Re: variable transformer for dc power supply

Hi Greg,
I think I get what you saying, its not a problem with grounding
the DC side, but assuming that the AC line coming from the street
is tame enough and of the correct relation to earth ground. If the
center tap is the secondary side to the transformer, then I whole
heartedly agree with you, ground it. That's very similar to way tube
guitar amps are wired, the chassis is connected to the center tap of
the secondary +/- 300vac legs, which each are run through diodes with
the cathodes connected to create DC plate voltages.

The original question I think was asking to bypass the 2 coiled
transformer topology with a single coil primary that
has a tap off of it for the output ( what a regular variac does is
make that tap movable to any point of the primary ). A transformer
analogy of a potentiometer.

It can be done, but it's something not for the inexperienced and
precautions are needed to make sure that hot and neutral are never
reversed and the neutral is always truely at ground potential.

You are right about isolation in that if (most) then needs to be
referenced to something, and chassis/earth ground is usually it. An
exception could be using an isolated secondary to add a voltage
supply on top of another potential voltage.
( I'll blather no further :-) and I dont mean to stir up arguement,
just things to ponder )
-Lee

--- In CAD_CAM_EDM_DRO@y..., "Greg Jackson" <greg@t...> wrote:
..., "Greg Jackson" <greg@t...> wrote:
I designed industrial machines with DC. Like I said we did it both
ways. I liked it more when we didn't let the DC float. On the larger
voltage systems we ALWAYS grounded the DC bus from a center tap on
the transformer.
Those were 600 volt DC systems so each side was either +300 VDC or -
300 VDC.
If we didn't ground the center tap and let it float through
isolation, then whenever a DC wire touched the machine frame the
other side was at a full 600 volts relative to ground. If we did
ground the center tap, then whenever a DC wire touched the machine
frame we would blow a breaker and everything stopped as it should.

To my way of thinking, isolation does not make the system safer. In
particular, don't think that by isolating the system you can be less
careful with the DC wiring. Wires should never get loose in the
system and any loose wire that contacts the machine should shut the
machine down, not electrify it.
>
> For anyone that is really concerned with safety, just get a $6 GFI
and wire
> it into the main supply of the machine. This is a standard
industrial
> practice on 115 vac industrial circuits and UL encourages it.
>
> GJ
>
> -----Original Message-----
> From: n4onl [mailto:umrk@b...]
> Sent: Tuesday, November 26, 2002 12:04 AM
> To: CAD_CAM_EDM_DRO@y...
> Subject: [CAD_CAM_EDM_DRO] Re: variable transformer for dc power
supply
>
>
> I bet if you check those industral machines with DC in them they
> aren't rectifying the AC line directly, their using an isolation
> transformer. They don't want to kill their customers.