Re: Power Supply Caps and Xformer

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "kmslinda" <gerry@z...> wrote:

> I am by no means an expert on transformers, but the instructions
> I received for a 1400 VA transformer kit from Toroid Corp of
> Maryland, states the the DC seconday Max Load is 880 Watts. In
> all the posts I have read concerning power supply size, no one
> has ever stated that power is lowered when rectified to DC. Is
> this correct?

VA is not power. Watts is, therefore VA is not exactly the same as
watts.

When you rectify ac with a full wave bridge, AND add a filter
capacitor, the measured voltage in DC volts is approximately 1.414 *
the secondary ac volts. Depending on the "regulation" of the
transformer which is the ability of the transformer to maintain
voltage under load, you may see a higher free wheeling value, even
with the couple of volts drop for the rectifiers.

A 20 volt AC secondary will thus yield about 28 vdc, plus or minus.

However the supply won't be "rated" above the VA, so the DC current
is also derated by a proportional amount ( .707). A 20 volt ac, 160
VA transformer would yield about 6 amps ( 5.7).

This doesn't mean that a significantly higher power surge cannot be
delivered. Charge up an electrolytic capacitor and discharge it
through a dc motor winding. Some of that power is available for
surge demand above and beyond the transformer rated load.


Fred Smith - IMService
http://www.cadcamcadcam.com/hobby

My transformers are finally in stock! 120vac primary, 750 VA toroids
with a 48v (15 amp) and a 6 v (3a) secondary. They are $55. Also
have 160VA, 120/20 for $35.