cap btween primary and secondary windings?

I'm sure I've seen schematics showing a cap between the primary and secodary windings of a power supply transformer. I don't recall where, the context, or why it is done. I imagine it is some sort of RF supression. Anyway, does anyone point mr toward an example of this? Also can anyone explain the intended purpose etc.

Mostly I'm just curious. It came into my head while connecting an X-cap between the hot and neutral leads at a primary, and I just wondered to myself why someone would do that.
 
I can only imagine one reason for doing so...:
To create a HF connection from the electronics to the mains ground (zero).
However, serious danger can occur if you connect anything to the live wire, and there is usually a lot of "noise" (HF garbage) on the live wore anyhow.
Where I live, most outlets are class II which means that there is no safety ground connection, and the plug pins are identical, so there is no way to tell which pin is live and which is neutral.

If the purpose should be something with audio, I would rather connect the enclosure to safety earth, rather than putting any cap between primary and secondary of the transformer.

Jennice
 
I recall it being a power transformer. I've been trying to find the schematic that I think I remember but without luck. I'm starting to suspect I may be delusional. The main reason it bugged me was that it seems like a remarkably bad idea, but maybe there is something I'm overlooking. Oh well, it's not like I had an ovewhelming compulsion to try it out -- at least not on a transformer I've paid for!
 
The power transformer for a PSU feeding an audio pre/amp would never need a primary to secondary connection. I cannot think of any exceptions.
It is common for a liberal sprinkling of caps around the input switch / transformer / rectifier to reduce the high frequency mush that gets added to the 50Hz/60Hz that we all require (excepting smps).
These caps are usually 10nF to 220nF and can tie Live(hot) to Neutral and/or live to earth(safety ground) and/or neutral to earth. All these on the primary side MUST BE RATED FOR MAINS use. In addition caps across the secondary before the rectifiers and across the diodes and across the DC before & after the smoothing all have a similar purpose - to reduce all that Hi Freq trash. A cap after an inductor is an effective 2 pole filter and almost any two parallel wire circuit form the inductor.

A word of caution;- live to earth and neutral to earth caps will transfer some current to the earth return. If your house is protected by RCCB (earth leakage/ residual current) then be carefull that each detector is not exposed to a continuous or repeated level of leakage that exceeds 25% (not sure of this %) of the rated trip level - usually 30mA in UK. i.e. each RCCB circuit should have a loading that generates less than 7.5mA of leakage.
I don't know of the reason for this but it must be good otherwise the UK authorities would not demand it. But a secondary advantage of sticking to this would be reduction of nuisance tripping.
 
I would keep this leakage current well below 7mA.
It's a power loss (=heat) in the cap, and this unit is not likely to be the only appliance with ground leakage currents in your house. Thus, your AND all other devices should keep the total leakage below 30mA (which is an international standard requirement, so many countries use this limit).

Jennice
 
Primary-to-Secondary Coupling Cap

Sam,

The only time I can think of this as being valid is if the SMPS is a SEPIC (Single-Ended Primary Inductance Converter). The cap couples two inductors to give the SMPS circuit the ability to maintain a steady output viltage regardless if the incoming DC voltage is above or below the output voltage.

Another name for the SEPIC is the Cuk converter (named after Slobodan Cuk). Magnetically coupling the two industors (winding them bifilar on the same core) increases converter efficiency.

This works for both high-voltage and low-voltage input DC-DC converters, but the output is NOT galvanically isolated from the input. Also, the coupling cap is a rather large value low-ESR hi-frequency electrolytic cap.

If your coupling cap looks like this, then perhaps you have a SEPIC power supply. If it is a small-valued cap, then I'm not really sure what it could be, other than an Output Grd-to-Earth Grd AC-coupling cap.

Hope this helps.

Steve