4
\$\begingroup\$

I built the following circuit, a reference VIPer22A circuit. VIPer22A Reference design

It works fine but I have a little problem. It gives an electric shock on the secondary side. It's not a "line" shock (sticking your fingers into an outlet), but rather, a "tingly" shock if you touch the device while you're grounded.

It's the same thing that happens when you have an PC power supply not connected to mains earth.

The obvious solution would be, of course, to connect the secondary GND to Earth. The problem I have with this is that this is an audio amplifier, so I'd rather leave GND floating.

Is there a way to eliminate this shock? Capacitively connecting GND to EARTH? Or should I just connect my circuit's ground to Earth?

\$\endgroup\$
  • 9
    \$\begingroup\$ Use a transformer with an internal electrostatic shield between primary and secondary, and ground that. Such transformers are frequently used in audio gear. \$\endgroup\$ – Dave Tweed Aug 29 '14 at 18:09
  • \$\begingroup\$ @DaveTweed that is very interesting. I've seen transformers with an external shield but I didn't know they had it inside too. \$\endgroup\$ – hjf Aug 29 '14 at 21:56
  • \$\begingroup\$ W1, C8 ...disconnect it, put it on the junk. all would be just fine. (your main AC circuit has gnd problem. not your or your circuit fault.) \$\endgroup\$ – Andy_Indonesia Aug 1 '17 at 21:02
  • 1
    \$\begingroup\$ Who made that schematic? It contains a copule of issues. C1 is polarized, C1-C3 don't have any voltage specified and the text says that it would be 50 V unless specified, and C8 doesn't mention that it has to be Y-rated. It would also be interesting to know what W1 is. \$\endgroup\$ – Oskar Skog Aug 1 '17 at 21:50
  • \$\begingroup\$ I agree the schematic isn't the greatest -- W1 is a zero-ohm jumper link though. \$\endgroup\$ – ThreePhaseEel Aug 1 '17 at 23:19
6
\$\begingroup\$

There is 4.7nF between primary and secondary (hopefully a Y rated capacitor for safety). If the N is connected to 230VAC relative to ground, it will pass about 400uA which may be perceptible.

From information on the net, Argentina appears to use Australian style 3pin plugs with 230VAC 50Hz one side hot wrt ground.

Maybe your plug connections are reversed or the outlet is wired incorrectly.

\$\endgroup\$
  • \$\begingroup\$ This is the board in question: i.imgur.com/uRiHbFV.jpg I don't know about the rating of the capacitor (blue one next to the transformer). But it's a 10nF one because I couldn't find a 4.7nF one. What if instead of connecting that capacitor to neutral, I connected it to Earth? \$\endgroup\$ – hjf Aug 30 '14 at 13:58
  • 2
    \$\begingroup\$ @hjf Well.. that explains the perceptible tingle- it's more like 1mA with 10nF. Can't you just swap the input wires? Nice work on the home-made transformer and board. \$\endgroup\$ – Spehro Pefhany Aug 30 '14 at 14:01
  • \$\begingroup\$ Most likely the pins in my circuit are swapped, as I've wired my house myself and all my outlets are strictly the right polarity. But I'd still like to have an extra "layer" of safety, in the event something like this happens (for example, before 1998 we used non-polarized outlets. What if I plug my device into one of these of unknown polarity? 50/50 chance of tingles!) \$\endgroup\$ – hjf Aug 30 '14 at 14:08
  • 1
    \$\begingroup\$ Also re: homemade transformer. I tried to make it as "safe" as I could. I used enameled wire of an unknown kind ("this is what they use for motors" said the seller). And I added a couple turns of thin mylar sheet between coils. No idea how safe this is, but I took apart one of those SMPS transformers to see how they were made and well, they were similar to mine. \$\endgroup\$ – hjf Aug 30 '14 at 14:11
  • 1
    \$\begingroup\$ @hjf -- the big key to making a safe transformer other than the mylar sheet (doubling up on it's not a bad plan at all by the way) is to keep the windings from encroaching on the edges of the insulation (can't get closer than 4mm to the edges otherwise you violate creepage) \$\endgroup\$ – ThreePhaseEel Aug 1 '17 at 23:21
2
\$\begingroup\$

i think the 4,7nF cap will be of the wrong type. as Y1 caps general don't come in 4,7nF

Y1 capacitors are general blue round disk shape capacitors

you can try a lower value

\$\endgroup\$
  • \$\begingroup\$ Here is a photo of the board: i.imgur.com/uRiHbFV.jpg Is the cap next to the transformer the one you are talking about? And yes, it's a 10nF cap because I couldn't find any 4.7nF ones. I figured at the frequency they work it wouldn't matter anyway. \$\endgroup\$ – hjf Aug 30 '14 at 13:56
  • \$\begingroup\$ the cap is right thats a Y1 \$\endgroup\$ – niels1 Aug 30 '14 at 16:17
1
\$\begingroup\$

You're probably experiencing capacitive coupling from the primary to secondary of the transformer.

Just out of curiosity... are you in a country where the mains are three-phase (e.g. Germany)?

\$\endgroup\$
  • \$\begingroup\$ No, I'm from Argentina. My house has single-phase 230V 50Hz service. \$\endgroup\$ – hjf Aug 30 '14 at 4:06
  • 1
    \$\begingroup\$ Do you really have three phase outlets (i.e. FIVE wires) in germany? I doubt that. \$\endgroup\$ – Vladimir Cravero Aug 30 '14 at 7:47
  • 2
    \$\begingroup\$ @Vladimir Cravero We have 3 phases in Serbia and we have 5-wire sockets for 3-phase appliances. Usually inside an apartment, the load is distributed among the 3 phases and there's a number of 3-phase sockets for say electrical stoves and similar high consumers. \$\endgroup\$ – AndrejaKo Aug 30 '14 at 8:21
  • 1
    \$\begingroup\$ That's awesome, didn't know that. Why would they do it, that's a mistery to me. \$\endgroup\$ – Vladimir Cravero Aug 30 '14 at 8:22
  • 1
    \$\begingroup\$ @VladimirCravero no the outlets in Germany are single-phase, but it's three-phase entering the house. As AndrejaKo describes, the outlets are distributed among the phases. I found that out when one leg went out and two thirds of the house was still up. \$\endgroup\$ – Eric Nelson Aug 30 '14 at 12:24
1
\$\begingroup\$

Standard Class II switchers leak slightly, yours is just a trifle leakier than the norm

All commercial grade (vs. medical grade) Class II switching supplies, such as the design you're building, will leak a few hundred microamps or so of current to ground from the secondary side thanks to the Y capacitor bridging primary and secondary (C8 in your design). This Y capacitor shunts EMI from the secondary back to to the primary to keep the DC output wires from radiating noise everywhere, but isn't exactly an open circuit to mains either. This is why Y capacitors don't go up very far in value compared to their X capacitor brethren (you can get X capacitors into the microfarads, but 10nF is a big Y cap, as you observed).

Since you used the 10nF cap for C8 instead of a 4.7nF or perhaps even a 2.2nF-or-so, your supply is just a bit leakier than normal (1mA vs ~500µA). It won't kill you (as long as C8 doesn't turn into a duplicate of W1 :) or trip a RCD/GFCI for that matter (it's too small for that), but as you observed, it can tingle. (Try putting an AC milliammeter/microammeter between the negative output of your supply and the receptacle earth ground lead.)

\$\endgroup\$
1
\$\begingroup\$

Capacitively connecting GND to EARTH? Or should I just connect my circuit's ground to Earth?

If you want to keep the EMC benefits of the capacitor, keep your output floating but divert leakage away from the output you should replace the capacitor with two capacitors connected in series. Then connect the mid-point of the two capacitors to mains earth.

For safety these should be "Y1" or "Y2" capacitors.

\$\endgroup\$
  • \$\begingroup\$ That's actually a rather intriguing way to design a SMPS if you ask me...I wonder what UL'd have to say about a supply that is configured that way, though. \$\endgroup\$ – ThreePhaseEel Aug 2 '17 at 2:15
  • \$\begingroup\$ I thought it was the normal way to configure a class 1 floating output SMPSU. \$\endgroup\$ – Peter Green Aug 2 '17 at 11:20
  • \$\begingroup\$ Ah -- my understanding of Class 1 supplies is that the output on them is grounded, but come to think of it -- you're probably right re: lab supplies and such. \$\endgroup\$ – ThreePhaseEel Aug 2 '17 at 11:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.