Authenticity opinions

[Brent Hilpert]

Ben wrote:
> 
> dwight elvey wrote:
> >
> >> From: philip at axeside.co.uk
...
> >>>
> >>> Yes Tony... I have done all of that. The secondary is dropping to nearly
> >>> 6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
> >>> to the primary. The ripple on the DC side is in excess of 1V with that
> >>> same 5A load. I have swapped in alternative filter caps temporarily
> >>> and see little improvement. I checked the bridge for shorted diodes or
> >>> excessive loss and it looks OK.
> >>
....
> >
> >   I should note that a transformer rated at 5A AC will not provide
> > 5A DC. Because the rectifiers only take power on the peaks of the
> > cycle the power in the transformers windings are much greater
> > than if it was a continuous load. One needs to derate the transformer
> > some.
> >
> > Dwight
> 
> Unless you have big choke hiding in the circuit. :)

.. and in which event one has to lower the voltage expectations.

Here, for example, is how Hammond derates some of their common
low-voltage transformers:

  Ratios of DC output from filter to AC transformer specs
  -------------------------------------------------------
                                    Voltage Current
                                     DC/AC   DC/AC 
   Bridge feeding cap-input filter    1.25    0.56
   Bridge feeding choke-input filter  0.88    1.0

Also to note, simply adding capacitance to a cap input filter cannot be relied
on as a solution, as it reduces the conduction angle of the rectifiers (the
portion of a cycle for which they conduct), leading to more transformer losses.