Huge interference on 24V supply when AC connected

[Yaroooo]

Hello,

I've a problem on a revision of a custom board that drives stepper motors. This board originally had a single power supply source from an AC 110V . Internally there's a rectifier plus capacitance and DC result goes to power side (mosfets and stepper phases) and a switching regulator that powers logic at 12V. This board never had particular issues.
After revision, power side and logic power side were separated. There's same AC that power mosfets but no longer the internal switching, now this one is powered by external DC at 24V. This two power supply sources have a common ground on PCB between rectifier low side output and 24V's gnd.
What I encounter now is a huge noise on 24V and on outputs from devices that are powered from this 24V that generate several problems.

Here's some images to explain:





I'm trying to figure out how to reduce this noise. If I should filter and how the 24V input or place a filter between the joint point of this two grounds (AC and DC).

Looking for any help.

[penfold]

If you probe with a very short ground lead, (like the spring that often comes with probes) connected to the probe ground near the tip, is it still there? If you connect both the probe ground and probe tip to the circuit ground does it still register?

[TheMG]

Is that 110V AC supply isolated? If not, then there's your problem! The negative side of the bridge rectifier is not at 0V with respect to AC ground, it's going to have quite some significant voltage there.

To do this properly when running some circuitry off of non-isolated mains, the low voltage control circuit need to be galvanically isolated from the high voltage power circuitry. This would normally be done through the use of optocouplers.

[Terry Bites]

Check for power loops. Close them off with CM chokes. I'm not sure anything you've added so far will help deal with the massive ground transients. Route/ wire all comons and gnd back to the transformer common.

[T3sl4co1l]

Zoom in on the noise.  What does the waveform look like?  Impulsive ringing?  What risetime, frequency, amplitude, repeat rate?

Tim