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
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
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.