I have a relatively cheap bench PSU - Model No QW-MS3010D (because someone is bound to ask). The DC output is fine but when it is switched on it creates a noticeable amount of noise on the AC side. Below are photos of my 'scope showing the trace with probe unconnected, in the first is picking up general mains AC, the second is the same situation with the PSU turned on. The last, for what its worth, is the inside of the PSU.




I am wondering if there is anything straight forward I can do to suppress the noise? It's not worth a lot of time and effort but before I relegate it to the subs bench I thought I'd ask

And yes, I know you wouldn't have bought one of these / you have rock solid PSU from 1957 / etc. This is the one I have and before I relegate it a simple fix, if possible, is worth a shot.

 

I have the same PSU and is used often without problems. I wouldn't bin it. It is quite useful.
You can try plugging the PSU into a power bar that has noise suppression filters.

 

Certainly there is plenty of room inside for an additional mains side filter circuit. The mains input filter components, and possibly the whole assembly, can be recovered from a scrapped desktop PC power supply.. Those supplies did require input filtering and many of them were built with more spacing between the parts. In many cases the circuit board can simply be cut to recover the input filter, which is all of the circuit up to the bridge rectifier.

 

Your scope is just displaying radiated electro magnetic interference. That really doesn't mean much. What really matters is how much AC noise is on the DC output of the supply and you mention that it is OK. Is the case of the supply connected to mains ground?

 

The effect of the radiated noise depends quite a bit on what it is effecting. It might be a problem with a nearby low-level audio system, or medical instrumentation, but not with power lighting.
AND, like I suggested, a mains power filter could be cheap and easy, but only if you understand enough electronics to install it.
The fact is that I know nothing about the TS skills and experience.. Some folks could add the filter just based on my description.

Certainly additional mains circuit filtering will reduce the noise signal.

 

In some circumstances it doesn't matter but it does affect nearby audio and I think it has an impact on nearby WiFI

 

Looking at the photo of the inside of the power supply shown in post #1, it is clear that the incoming power filter is not very big.
AND certainly there is adequate space on the rear panel of the enclosure to add a more effective filter to block more of the mains supply signal.
Do others have any suggestions??

 

You could add this AC line filter:




Or you could replace the receptacle with this:



Make sure that is matches your line frequency and voltage.

 

Why do you think adding power input filtering is going to make a difference? The noise is most likely to be radiating from the switching transformers. Is the case plastic or metal? I don't see any conductive coating on the inside of the supply's case.

 

The power entrance filter might actually fit in place of the present power entrance connector.
It looks to me like the case is metal, quite probably steel. BUT it is clear that there was no effort to assure any connectivity between the two parts of the case. So it is rather obvious that not much effort was taken in the package design to reduce radiated energy. Certainly the enclosure connectivity could be improved quite a bit.
BUT, since there has been no comments about adding the filters, is it really a big concern ti the TS???

 

You could add this AC line filter:

View attachment 361365


Or you could replace the receptacle with this:

View attachment 361366

Make sure that is matches your line frequency and voltage.

Yes use this, or make good earth connection with the psu metal case

 

By the way any machine around have transformer inside?, within the room or area where is the psu?

 

By the way any machine around have transformer inside?, within the room or area where is the psu?

What possible effect of a transformer inside another piece of equipment have, relative to this discussion?? How would it relate to the electrical noise flowing out of this DC power supply??? OR what other effect might it produce??

I find the question rather puzzling. Would " gaber2611" please explain the reason for the question.

 

What possible effect of a transformer inside another piece of equipment have, relative to this discussion?? How would it relate to the electrical noise flowing out of this DC power supply??? OR what other effect might it produce??

I find the question rather puzzling. Would " gaber2611" please explain the reason for the question.

Okay, it happened with me before
I made simple circuit so my oscilliscope work as curve tracer, and on the same room I had line interactive UPS machine, " Small capacity, about 1200va ", and once I turned the UPS on, I've noticed the oscilloscope signal for the curve tracer came up with noise, and after some check I lately found the reason is the UPS
So it was electro-magnetics interface between oscilliscope and the UPS transformer
So why not be similar issue here?
By the way, once I turned off the UPS, the oscilliscope worked fine

 

Some pieces of equipment are poorly shielded and radiate a lot of noise. And some scope leads will pick up noise.
Usually transformers only might disturb scope the trace by magnetic flux leakage for a very short distance.

 

Thanks all, I've been a bit busy, I'm intending to try one of those filtered inlet connectors. The case is steel with a handful of screws joining the two halves but it is painted/coated so there isn't much connection between the halves, there is a good earth connection to the case. I might try to improve the connection between the halves but there is a big hole in the back for the fan so making a complete shield is probably not possible.