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.

Transient1 Transient2 Duration

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 PSU 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.

5VsbRamp 5VsbSpikes 5VsbSpike 5VsbLowSpikes 5VsbLowSpike 5VsbHighSpike

  • 1
    \$\begingroup\$ How do you measure? Show setup. Show schematic. Show layout. \$\endgroup\$
    – winny
    Sep 11, 2019 at 8:44
  • 1
    \$\begingroup\$ The devil is in the details. Also, schematic is king here and words are, well, not. \$\endgroup\$
    – winny
    Sep 11, 2019 at 15:27
  • 1
    \$\begingroup\$ Show probe setup. \$\endgroup\$
    – winny
    Sep 11, 2019 at 16:09
  • 1
    \$\begingroup\$ @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. \$\endgroup\$
    – mrbean
    Sep 11, 2019 at 16:25
  • 1
    \$\begingroup\$ ”I'm assuming the ground lead is a bit long based on the frequency of the "ringing"” Ding ding ding! \$\endgroup\$
    – winny
    Sep 11, 2019 at 16:51

1 Answer 1


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.

Thus making it difficult to help you.

Your problems looks a lot like switch bounce combined with the undampened beahviour of an unloaded supply.

What I would suggest you to do:

  • Do not put an EMI filter (this would help if other noisy equipment shared the AC voltage, but not for the switch.)
  • An NTC is always a good idea following a switch on an AC line voltage (as you mentionned it will helps with the current inrush if you switch on the highest portion of the sine and if your redressing caps are completely deplated and if your wire has very little resistance.)
  • 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 so as not to set your house on fire, \$P=U^2/R < P_{rating-of-your-resistor} \$.)
  • Try a low pass filter at the output of your power supply (a simple capacitor in parrallel with the output might add some dampening of the regulation loop inside you PSU, note that this will worsen the dynamic behaviour of the voltage tracking of the supply, but it might help with the stability a little bit.)
  • \$\begingroup\$ The 5V standby output stays up for a long period of time (20-30 seconds) even after AC power is removed. That's why there's 5VDC in the scope captures. I was toggling the switch and then capturing the transient on the scope. Thanks, I'll try your suggestions and report back. \$\endgroup\$
    – mrbean
    Sep 11, 2019 at 15:05

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