impedance, but still at 0V. When you buffer this 0 V, you get a low signal ground for a opamp supply.

Opamp Supply on Buffered Virtual Ground

This gnd. can sink and source in a couple of mA. You can use it with low power opamp circuits for portable battery operated devices. This creates a virtual +/- 6 V dual supply from a 12V battery. This may be needed in cases where some instrumentation opamps need the negative supply or your design demands a measurement around zero. You may get a more loadable ground using a Power Opamp, i have not tried. The above circuit gnd cannot be used as a return path for LED’s or Relays. You can drive these, between VCC-VDD, but translate levels to drive them.