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Monday, January 29, 2018

Interface a PC to Measure or Control Temperature

Thermocouple, RTD, Temperature monitoring is central to the majority of data acquisition systems, be it to save energy costs, increase safety, reduce testing time…whatever your reasons, you will need a device to measure the temperature - a temperature sensor.

How to Use Your PC to Measure Temperature

Thermocouples, resistance temperature devices (RTDs), thermistors and infrared thermometers are all types of temperature sensor. Which you choose depends on factors like your expected maximum and minimum temperatures, the accuracy you need and your environmental conditions.

Interface a PC to Measure or Control Temperature

Windmill data acquisition software with Microlink hardware

752-RTD: RTD Resistance Temperature Measurement, Digital I/O, Counting, Analog Output
  • Measure temperature using RTDs or Pt100 sensors, and voltage signals
  • With extra hardware monitor thermocouples and strain gauge bridges
  • Includes the powerful yet easy-to-use Windmill software for Windows: no programming required
  • Connect up to 8 USB units to one PC
Here is a way you can learn, How To Interface PC to the Outside World.

Temperature Control with a PC

delabs Notes

This is a project that will help students learn both hardware and software aspects of a control system. Here you can test out an automation program on a PC with minimum external hardware.

When you implement in the field, the system has to be a dedicated real time uC based product on custom firmware. Systems with OS + Software combination are not known to be reliable in Process Control. But OS + Software is good for Supervisory Systems which get their data from Real Time Controllers.

So a front end controller must be Rugged, Non-PC based and Non-OS based embedded system. With a Robust and Compact (means fast) firmware.

The firmware should be Polling or Interrupt based but with both internal and external watchdog. Most important a Power supply Management that resets system properly over Brown-outs and Black-outs. Even today some Embedded Systems i have to turn off and on after power resumes.

Don't Forget, Multiple Watch Dogs and Multiple Power Management for graceful start of system !! Use one big SuperCap if you want. Else people will go back to Thermostats.

Thermocouple and Current Input 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.

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

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

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).

Wednesday, January 24, 2018

PCB SSR Module - IO Module

This is a PCB Mount SSR for Control of a Small AC load like a Lamp or a Contacter. It has Optical Isolation and has a Thyristor for output. This was in the I/O Module Standard.

PCB SSR Module - Enclosure

The MOC3041 A zero crossover optical triac driver is used here. The footprint is very small, many can be stacked in one small PCB.

Read More - PCB AC Load SSR with DC control.

You can see in the PCBs below  the distance between the input and output tracks is more than 5mm. The opto is itself a few kilovolts of isolation and we should take care of the creepage distance on the PCB and surface.

This thing is Hermetically Sealed using a Alumina Powder filled Epoxy. This combination gives good thermal conductivity and Excellent Insulation.