This is the RTD Signal Amplifier part of Temperature Controller or Indicator. The card also contains a relay on-off control circuit.
The Circuits are –
The PCB Boards –
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.
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
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
This is the Display Circuit and PCB part of of section Temperature Control.
The above circuit is powered by +5 and -5 from a LM7805 and LM7905 pair. If +/- 12V or +/- 7.5V is used in opamp or digital parts, then use below circuit for the DPM section.
The PCB for above
The PDF Circuit for above Display Card STC1000
This is a generic or standard controller for any type of process. Shown here is a temperature controller.
The STC1000 could be used for Ovens or furnaces, liquid baths, heating or environment chambers. It has a single analog input and an analog or relay output.
Analog inputs could be like 4-20 mA, thermocouple or RTD. Analog outputs can be 0-10 V for a thyristor drive, SSR drive output or potential free contacts of a relay to operate external 3 phase contacters.
The closed loop control could be On-Off or Time-Proportional. It was known as digital temperature indicating controller. Sometimes it would be used just as a Alarm or in a Trip circuit for protecting a system, when a uP or uC based PID controller fails.
This is used in the processing of plastics, rubber, metals and in chemical plants too. Here is a checklist that was made for helping its fabrication. You can see the limitations in which we manufactured these products. An optimum quality was evolved, defined by a user’s affordability of the product.
Production Checklist of STC1000
- Has Transformer been soldered properly with reinforced pads – Scrape with blade area around pad and make lead bridges to support transformer.Front Card and Rear Card must have very thick solder bridges after fixing sq. post.
- Is the Front and back stickers and SL. No Sticker properly stuck – In Front Sticker the holes for switch, pot LED etc. must be without Burrs file and improve. Acrylic should not have cynoacralate stains. In the back sticker there should not be any wrinkles use rubber solution (fevibond) to stick properly.
- Are all the Threaded parts and Plastic parts fixed with fevibond – Use Fevibond (very little) for the CJ box in rear panel all Metal screws and loose mechanical parts in Pots, plastic threads and pillars. This prevents parts from breaking loose during transportation or vibrations, All Nuts / Bolts fix tightly .
- Are all the Pots. Turning freely and is direction proper – Clockwise is always increment or increase in a parameter and anti-clockwise is decrement of any control or parameter. All pots should turn without friction.
- Are all the Front panel components Fixed properly – All Four Displays are required Fourth is for Overrange and Polarity. LED’s are for Load On or Process on and Should be neatly visible above sticker and Switch should not get stuck after cabinet is closed and screwed.
- Only two screws must be visible on the back panel – Cover Pot clamp screw, card fixing screw etc. by Black insulation tape.Use Ni. plated Phillips / Universal head Screws for fixing back plate.
- Are all other items in the final packing – Check by fixing both side clamps and file if required add Instruction manual in Cover or with Invoice and DC.
- Sources of omissions and neglect checked by Visual Inspection – Gnd to Earth Capacitor to be added 0.01uF 103 1kV, Relay OEN only, Polarity of Electrolytic and diodes , Regulator tab touching any conductor, Loose wire strands, Solder Bridges absent at places where it is required.
- Troubleshooting when a card does not work – Keep always a 3D reference of a working unit / card compare component to component by visual inspection wrong polarity of diodes / Tx / IC, wrong value of Resistor or Low value capacitors missing jumpers solder bridges or hairline shorts at edge of board or pad to pad lead streaks.