60 Hz signal on DC power supply output

Thread Starter

copper dog

Joined Apr 15, 2013
11
EMI from 60Hz sources is almost always around however I've noticed a strong 60Hz signal on my DC power supply output wires. I can see it on the oscilloscope without hooking up the probes. If I unplug the the output cables or the power supply it diminishes significantly so its broadcasting off the output wires.

How is this getting through from the house outlet? I've tried turning off some things around the house to see if somethings leaking but its not that obvious a source. Could it be the supply itself leaking on the AC side even when turned off?

How to trouble shoot?
 

Tonyr1084

Joined Sep 24, 2015
3,470
IDK - - - maybe it's a cheap power supply?

I can take my scope probes and just touching the probe with my finger I can see a fairly significant AC wave. Put the probe - or any part of my body - near an appliance or lightbulb and that signal goes way up. A GOOD power supply should have filtering for that. If you're seeing it on your DC line then something is not shielded properly. OR it could be a bad ground in your home wiring or the plug going to the AC outlet.
 

Thread Starter

copper dog

Joined Apr 15, 2013
11
Well its odd.

With the device on and supplying 12 volts, for instance, I cannot detect any significant AC signal on the output (AC coupled) however if I turn the supply off the 60Hz shows up. If I then unplug the power supply the signal fades to the typical background 60 Hz that a circuit might pick up from lights etc.
 

MaxHeadRoom

Joined Jul 18, 2013
18,864
Stray Inductive-Capacitive coupling, there is no guarantee as to what very high impedance sources are out there.!;)
Max.
 

crutschow

Joined Mar 14, 2008
23,152
The only AC signal of concern is that from a direct connection to the power supply when it is turned on.
All the rest is just typical inductive/capacitive pickup from the power lines.
Why are you worried about that?
 

Thread Starter

copper dog

Joined Apr 15, 2013
11
Stray Inductive-Capacitive coupling, there is no guarantee as to what very high impedance sources are out there.!;)
Max.
Thanks for the replies

So you're suggesting it could be something in the electrical neighborhood? I'm seeing PK to PK of 1.8V just holding the probe near the power supply when it's off and plugged in and a only few mV when it's unplugged and I'm guessing it disappears with a DC load because the power supplies filters are doing their job.

Could the AC side of the transformer in the supply be broadcasting ?
 

BobaMosfet

Joined Jul 1, 2009
756
EMI... 60Hz is everywhere and couples into everything in the US.

P.S.- Anybody. If your mother is alive, call her today and tell her you love her. Because when she's gone you'll regret all those days you didn't call.
 

MrSoftware

Joined Oct 29, 2013
1,511
Any wire will act as an antenna and pick that up, and all the power wires throughout your house are broadcasting it. If your scope probes have the little spring loaded hooks on them, those are excellent antennas for picking up 60Hz. I bet just by removing those so that you have only a short stubby pointy probe, the signal will decrease significantly.
 

Thread Starter

copper dog

Joined Apr 15, 2013
11
Thanks for the info everyone. It's not that I'm concerned about it really since it's not affecting anything. I'm just surprised at the strength of it on this particular unit. It's not that significant on a homemade supply I made years ago.
 

BillB3857

Joined Feb 28, 2009
2,493
Is the power plug for the power supply a polarized plug or a 3 prong plug? Can it be reversed in the wall outlet? If so, it could be that when in one orientation, the power switch is actually in the common (grounded) lead and when open, the entire power supply is following the incoming power through capacitive coupling. Think of the old 5 tube superhet radios with one power lead tied directly to the chassis. Plug it in one way and all was good. Plug it in the other way and be very glad they had plastic knobs on the tuning shaft and volume control.
 

crutschow

Joined Mar 14, 2008
23,152
Is the power supply case connected to the mains safety ground?
That connection should reduce any radiation from the supply.
 
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