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Discussion Starter · #1 ·
my boss told me to connect three thermocouples so that he can view them at the hmi via a plc using siemens S7 300 programme

I have a thermocouple type k. I connect it to a temperature controller that has a 4-20mA as an output.can I connect this output direct to an analogue input card or a thermocouople module of a plc?

2nd instance I have a thermocouple and a temperature transmitter.do I connect the output of the temprature transmitter direct to an anologue input module or to a thermocouple module?





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my boss told me to connect three thermocouples so that he can view them at the hmi via a plc using siemens S7 300 programme

I have a thermocouple type k. I connect it to a temperature controller that has a 4-20mA as an output.can I connect this output direct to an analogue input card or a thermocouople module of a plc?

2nd instance I have a thermocouple and a temperature transmitter.do I connect the output of the temprature transmitter direct to an anologue input module or to a thermocouple module?





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I have no idea..:laughing:

But I bumped your thread. Maybe one of the controls guys will jump in:)
 

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Discussion Starter · #3 ·
I have no idea..:laughing:

But I bumped your thread. Maybe one of the controls guys will jump in:)
hope so.am trying to learn plc and am getting confused.I know temprature transmitters amplify the signal but I dnt know if a thermocouple module does the same thing

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Hello Nickson


Information about the Siemens PLC

Temperature Signal loop



Thermocouple : signal conditioner and transmitter : Control equipment (PLC)

The temperature transmitter will amplified and condition the thermocouple voltage.
The temperature transmitter will then convert this amplified voltage from the thermocouple and convert it to a standard 4-20ma signal. The current signal (4-20ma) can be transmitted on a single pair shielded wire over a much greater distance than the millivolts signal from the thermocouple.





The Siemens PLc (or any other plc) must be equipped with an analog input card. The input card shall be equipped with voltage input capability or current input capability.

The input card from the plc shall be (if possible) accepting the current signal.

If the only available option for this PLc is a voltage input card, a 50ohm resistor is required to transform the 4-20ma to a voltage signal of 1 to 5 vdc


Hope this help

Abel
 

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A transmitter should always be run to an analog input card, if available. As a previous poster said, you can add a resistor to change the 4-20ma signal to a 1-5VDC signal if all you have is a voltage input card.

If you were connecting the thermocouples directly to the PLC, you would use the thermocouple module. I've never used one, I've always seen RTD modules, but I'm sure they are pretty much the same. It's simple.

Good luck. Just remember, thermocouples and transducers create a millivolt signal. Transmitters condition this mv signal into a standard mA signal. mA is analog.
 

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Bilge Rat
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If you have a thermocouple, it can be connected directly to a thermocouple input card in a PLC, but you'll need to use thermocouple wire the entire distance and very likely thermocouple terminal blocks for any splices. This wire must match the type of thermocouple; in your case (type K) it'll be red and yellow. Red is negative.

If the thermocouple is connected to a signal conditioner that has a 4-20ma output, then it is connected to an analog input card in the PLC. From the thermocouple to the conditioner needs to be thermocouple wire, but from the conditioner to the PLC can be just about any two conductor shielded wire. 2 conductor shielded wire is a LOT less expensive than thermocouple wire.

If the conditioner has no power input, then it is loop-powered, meaning it gets its power from the 4-20 input card in the PLC. The problem here is that some PLCs have power at the cards while most don't. So you might need to connect a 24DC power source in series with the loop. This will power the conditioner as well as the card input.

If the conditioner has a power supply plus a 4-20 output, then the output goes directly to the analog input card, and the card cannot be internally powered, because the conditioner sends power to the PLC card.

Confused.....welcome to the wonderful world of PLCs; it gets rough from here.....lol.

Rob
 

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... So you might need to connect a 24DC power source in series with the loop. This will power the conditioner as well as the card input. ...
Rob
This where I have seen many electricians have problems. It is a series circuit, and just like when you put two batteries in series, there will be a point where the positive directly connects to the negative. Really a simple concept, but it trips a lot of guys up.
 

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Discussion Starter · #11 ·
Hello Nickson


Information about the Siemens PLC

Temperature Signal loop



Thermocouple : signal conditioner and transmitter : Control equipment (PLC)

The temperature transmitter will amplified and condition the thermocouple voltage.
The temperature transmitter will then convert this amplified voltage from the thermocouple and convert it to a standard 4-20ma signal. The current signal (4-20ma) can be transmitted on a single pair shielded wire over a much greater distance than the millivolts signal from the thermocouple.





The Siemens PLc (or any other plc) must be equipped with an analog input card. The input card shall be equipped with voltage input capability or current input capability.

The input card from the plc shall be (if possible) accepting the current signal.

If the only available option for this PLc is a voltage input card, a 50ohm resistor is required to transform the 4-20ma to a voltage signal of 1 to 5 vdc


Hope this help

Abel
so this is what am getting from you,if I have a temperature transmitter then I will just connect to an analogue input card of a plc not a thermocouple module right?

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Bilge Rat
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This where I have seen many electricians have problems. It is a series circuit, and just like when you put two batteries in series, there will be a point where the positive directly connects to the negative. Really a simple concept, but it trips a lot of guys up.
True, one cannot look at polarity as absolute, it is relative.

For example, in a series circuit with a power supply, a loop-powered device and an analog input card, the positive of the power supply connects to the positive of the device. The negative of the device connects to the positive of the card and the negative of the card connects to the negative of the power supply.

The positive of the device is of course, positive, but the negative is actually positive relative to the input card, because the negative of the card is actually negative.

Yep, it's simple.....until you go to hook it up......lol.
 

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Bilge Rat
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so this is what am getting from you,if I have a temperature transmitter then I will just connect to an analogue input card of a plc not a thermocouple module right?

Sent from my HUAWEI Y210-0100 using Tapatalk 2
Yes, the thermocouple connects to the transmitter (red is +, yellow is -), and the output of the transmitter connects to the PLC analog input card.

If neither the transmitter nor the card outputs current, then you'll need to connect a 24DC source in series with the circuit.
 

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Discussion Starter · #14 ·
The only thing that connects to a thermocouple module is the thermocouple wire. If you have a transmitter or controller, it will connect to an analog input module.
ok so if I dont have a transmitter I just connect the thermocouple wire direct to the thermocouple module of the plc right?

so a transmitter does the same thing a thermocouple module will do?

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Discussion Starter · #16 ·
The only thing that connects to a thermocouple module is the thermocouple wire. If you have a transmitter or controller, it will connect to an analog input module.
this is very helpful

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ok so if I dont have a transmitter I just connect the thermocouple wire direct to the thermocouple module of the plc right?
Yes, making sure you use thermocouple wire or thermocouple extension wire and thermocouple terminal blocks for any splices as Rob said in his post.

so a transmitter does the same thing a thermocouple module will do?

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It does not do the same thing, but the end result is the same...the PLC knows what the temperature is.
 

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Discussion Starter · #18 · (Edited)
If you have a thermocouple, it can be connected directly to a thermocouple input card in a PLC, but you'll need to use thermocouple wire the entire distance and very likely thermocouple terminal blocks for any splices. This wire must match the type of thermocouple; in your case (type K) it'll be red and yellow. Red is negative.

If the thermocouple is connected to a signal conditioner that has a 4-20ma output, then it is connected to an analog input card in the PLC. From the thermocouple to the conditioner needs to be thermocouple wire, but from the conditioner to the PLC can be just about any two conductor shielded wire. 2 conductor shielded wire is a LOT less expensive than thermocouple wire.

If the conditioner has no power input, then it is loop-powered, meaning it gets its power from the 4-20 input card in the PLC. The problem here is that some PLCs have power at the cards while most don't. So you might need to connect a 24DC power source in series with the loop. This will power the conditioner as well as the card input.

If the conditioner has a power supply plus a 4-20 output, then the output goes directly to the analog input card, and the card cannot be internally powered, because the conditioner sends power to the PLC card.

Confused.....welcome to the wonderful world of PLCs; it gets rough from here.....lol.

Rob
another thing rob,a thermocouple is pair of two dissimilar metals joined together to great a junction

in an RTD resistance changes with increase or decrease in temp.

if I have a thermocouple and an RTD in my hand can you tell the difference by just looking at both of them

why I ask is that we deal with both kinds of temperature sensors and I think the two have been confused or maybe iam

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Discussion Starter · #19 ·
ok I get this thanks.
iam in a company where everything is Siemens.do you have a site where I can get the programming aspects of Step 7 300,400?

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