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
Proportional Temperature Controller
This is a Proportional Controller where the setpoint is derived from a Thumbwheel switch.
The conversion of Thumb-wheel Digital Data to Analog mV is similar to R-2R Weighted Resistor Network. In this case it is a 1-2-4-8 Binary Weighted Resistor Network. It has no Digital Components.
You can see an example circuit below for digit weights you just use like 10K-100K-1M etc. There is a problem of procuring 8M Resistors, so use series parallel combinations, avoid open presets. Trimpots can be used but then it raises the BOM cost.
1-2-4-8 Kilo Ohms may load opamp for high output levels. 1-2-4-8 Mega Ohms may be ok in the lowest digit. Greater than 10M designs are possible only in lab, not in commercial or industrial domains.
Make such R networks, solder array on thumbwheel, in some thumbwheels remove diodes or other connections. Club all of them, thumbwheels. One opamp will do. Use -2.5 V for positive mV output. The resistors should be close to 0.3% at least.
This Binary Resistor Network can also work with Digital CMOS Chips like CD4029. Use these chips on a separate supply, which is just a LM336 – 5V device. A digital thumb-wheel also can be used.
In this controller you can see a sensor open indication. When the sensor breaks, the temperature controller may continue to turn on the actuators or heaters, It may even Oscillate. So when a high impedance is detected in the sensor input terminals, the output relay is shut off and a LED is turned on to simplify operator’s diagnosis.
Mount the controller a distance away from heaters, ac-drives and vibrating parts. Avoid direct sunlight on controller, fix controller in a sealed control panel. Earth the point where the thermocouple senses heat. Some heaters leak. The machine has to be earthed.
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.
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.
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.
PCB Boards for Blind Controller –