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

I´ve been looking at the mini-three wire 4-20mA transmiter 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

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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

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.

delabs

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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 - Spain

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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.

delabs

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Related Reading

• How to Connect RTD to transmitter
• How does RTD Work
• Significance of Current Loop 4 to 20 mA Standard

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Battery Management and Supply Design

Continued from uC Board is Misbehaving on Brown Out

Read articles in the link you provided, thanks for that!

As you have have pointed out, my circuit is not driving any motor or such loads...but it is driving a WIRELESS MODULE which draws about 165mA to 240mA from my circuit during transmission[for 4 sec] and normally it takes upto 114mA. After a time-out of 1min, I put my uC P89V51RD2 to powerdown mode[200uA in this state].

My other problem is that my batteries get supercharged upto 5.8V, and as per spec's of most IC' they are bound to operate faithfully upto 5.25[max 5.5v], so how do I solve this issue? I want that my boards get only 5.25v even if my battery is having voltages 5.25+, without increasing my existing current consumption to noticeable value.

Moreover I'm in need of a cutt-off circuit which will take away the load from my batteries when my battery has reached 4.00V, & the charger charges my battery independently. Do you have any thing which will suite my purpose?

Mail from MO

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Have two supplies, the one for uC can be a small Ni-Cad battery charged by a diode by the bigger battery which supplies the 200mA for RF drivers.

So when the big battery is loaded and the supply dips the shock is not felt by uC. or isolate it by SMPS and isolate the grounds. The ground return current of RF module 200mA must be lifting the ground of uC or some Logic.

Start by trying to locate the problem, first power RF and uC boards with isolated lines from two lab-test 10A power supplies. If both are in same board, cut tracks and power them separately. The grounds should meet at point the two sections interface.

If that works without problem, you know it is related to supply, now step-by-step go back to the battery and charger you have built keeping isolation intact. Look for spikes-glitches on power lines of uF when RF module active. Check if that is close to or leaking RF to uC circuits.

The battery charger circuit should limit voltage and current to battery specs. If you are not using a charger chip then while testing make sure the charger has a regulator or use LM317 to clamp voltage to 5.6 or 5.5 as per your design.

There are charger chips from maxim, national for battery charge management. If you have a need for extra voltage the you have to use SMPS boost switcher. which may take the low battery 4 to 8 which can be regulated to 5 using 7805.

This may be getting bit complex, first try to find what is the problem, it may be just the board design.

delabs

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Binary ASK modem - Power line Modem

Want hlp in dsigning of a binary ASK modem or Power Line Modem.

Mail from AM UK

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Study these links ....

• ASK modem from NXP Semiconductors
• Computer Telephone Interface
• Amplitude-shift keying (ASK)
• Power-line transceivers - ST
• OFDM Power Line Modem - TI
• OFDM-Based Power Line Communication Modem

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How to Connect RTD to transmitter

Can you explain me were i have to conect the three cables of the PT100 in the Circuit RTD-Pt-100-Transmitter Circuit?

RTD PT100 Transmitter and Multiplexer

Comment from CV of Spain

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You have a three wire RTD there are even 4 wire RTD, This circuit can be used for both.

In the 3W RTD you have two wires coming from the same point (zero ohms between them) the third wire will be 100 ohms from these two. From the two wires that are at same point take one wire and give 1mA from Q3 + OUT. The other wire of the pair connect to input +..... input - can be connected to the 100E end.

Learn more

• The pros and cons ofRTDs and thermocouples
• RTD Resistance Temperature Detectors
• Kelvin (4-wire) resistance measurement

delabs

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So imagine that I have three wires one white and two blacks.

One black to +OUT 1ma
Other black to input-

And waht about the withe one that is the one that give me arround 100ohms??

Reply from CV

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Black 1 - +Out 1mA
Black 2 - +Input
--------------
White goes to -Input

See NI Developer Zone - RTD Connections

delabs

Leakage of SSR driving AC Solenoid

message: I have a 115vac ssr powering a solenoid only pulling a 160ma. upon activation 115vac is applied, but upon deactivation 78vac is still present. I'm told I need a bleeder resistor, but no one knows exactly what size? Can you help?

Mail from GH USA

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When a solenoid draws 160mA, you dont need a bleeder. That itself is a bleeder. The leakage of the ssr is very less, it is mainly the current thru the snubber caps.

The SSR cannot leak so much, Is the solenoid a simple coil solenoid or has circuitry? A coil solenoid is also a bleeder for the 1 or 2mA max AC current from caps.

If the solenoid has some control circuit then you may need a bleeder, put a 100K 250V 1W. It also could be a defective SSR or a NC (normally closed) SSR with improper control signal.

Check if control and load circuits are isolated too, make careful measurements across the ssr. Tell me if you still are in doubt,

delabs

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Sourcing the SP646 Solid State Relay

I have a customer that call out a Solid State relay and i cannot find any information on it. Can you help? It is a SP-646. I need a quote on 60 and 120 pieces.

Mail from PG US

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Have a look at this, is this what you want. SP646 1 Form A Solid State Relay. contact them if the specs are right, ssousa.com -contact

Description

The SP646 is a bi-directional, single-pole, single-throw, normally open multipurpose relay. The circuit is composed of one LED on the input side which activates an optically coupled IC on the output - controlling the firing angle of two back-to-back SCRs. This circuit assures no false triggering under most adverse conditions, and a tight zero-volt window not exceeding 5V.

Features

• Inverse parallel SCR output
• High transient immunity
• 400V blocking voltage
• 1.2A maximum continuous current
• Low input control current
• High input-to-output isolation
• Solid state reliability
• Zero-volt switching

Applications

Programmable controls, Valve control, Solenoids, Remote switching, Home appliances, Metering equipment, Heating elements, Gas pump control circuitry,

The SP646 Data Sheet includes the Complete Overview, Electrical Characteristics, and Schematic Diagrams.

Even if you dont find the same number, Digikey or Mouser may have an Equivalent. Just match the Electrical Specs and PCB Footprint, pitch etc.

mouser - digikey - jameco see list here Components Suppliers

delabs

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