Problem of decreased or increased current signal at the output terminals of the 4-20MM single-input two-output isolated safety barrier.
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I encountered the following issues during on-site debugging; I haven’t figured them out yet, so I hope for some guidance. . . One of the toxic gas detectors on site is wired in a three-wire configuration; the original wiring of the system is shown in the image (the PLC uses passive wiring for signal input). After removing the original 2 signal wires from the system that led to the PLC and connecting them to the input terminals of the isolation safety gate (with the safety gate powered separately by 24V), it was found that the signal strength at the output increased, going from 3.7 mA to 4.5 mA. When an isolation gate with 1 input and 2 outputs was used, the signal strength at both the input and output remained the same. Upon returning to the site for another measurement 2 days later, it was found that the previous isolation barrier could no longer keep the input and output signals equal; at this point, the input was 4.1 mA while the output became 3.7 mA. After replacing it with the previous isolated safety barrier and testing again, it was found that the signal also decreased. By directly supplying a signal to the input terminal of the safety gate through a signal generator, it is ensured that the signal is identical to the input signal. I can’t figure it out! ! !
Drink, drink, drink! Hey, that person—big bro, sir, please explain it
Drink, drink, drink! Hey, that person—big bro, sir, please explain it
You can check the safety barrier setting to see whether it is 0-20MA or 4-20MA. The hardware should be fine; check the settings
The hardware is definitely 4-20MA; there’s no doubt about that
Also, I would appreciate it if some expert could advise me on how to match the impedance of the safety barrier with that of the field instruments Are there any study* reference materials? Regarding the issue mentioned above, I connected the toxic gas alarm directly to a three-wire safety barrier; after the instrument was powered by the safety barrier, I found that the voltage at the connection point between the safety barrier and the instrument dropped to 6V, preventing the instrument from functioning properly. I suspect that the load-handling capacity of the power supply side of this three-wire safety barrier is insufficient to power the toxic gas alarm.
Why is a safety barrier needed? This combustible or toxic gas detector should merely be explosion-proof; it rarely meets the intrinsically safe requirements. Flameproof gas detectors consume more power; safety barriers cannot be used with them, and installing a safety barrier is unnecessary.