On Thu, 12 Dec 1996, Andy Burgess wrote:

> >>The problem with using any type of transformerless power supply in a device
> >>is that ANY electrical connection (intentional or unintentional) between
> >>that device and anything that's ground- or mains-relative (or between that
> >>device and any other device with an intentional or unintentional connection,
> >>etc...) poses a severe risk of blowing up one or both devices.
>
> I am confused by this. Aren't PC power supplies transformerless?
> Why aren't they dangerous?
>
> --
> Best regards,
> Andy Burgess
> aab@cichlid.com
>
There is a SMALL transformer in the power supplies that provides isolation
and voltage reduction.  The AC mains input is first rectified and filtered
to produce a high DC voltage and then a switching transistor is used to
produce a high frequency AC input to the transformer. Feedback for
regulation and control is done with either an optoisolator and/or another
tiny transformer.

The high switching frequency allows using such a small transformer.

73 de WA0ZNL
------------------------
Lynn Richardson
Progress Intrument, Inc.