Wednesday, January 18, 2017

Room Temperature Monitor and Alarm

I am required to design an electronic system that monitors the temperature between(22-24 degrees Celsius). Given are power supply, Analogue subsystems and digital subsystems. the accuracy of the digital control circuit is limited by the accuracy of the temperature sensors. A green LED is lit to indicate correct temp range.

Room Temperature Monitor and Alarm

A red LED indicates out of range temperature and in sustained events( where this indicates out of range temp which is counted, displayed and timed) a large LED is lit. In a event where the sustained event exceeds 6 days an alarm is sounded to indicate a possibility of a system failure.

Now I need help on how to come up with the correct circuit diagrams which can work , how to simulate and to get the project done, please help, even direct me to books on the project or other sites please...

Mail from AM South Africa

First acquire the room temperature as an analog value. This requires a sensor for the range you indicated a Semiconductor sensor like LM335 will do fine.

Use the ideas on this page - Circuit Design Service and Projects 

Now monitor and generate Logic outputs for temperatures out of range using a Window Comparator. Then for the timed events you need a Timer circuit. And a Logic to set off the Alarm or Annunciator.

All this can also be done by a PIC circuit if you can write some code. I have given some pages to see, study them, that will be sufficient

Tuesday, January 17, 2017

RTD and Connections - Mini 3W 4-20mA transmiter

I´ve been looking at the mini-three wire 4-20mA transmitter and it is everything I need to try a new way of measuring the temperature. It is great but there are a few questions about it I hope you can answer them:

- What is the meaning of RYIN and RXIN?

- You say that it is a three wire circuit but I can only seen two points of connection for the PT100. Is it because the signal conditioner of the three wire PT100 isn´t drawing?

- The OUTPUT connector is where I can find the 4-20 mA?

I hope there is no problem to answer these quetions to me, I would be really pleased if you can do it and I´d appreciate it.

Many thanks and hope to hear soon from you.

Mail from AD - Spain

All the transmitter circuits can be seen here. Industrial Process Control Circuits

RTD and Connections - Mini 3W 4-20mA transmiter

As this circuit was popular i have made a separate page for it. Mini RTD Pt-100 Three Wire Transmitter

A ex-customer of mine, (ex because i don't manufacture now.) wanted a low cost transmitter, so i made this.

What is the meaning of RYIN and RXIN? - Connect it to any two wire RTD

For RTD read more RTD Omega - RTD Intro

You say that it is a three wire circuit ....

+V .... -V .....OUTPUT ...... on the left are the three wires.
+V and -V is the Supply. OUTPUT is a Current Sink Directly Proportional to Temperature on RTD.

In the Costlier 2 Wire systems, Current Drawn by Transmitter is Directly Proportional to Temperature on RTD. Two wire systems can be just looped, 3 Wire like mine has to be wired, more wire.

The OUTPUT actually drinks 4-20mA prop to Temp. Put a Ammeter (fused) in 0.2A range in Series with a 50 Ohm Resistor. Black Lead of ammeter to this OUTPUT. Red Lead to one end of 50 Ohm. The other End of 50 E to +V.


Thank you for your quick answer. I think I´ve got it. Although there is another question that has come up to me while reading your email. +V and -V is the supply I apply to the PT100? I am applying 24DCV so I suppose the +V is the 24V and -V is the ground, isn´t it?

Reply from AD

YES - +V is the 24V and -V is the ground, anything above 12V will work.

The 24 V should be applied to +V and -V only. Not to RTD Directly. RTD Directly Connected to 24V will BURN the RTD.

RTD is a Thin Platinum Film or Wire of 100 Ohms worth. The Resistance varies with temperature.

A 1mA ONLY Constant Current is sent thru the RTD to measure the Voltage across it. From the measured voltage 100mV for the 100 Ohms is subtracted using opamps.

The balance mV is Directly Proportional to Temperature. This mV is converted to 4-20 mA. 4-20 mA is a Strong Signal for Current Loop Transmission.

4mA is 0% 12mA is 50% and 20mA is 100% of the range you calibrated.

Related Reading

Monday, January 09, 2017

Comparison of Thermocouple and RTD

message: what is the pros and cons of rtd,over thermocouple

Mail from MU

Read the pages in links below, that ought to clarify your doubts. Last edited February 16, 2007

Comparison of Thermocouple and RTD

Comparison of Thermistors, Thermocouples and RTD's ... Unlike RTD's and thermistors thermocouples have plus and minus legs so polarity must be observed.
Comparision of Thermistors, Thermocouples and RTD's

Thermocouple - No resistance lead wire problems, Fastest response;  RTD - Most stable, accurate, Contamination resistant,; Thermistor - High output, fast, Point temperature sensing.
Watlow - Temperature Sensor Comparison Guide

Since thermocouples measure wide temperature ranges and are relatively rugged, they are very often used industrial and process applications where accuracy may be a less important factor.
RTD vs. Thermocouple comparison chart