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Old 06-09-2021, 12:01 PM   #1
Per Hansson
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Exclamation WARNING: Live heatsinks & single pole on/off switches in switching power supplies

This is a warning to inexperienced and experienced techs alike:
The De-facto standard is to have the primary side heatsink live in switching power supplies.
That is the primary side heatsink is referenced to the negative of the bulk filtering capacitor.
Therefore never ever touch the heatsink if the device is plugged in.
And even if it is unplugged let the capacitor discharge first!

Another really serious concern is that the power switch also by De-facto standard is single pole.
This means it only cuts power to the phase or neutral (not both) depending which way you have it plugged in if you have a Schuko plug or another non-polarized plug.
This means that when the device is switched off you will read 230VAC on the primary side heatsink to ground! (Or 115VAC if that is your mains voltage).

In the following picture I show a Fluke 113 measuring the voltage, I'm using this as it has a 3KΩ measurement impedance, so it presents a quite heavy load to the circuit being measured.
This is to allow you to understand that it is a real voltage we are speaking about and not a phantom floating voltage as you can sometimes see in house wiring.
Indeed it is large enough that I can't use the 30mA RCD you also see in the picture: because it will trip!
Then I also use a Fluke 225C Scopemeter to show the actual waveform.

Here is the voltage of the primary heatsink with relation to ground with the unit switched off on the single pole switch:
As you can see we are reading 237VAC (our line voltage)

And here is the voltage waveform of the primary heatsink with relation to ground with the unit switched on:
(As you can see the meters show a voltage of roughly 167VAC but this is a RMS approximation that is not so useful).

The actual DC voltage is of course our rectified AC voltage that is close to 340VDC due to my slightly high line voltage (√2 x 240VAC = 339.4VDC)
I've added some cursors here to better clarify what I mean:

It is also extremely important to point out that the only safe way to measure on the primary side with an oscilloscope is using a differential probe or with a Scopemeter that is not referenced to mains ground.
Because if we wanted to get a useful reading here we would need to clip the negative lead to the primary side heatsink since that is the negative of the primary side.
And I hope you understand now why that would end extremely badly, if not please watch EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
Note: if you wish to discuss this thread please use the original located here:
Attached Images
File Type: jpg PSU Switch Off.jpg (1.87 MB, 15 views)
File Type: jpg PSU Switch On.jpg (2.07 MB, 9 views)
File Type: png PSU Switch Off.png (8.6 KB, 7 views)
File Type: png PSU Switch On.png (7.5 KB, 5 views)
File Type: png PSU Switch On with cursors.png (8.0 KB, 7 views)
"The one who says it cannot be done should never interrupt the one who is doing it."

Last edited by Per Hansson; 06-11-2021 at 12:55 PM..
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