Re: [CAD_CAM_EDM_DRO] Re: What size capacitor should I use? resistor or not ??

turbulatordude wrote:

>OK Scott and Wally, this a definate design point in which a person
>with a hgh level of electical knoledge would be needed. Most of us
>are hobbiests and not EE's.
>
>If either of you guys want to make a spreadsheet and white paper on
>how to impelment unregulated power supplies eq: bridge and cap
>selection, that would be a really great resource.
>
>>From my watching the list, the discussion of the inrush surge being
>either harmful or just un-nerving, came in to play as the
>transformers got huge, not the 'average' 10 amps ones we see so often.
>
>

No, it can actually be pretty significant even in these roughly 1 KW
transformers.
There are two components of the surge. The obvious one is the charging of
the capacitors. but, there's another, too! That is the charging of the
magnetic
circuit in the transformer. For cost reasons, transformers are built
with "just
enough" iron. Every half cycle of the line, the voltage builds the
magnetic flux
in the core, and then "unwinds" that field, and the total flux stays
balanced around
zero. But, if you turn on a transformer at the voltage zero crossing,
it starts a
half cycle from zero, rather than starting with the maximum reverse flux
in the core.
This would require twice the iron that the core actually has, and so it
becomes
saturated right after the voltage peak, and stays saturated for the
remaining
quarter cycle as the voltage falls to zero. This is why when turning on
big transformers
you can get a loud, sharp buzz right after turning it on. So, an inrush
surge
limiter should actually go in the transformer primary circuit, not the
secondary.

>At what point would the resistor be needed to prevent damage ? at
>what point would it help to prolong the life of the components, but
>otherwise not be apparent it is needed. (no noise or whatever) and
>when is it just not needed at all ?
>

Well, it is why we put 50 A, 400 V rectifiers in 70 V, 10 A power supplies.

Jon