Re: Power Supply for Servo System (Hot Supply using Variac)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, wanliker@a... wrote:

> In a message dated 7/29/2003 7:59:14 PM Mountain Daylight Time,
> engineer@s... writes:
>
> > The only thing I thought of was no isolation but what is that

buying

> > me?
> >
> >
>
> A good chance to make your wife a widow, and your children have no

father.

>
> When you use a variac you have no line isolation, and if any thing

goes wrong

> you can be killed quite quickly.
> If the neutral wire to the variac breaks, the entire transformer

element goes

> to line potentional, and it hurts, ask me how I know.......
>
> If you insist on a variac, then put a isolations transformer in

front of it.

> bill
>

Okay, Bill, how DO you know?

Seriously 160VDC is pretty nasty stuff ANYWHERE you find it.
Floating around on components that it shouldn't be on is even
nastier. Even with an isolation transformer if you get across the
secondary or the resultant DC, you will still get a good dose of
juice.

If you were to take exceptional care that all exposed components were
at safety ground and there was no open wires to get across, the old
SCR (light dimmer) method and bridge rectifier would work. I don't
consider it prudent for anyone but an electronics or electrical
expert to attempt. I have several small SCR controlled DC motor
controllers 0 - 130VDC made by Reliance and others, that connect
directly to the AC line. It is just a descrete bridge and power
components on a small PC board. I have seen lots of circuits like
that for running high voltage DC motors.

This will date me but millions of cheap tube type (ACDC 5 tube)
radios were sold that had no power transformers. The filaments of the
tubes were in series and dropped the rectified line voltage across
them. All of that BEFORE 3 conductor plugs were common. They solved
the problem by making sure the cases, knobs and switches were all non-
conductive.

Modern computer power supplies rectify the AC line, double it to
300VDC and switch it at high frequency before it goes through the
small core transformer to be reduced to 5 and 12 VDC. You go probing
around in the primary side of one of those and you have a formula for
memory loss (or worse).