Re: LF: Switch mode power supply nasty

[Markus Vester <markusvester@aol.com>]

Hi John,

 

Your supply is probably working as designed - don't dump it, ground it!

 

The "tingling" this is a fairly common effect of 50 Hz leakage current. I find that sliding the dry backside of a finger across metal parts can produce a "rubbery" or vibrating sensation with any of my laptops and laptop supplies.

 

Most small switchmode power supplys are nowadays equipped with two terminal mains plugs, eliminating cost for country-dependent and clumsy three terminal plugs with a protective earth pin. To reduce RF emissions, they usually contain a shunt capacitor between output and one (or sometimes both) of the mains lines. This is meant to minimize radiation of switching harmonics from the oputput DC lines due to capacitive leakage in the ferrite transformer. As there is no ground to shunt to, going to mains primary is the only option. The capacitor is usually several 100 pF to a couple of nF, a tradeoff between EMC (especially at low frequencies) and 50 Hz leakage currents to the output, and being a safety-critical component it needs to be rated for high surge voltages. Whether the leakage current is present or not may depend on the polarity of the plug - try reversing it!

 

The best solution is to externally ground the output or connected devices to protective earth, which will eliminate the 50 Hz leakage, and very likely improve LF interference a lot. 

 

I haven't seen 25 Hz periodics from one of my power supplies yet. But it may be related to a method of improving light-load conversion efficiency by intermittent switcher operation. 

Best 73,

Markus (DF6NM)

 

-----Ursprüngliche Mitteilung----- 
Von: John Rabson <john.rabson07@gmail.com>
An: GQRP <GQRP@yahoogroups.com>; rsgb_lf_group <rsgb_lf_group@blacksheep.org>; rsgb_lf_group <rsgb_lf_group@yahoogroups.co.uk>
Verschickt: So, 1 Jun 2014 11:58 am
Betreff: LF: Switch mode power supply nasty

While chasing some QRM on LF, the ordinary wall wart which powered my receiver died. I replaced it with a switch mode power supply which I had previously used with a small computer. This turned out not to be a good idea for two reasons:

• The new PSU generated even more interference, which seemed to have a fundamental frequency of 25 Hz (half that of the mains),
•  More importantly, I got a tingling sensation when I touched the metal case of the receiver. Although my AC voltmeter did not indicate that the case was live, my trusty electrician’s screwdriver with built-in neon bulb lit quite brightly.

This PSU is going to the electronics disposal unit at the local waste management site. Even if the stray potential is not a simple electrical hazard, it could easily provoke an involuntary movement resulting in damage to some piece of equipment. I seem to recall the late G3VA reporting in the Bulletin that he had lost an AVO 8 that way.

John F5VLF