6 Jun
2010
6 Jun
'10
5:53 p.m.
Ben wrote:
.. 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.
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. :)