Re: variable transformer for dc power supply

Without isolation your hooked directly to the AC line through the
variac. To many people have been electrocuted that way. If a ground
comes loose, you can end up with a floating voltage on the chassis,
Bad News. Another possibility is part if the winding opens up, then
you have Full AC voltage going to the rectifier. Plug the Power
Supply into an outlet thats wired wrong and you have 120vac on the
chassis!

Older Black and white TVs use to rectify the AC line, they Got Away
With It because Nothing with possible electrical potential, including
ground, was exposed.

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.

If you have a machine with 240vac going to the spindle, then the cost
of an isolation transformer is very cheap for the safety it provides.

If someone wants to use a variac to get their voltage set, thats
fine, IF they use an isolation transformer in front of it and lock
the adjustment knob.

Dont get me wrong, i'm not a safety freak, I drive fast, don't wear a
seatbelt... and generally laugh at what most folks do in the name of
safety these days, but the thought if not using isolation scares me.

mike

> Tim,
>
> How big of a deal do you think it is? I suppose it may be more

important on

> a little desk top like a Sherline, but a larger machine will

already have

> 240 VAC in the spindle motor. The iron should be grounded and any

lose

> wires should simply short.
>
> Isolation is a fine idea, but I had used DC in lots of larger

industrial

> machines. Sometimes we let the DC float, but more often I tried to

tie it

> down. I like to know where things are, particularly voltages.
>
> GTJ