DC noise filtering

Thread Starter


Joined Apr 23, 2019
I have a water chiller with 4 compressors. Recently it was upgraded with a new PLC. The PLC has 110V outputs that make/brake compressors' contactors. The problem is when all 4 compressors kick in, PLC breaks them after about a second. No alert, no change in input signals. At first i though it was because all four compressors kick in at the same time, so i tried putting in a time delay. That didn't help. I started checking all the inputs with an o-scope, thinking theres's a bad input, and noticed that when 4th compressor kicks in DC voltage shortly drops to 11-14V (this spike repeats on the PLC inputs). Is this drop low enough to be read as logical 0 by the PLC. And if yes could you recommend appropriate filtration device.

Noise when 1-3 compressors kick-in:

Noise when 4th compressor kicks in:


20200728_095040.jpg24V DC supply: