Re: [CAD_CAM_EDM_DRO] Re: current rating of transformers

turbulatordude wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
>>turbulatordude wrote:
>>
>>
>>
>>>Hi,
>>>
>>>If I have a 24 volt, (12V-0-12) center tap transformer like the
>>>
>>>
>MPJA
>
>
>>>unit.
>>>http://www.mpja.com/productview.asp?product=7846+TR
>>>
>>>And isolate the center tap I get 24 volts.
>>>
>>>But when rectified, doesn't the current change also ?
>>>
>>>There is a note in the catalogue that for this wiring the amps are
>>>1.8x
>>>
>>>The catalogue refers to required ouput amps
>>>
>>>
>>>full wave bridge cap input filter 1.8x to required output amps
>>>full wave choke input filter = required output amps
>>>full wave, center tap, cap = 1.2x roa
>>>full wave, center tap, choke = 0.7x roa
>>>
>>>
>>>
>>>
>>Using the full winding means current flows in both ends of the
>>
>>
>winding
>
>
>>all the time. So, the current rating must go down. With full-wave
>>center tap, each half of the winding gets a rest 50% of the time, so
>>current rating goes up. I guess what they are saying here is that
>>
>>
>you
>
>
>>take the load current and multiply by these factors to get the
>>
>>
>rating
>
>
>>of the transformer.
>>
>>Jon
>>
>>
>
>
>This is where I'm confused.
>
>If I use the full winding, 24 volts, do I really get 5.8 amps ?
>and with center tap, 12 volts, 8 amps
>but using a choke at 24 volts, I can get the full 10 amps ?
>

Yes, probably. But, inductor input filters usually have poor regulation,
meaning the voltage droops substantially under load. But, the peak currents
in the transformer will be greatly smoothed out, so the RMS rating of the
transformer is fully available. Capacitor input filters draw strong pulses
of current right around the voltage peak, and this is a worst-case situation
for copper losses. So, all capacitor input filters will deliver less
current
from the transformer for the same heating.

Jon