# Mechanical Switch Causes Transients from AC-DC Power Supply

I'm having trouble getting rid of output transients from this Mean Well PSU right after the AC input is switched.

The PSU is a RPS-500 27VDC with fan. The 120 VAC is coming from an industrial power strip and the switch is a mechanical DPST bat handle style.

The switch transients are showing up on all the outputs: 5Vsb, Power Good (PG), and 27VDC output.

I captured some scope shots from the 5Vsb pin w/out load.

Occasionally the transients aren't noticeable.

Would a multi-stage AC/DC EMI filter and perhaps a NTC resistor (to limit inrush) get rid of this?

My guess is that these are due to inrush current or the switch making contact at different phases of the AC input.

Edit 1:

Bench setup schematic

RPS-500 block diagram

Edit 2:

Scope captures from an MSO4054B using TPP0500B probe with spring tip ground. Measured at 2mm double-row connector, no load (for now).

These are showing the 5Vsb ramp and the voltage transients when the switch is toggled.

• How do you measure? Show setup. Show schematic. Show layout. – winny Sep 11 at 8:44
• The devil is in the details. Also, schematic is king here and words are, well, not. – winny Sep 11 at 15:27
• Show probe setup. – winny Sep 11 at 16:09
• @winny I'm assuming the ground lead is a bit long based on the frequency of the "ringing". A short ground lead or FET probe might be better. Would scope captures using a better probe help? I'd be happy to get some. – mrbean Sep 11 at 16:25
• ”I'm assuming the ground lead is a bit long based on the frequency of the "ringing"” Ding ding ding! – winny Sep 11 at 16:51

What I think you are observing is the settling of your output unloaded.

You say the "noise" is occurring right after switch on of the AC input but on your oscilloscope we don't see the rise of the output voltage from 0 to 5V.

• Load your supply at least with a small load do not just put its output on a HiZ such has your oscilloscope, it will mostelikely not behave the same in your final setup (choose your resistor carefully not to set your house on fire, $$P=U^2/R < P_{rating-of-your-resistor}$$)