Issues regarding signal input when a PLC is connected to a level transmitter
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Our plant’s water treatment process is controlled by a PLC. These past few days, we’ve encountered a problem: the level transmitter in the intermediate water tank has been damaged, which has resulted in the high and low level alarms in the PLC becoming ineffective. Upon inspection, this level transmitter is connected directly to the PLC for evaluation. The specific wiring diagram is below. For the power connections, connect DC24V+1 to the positive terminal of the level gauge, DC24V-1 to V1-, short-circuit V1+ and I1+, and then use a yellow wire to connect it to the negative terminal of the level gauge. Now here’s the problem: when I use a voltage tester to measure DC24V+1 and DC24V-1, the neon bulb keeps flashing; the same thing happens with the AD041 device as well – its neon bulb keeps flashing. When I measure DC24V+1 in relation to the yellow wire, the voltage is 24V. However, when I measure just one of those terminals separately, the voltage keeps changing, ranging from up to 220V to around 50V at the lowest. Currently, an immersion type level gauge is being used, which can issue alarms properly. Excuse me, everyone – is this phenomenon normal?
A voltage of 50–220V appearing may be an interference signal.
It’s necessary to check the documentation of the card in order to understand the circuit relationships between V+, V- and I+. Once that is known, the problem will be easily resolved. Which brand of PLC is being used?
:Sleepy: Go and look at the PLC materials first. Is this point a passive point or an active point? Does it match the on-site level gauge? Regarding instruments, what do you measure with a multimeter? A multimeter is used to measure current, voltage, and continuity.
It seems that the entire measurement circuit lacks a ground point; the induced voltage from the switching power supply is applied to the DC output. If this is a problem, connecting a resistor and capacitor between the DC 24V supply and ground should help improve the situation.
It means I don’t understand whether it’s a four-wire or two-wire system; checking the instrument manual and the PLC manual will make it clear
https://wenku.baidu.com/view/8e1b7df8aef8941ea76e051b.html This is the manual for the AD041 card; you can take a look at it – it contains information on the wiring methods as well as solutions to related interference issues
Could you provide more detailed background information?