Changing AI Card. Expert opinion needed

i have an issue regarding Analog input card

Right now i have a system an old system of Siemens (S5-150CPU) with 460-4UA11 anlaog input modules. As per an old documentation the Analog input resistance of these Analog modules is 33 ohm.

Now i m replacing this system with AB Control Logix system with Analog input cards 1756-IF16 with input resistance of 250 ohm.

I am wondering that due to this change in input resistance , is it possible that the transmitters signals can't be read?. I have lots of different transmitters of different companies. All transmitters can support different loop resistance.

For example i have one with 4-20 mA with 0 to 500 ohm
other i have with 4 to 20mA with max 400 working resistance


Can any specify me exactly "What is the relation of transmitter's output resistance with card input resistance". Why we use impedeance transformation devices in the loops?.

Please specify me the exact parameters to look in all transmitters datasheet before "ordering" 250ohm input resistance 1756-IF16 module?
Please help

I'm no expert, but we have many different brands of transmitters feeding back to AB PLC, SLC, and CLX systems that are all 250 ohm resistance. We have never had an issue.

Current loop transmitters, 4-20 ma, have a maximum resistance rating. It will start at a very low resistance up to the maximum total allowed. Add up all the devices in series and just make sure that that total is less than what the transmitter is rated for. If the transmitter is loop powered the power supply should be taken into account. Stay with a typical 24vdc power source to get close to the maximum resistance capable.

salman111a said:

Can any specify me exactly "What is the relation of transmitter's output resistance with card input resistance". Why we use impedeance transformation devices in the loops?.

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Analog cards read voltages. The input card resistor is needed to convert the current to a voltage, per Ohm's Law: E = IR, or the voltage drop across the resistor is the product of the current and the resistance.

salman111a said:

Please specify me the exact parameters to look in all transmitters datasheet before "ordering" 250ohm input resistance 1756-IF16 module?
Please help

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Every transmitter comes with some sort of chart or statement of maximum resistance at a given power supply voltage.



Or some spec sheets have a statement like:
Max. Loop Resistance = 43.5 (Power Supply Voltage – 10.5)

As John Soltesz says, add up the loop resistances: copper wire resistance, input resistance of device #1, input resistance of device #2, and make sure that you don't exceed the maximum resistance.

You can do the arithmetic, or go by experience: It's my experience that with short runs of copper wire <100m, that any transmitter will drive 250ohms resistance with a 24Vdc power supply.

salman111a said:

I am wondering that due to this change in input resistance , is it possible that the transmitters signals can't be read?.

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It is highly unlikely that any commercial 2 wire loop powered transmitter built in the last 20 years won't drive 250 ohms at 24Vdc.

Is your power supply 24Vdc?

Dan

YEs my supply is 24VDC

But i have two things in mind

1- Why in many transmitters sheets its given "minimum 250 ohms required". It means is there is a range of loop resistance on which transmitters can operate. For example in above chart we have
650 ohms at 25VDC loop supply
Also its given that we need loop resitance >250 ohms

Now my range will be from 250 ohms to 650 ohms?
or
I have to select loop resistance equal or greater than 650 ohms?

2- Also can any body help me why we use Galvonic isolators in loops . Whats kind of impedeance matching is acheived by them.

Thanks for quick reply

We have one of the transmitters with the following status
We are using DAW1-FSU-2 frequency to current converter to interface with our Control logix analog input card 1756-IF8 . As per the datasheet FSU-2 the max load resistance for analog output is 0-115 ohms. But our analog input card input resistance is 250 ohms. IS it true that our FSU-2 transmitters cannot drive our control logix analog input card?. Do we have to use galvanic isolators?”

One reason for a specific minimum of 250 ohms could be HART communications. This provides a minimum signal to use HART and only applies to the HART reciever/modem. I would say not the total loop. This may not be your case. I have not seen any transmitters with the max resistance below ~300 ohms. If there is the 115 ohm max on a transmitter then look for a universal type converter that would have a low input impeadance to allow you to use the AB AI card with 250 ohm input. I have used Moore Industries isolators for ground loop problems. At least one model of ECT shows a 25 ohm 4-20 ma input with a 1200 ohm max 4-20 ma output. I am sure there are other brands in your area that will do the same.

salman111a said:

1- Why in many transmitters sheets its given "minimum 250 ohms required".

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Like John says, a minimum 250 ohms is needed IF HART communications is needed.

salman111a said:

It means is there is a range of loop resistance on which transmitters can operate.

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Yes.

salman111a said:

For example in above chart we have 650 ohms at 25VDC loop supply

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Which means that 25Vdc will drive up to a maximum of 650 ohms.

salman111a said:

Also its given that we need loop resitance >250 ohms

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Only when HART is required. The HART 275 communicator had jacks for a 250 ohm resistor plug to be installed "in the loop" while performing HART communications because sometimes loops don't have the minimum 250 ohms needed. A loop might work fine at whatever the total resistance is, it's just less than 250 ohms.

salman111a said:

Now my range will be from 250 ohms to 650 ohms?

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The range of loop resistance 24V can drive is 250 ohms to 650 ohms (for the example). Your loop resistance will be 250 ohms (the AI input reistance) plus copper wiring resistance plus any other device (indicator, perhaps) in the loop. The sum can get up to 650 ohms (in the example) before the power supply fails to deliver enough voltage potential to sustain the current in the circuit.

salman111a said:

I have to select loop resistance equal or greater than 650 ohms?

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No. You don't 'select' a loop resistance. The AI card manufacturer 'specifies' its required input resistance. Some SI cards have the resistor built into the card, others use an external resistor. But the AI card manufacturer specs the resistor, you don't select it. The remaining loop resistance comes from the copper wire resistance and any other device in the loop.

salman111a said:

2- Also can any body help me why we use Galvonic isolators in loops . Whats kind of impedeance matching is acheived by them.

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Like John says, isolators are used to prevent ground loops. In your case, the commercial product that accomplished impedance matching happens to be an isolator, I don't think impedance matching is the reason isolators survive as a commercial product.

salman111a said:

As per the datasheet FSU-2 the max load resistance for analog output is 0-115 ohms. But our analog input card input resistance is 250 ohms. Do we have to use galvanic isolators?”

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Some form of impedance matching is needed. A retransmitter or an isolator are the commonly available commercial products that do such. John's suggestions are on target.