Problem with grounding??

[JohnW2]

unless you are saying that, in the latter case, the 'unused earth terminal' is connected to something else internally.

the "ground" is also the 0 volt of the switching system which is connected to the primary winding of the output isolating transformer.

In that case, where does the N of the supply go? Are the N and this 'earthg terminal'connected?!!

Kind Regards, John

 

[JohnW2]

I'm getting increasingly confused. What 'earth terminal' are you talking about?

The earth "pin" on the PSU - which has an oddball crackpot connector dreamt up by Apple - because clearly, for Apple, there aren't enough standards they don't already own.

Intriguing though that is, it wasn't the physical nature of the 'earth terminal' I was asking about but, rather, to what (if anything) it is internally connected.

It's an older version of this YF-3503 which I was surprised to find as what looks like a current product. The version I have is third down here.

Thanks. Not a range I'm familiar with. Most I know don't attempt to measure AC current at all - I'd be a little intrigued to know how it does it.

So a quick measurement later, and I can tell you that in my supply, there is something in the order of 250pF between each mains pin of the supply and the earth stud.

Clearly not in the 'stray capacitance' ballpark, but fairly low in terms of filter capacitors. However, there could well be things other than capacitance between those measurement points, so I'm not sure that one can necessary trust those measurements.

Kind Regards, John

 

[SimonH2]

the "ground" is also the 0 volt of the switching system which is connected to the primary winding of the output isolating transformer.

In that case, where does the N of the supply go? Are the N and this 'earthg terminal'connected?!!

OK, lets take a step back ...

"Traditionally" most of us are used to supplies with a 3 pin mains connector with L, N, and E connections. Internally, L&N go to a rectifier which creates a high voltage DC in capacitors and this is then chopped by a switching circuit to drive a transformer at high frequency to generate an isolated output.

Often, the -ve or common of the output is connected (either directly or via a resistor) to the earth pin of the input - making the output linked to the supply earth. Properly double insulated devices may have completely floating outputs. Also, many "dubious" units have floating outputs but don't (I suspect) actually meet the requirements to do so safely.

To reduce the amount of noise going back up the supply, a filter is placed at the inlet. Depending on the design, and how cheapskate the manufacturer is, this may have various components - but a common factor would be 3 caps : between L&N, between L&E, and between N&E.
Actually, these days the requirements on current waveform & power factor require active PF correction on the inputs for all but the smallest units. For years, there have been chips which do a switch mode supply for the switch mode supply - effectively using a switch mode circuit instead of a simple rectifier to charge the high voltage reservoir caps.

Now we take such a supply, and leave the earth unconnected. We now have a cap between L& E*, and between E* and N. Since E* isn't connected to anything, we now have a system where the internal "earth point", and often the output as well, is actually designed to be around 1/2 the supply voltage unless some external influence affects it

Thanks. Not a range I'm familiar with. Most I know don't attempt to measure AC current at all - I'd be a little intrigued to know how it does it.

With about one chip ! There really isn't much inside them - but no, I don't know how they work. I assume a shunt folloowed by switchable gain amp (to get the ranges), then by the same measurement system as for voltage. IIRC this is supped to be a true RMS meter but it's a long time since I looked at the specs.

Clearly not in the 'stray capacitance' ballpark, but fairly low in terms of filter capacitors. However, there could well be things other than capacitance between those measurement points, so I'm not sure that one can necessary trust those measurements.

Indeed, which is why I wrote "something in the order of". Just the stray capacitance from the test leads and my hands is enough to alter the readings.

 

[JohnW2]

"Traditionally" ... Often, the -ve or common of the output is connected (either directly or via a resistor) to the earth pin of the input - making the output linked to the supply earth. ... Now we take such a supply, and leave the earth unconnected. We now have a cap between L& E*, and between E* and N. Since E* isn't connected to anything, we now have a system where the internal "earth point", and often the output as well, is actually designed to be around 1/2 the supply voltage unless some external influence affects it

Well yes, of course, if there is an explicit connection between an 'earth point' (which is not connected to earth) and one side of the output, then what you say will inevitably be the case. However, I wasn't thinking of such an explicit connection being present because, if there were a facility for connecting that terminal to earth, that would presumably make is a FELV, rather than SELV, device.

... but no, I don't know how they work. I assume a shunt folloowed by switchable gain amp (to get the ranges), then by the same measurement system as for voltage.

Yes, on reflection, I suppose one can use just the same shunts as for DC cirrent measurement and then use the AC, rather than DC, measurement system to look at the voltage across the shunt. I've never really thought about that before, but it makes me wonder why AC current measurements are so uncommonly implemented in multimeters, certainly cheap ones. Maybe I'm missing something.

IIRC this is supped to be a true RMS meter but it's a long time since I looked at the specs.

Interesting. Basic measurement systems will obviously usually give an average value. If one knows one has a true sine wave, it's trivial to convert that to RMS, but for the general case of an unknown waveform, it's far less straightforward. Apart from using high frequency sampling, I'm not sure how one would get true RMS of an 'unknown' waveform. I must do some reading!

Kind Regards, John

 

[Sam Gangee]

Take a look at the "MOXTONE LAB" diagram on this page:
http://www.diyaudio.com/forums/power-supplies/200474-iso-trans-ac-mains-filtering-advice-please.html

To prevent interference from getting back into the mains power supply (left) there's a 1uF capacitor across L+N, an inductor in series with both L and N, a 4u7 capacitor between L+N and two capacitor connected respectively between L+N and chassis "ground".

(In "double insulated" equipment, it's common practice to connect these capacitors to a "floating chassis". The leakage current is "within acceptable limits". My personal fix for "tingling" equipment is a 1M resistor connected from the chassis to the nearest earth point.)

There's also a 150K resistor* whose function is to discharge the capacitors when the power is off so that you don't get a shock by touching the pins of the mains power plug.

I hasten to add that this circuit is a "belt 'n' braces" design for high power usage. A filter circuit in a typical SMPSU would use fewer parts (no CX2, CX3 or L2) and the capacitors would have smaller values.

*The resistor is usually a Metal film type rated at 350 volts. Ordinary 0.25w resistors break down at mains voltage as they are rated at about 100 volts peak.