Regarding the 4-20mA signal shunting issue
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As shown in the diagram, an electrical current signal passes through a signal collector in series before reaching the cabinet components. The mA values measured at the collector and at the cabinet are different, indicating some attenuation. What is causing this?
If the collector and the signal source share the same power supply, there may be a leakage inside the collector； Check whether there are any interferences in the testing instruments, testing methods, and measurement environment ； The current in the same circuit should be equal everywhere; it can only be divided up, but not attenuated.
The internal resistance of the signal collector does not meet the requirements, resulting in shunting. If two separate signal paths are needed, it is recommended to use a signal distributor or a signal isolator.
Use a signal isolator – it’s simple and quick
Thank you all for your answers! Let’s try using a one-in-two-out safety barrier first!
"The claim that \"the internal resistance of the signal collector is not within the required range, resulting in shunting\" is incorrect: only leakage current (such as severe insulation defects) can cause shunting； "The internal resistance of the signal collector does not meet the requirements; if it is too high, it will exceed the impedance requirements for 4-20mA signals, resulting in incorrect signal values in the circuit (misalignment), and in severe cases, the signal cannot be transmitted.
The last edit to this post was made by 1111111 on 2019-1-30 at 20:39. 1. I guess what the original poster means by “attenuation” is that, after a \"collector\" is connected in series within the current circuit, the current value measured at the cabinet becomes smaller. 2. If that is indeed the case, it is because the sum of the values of the current sensing resistors r1 in the cabinet and r2 in the collector, namely (r1+r2), exceeds the load resistance value R allowed by the transmitter. 3. When (r1+r2) > R, whenever the theoretical output current value l of the transmitter is high, the voltage division effect of (r1+r2) causes the voltage V at the transmitter terminals to be lower than the minimum operating voltage U required by the transmitter. This results in the transmitter being unable to output its theoretical current value I; instead, it can only output a current value i that is less than l. In other words, the output current is forced to decrease in order for the transmitter to achieve the minimum operating voltage U. 4. For reference when thinking about this topic, you can check out the popular science post written by my brother: https://bbs.hcbbs.com/thread-1365184-1-1.html.