Greetings everyone, I am seeking assistance with a specific application involving a 4-20 mA loop-powered transmitter. Currently, I have a Siemens pressure transducer, which has an approximate resistance of 650 ohms according to its user manual. My goal is to connect its current output to two analog input (AI) channels utilizing an SM331 AI card with an input resistance of about 250 ohms. The system operates as a 2-wire loop (2DMU), so the AI channel supplies power to the transmitter. However, I need to replicate the same current signal for an additional SM331 on a different PLC. To achieve this, I plan to incorporate a Siemens loop-powered passive isolator, which is specified to have a maximum load resistance of 400 ohms. It is important to note that the second channel must remain passive (4DMU). My main concern is whether this setup will cause any issues with the loop load. Alternatively, I am considering this separator: [FICHTECH CAL25IG](http://www.loreme.fr/fichtech/CAL25ig_eng.pdf). I believe that utilizing a galvanic separator with higher resistance (load) would be advantageous. Is this an effective approach to solving my problem? How do others tackle similar challenges involving one transmitter interfacing with two channels? Thank you for your insights!
Based on my interpretation of your document, it appears that what you have is not a traditional "signal splitter." Instead, it consists of two distinct channels housed within a single unit. Although it specifies a load capacity of 1300 Ohm, you may be able to connect its output in series to both PLC inputs. If the PLC inputs are of the isolated type, this configuration should function properly. However, if they are not isolated, I recommend considering a 4-20mA signal splitter for guaranteed compatibility. You can explore options available here: [Acromag Signal Splitters](https://www.acromag.com/catalog/219...ignal-splitters/2-wire-loop-powered-splitters).
Hello OkiePC, thank you for your response. I am looking to utilize the galvanic separator in the loop configuration shown in the attached photo. I have concerns regarding the potential voltage drop and the accuracy of the setup.
It appears that your solution is promising. While I believe your accuracy will remain intact, the only definitive way to confirm this is through testing. Do you have access to a calibrator? If not, I recommend obtaining one. Connect your calibrator in place of the transmitter and switch it to simulation mode. Then, transmit a variety of signals through your system while monitoring the outcomes in both PLCs (Programmable Logic Controllers). If necessary, adjust the scaling settings in the PLCs to achieve the required level of accuracy. By following these steps, you can ensure optimal performance and precise measurements in your system.
It’s essential to understand that this product is no longer in production. Additionally, if you experience a wire break or card malfunction in your second loop, your input side artificial intelligence (AI) may not register the milliamp loop. This issue arises because passive systems, like the Sirius model, require a load on the output side to function correctly. However, if you opt for amplifier models, you won't encounter this issue.
I may not have the exact calculations, but here's what I found regarding current measurements: - The maximum current for just the input and transmitter at 24VDC can be calculated using the formula: 24VDC / (650 Ohms + 250 Ohms) = 26.67mA. - When considering the input, transmitter, and isolator at 24VDC, the maximum current results in: 24VDC / (650 Ohms + 250 Ohms + 400 Ohms) = 18.46mA. Can this setup achieve 20mA at 24VDC?
Hi there! It sounds like you’ve got a pretty solid plan with the Siemens loop-powered isolator, but I would definitely be cautious about the resistances in the loop. Since you need to ensure that the total load on the transmitter doesn’t exceed its output capabilities, double-checking all the resistances involved is crucial. If the isolator has a max load of 400 ohms and your AI input resistance is 250 ohms, that’s already 650 ohms on the primary channel, which is pushing it close to the transmitter's limits. The FICHTECH CAL25IG might be a better option for your needs since it can handle higher load resistances and can effectively separate the channels while maintaining signal integrity. Just ensure that everything stays within the allowable ranges, and you should be good to go! Good luck!
Hi there! It sounds like you're on the right track, but I’d advise double-checking the total load resistance in your loop. Given that your Siemens transmitter has a load of 650 ohms and the SM331 card adds another 250 ohms, that's 900 ohms total, which exceeds the 400-ohm limit of the isolator you’re considering. You’ll need to ensure the isolator doesn't introduce too much resistance, especially if your second channel is truly passive. A galvanic separator like the FICHTECH option could provide better isolation and help manage load issues, but just make sure its specs align with your needs. Have you thought about using a signal splitter specifically designed for 4-20 mA signals? That might simplify the setup and ensure the transmitter's performance isn’t compromised. Good luck!
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