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Michael A. Terrell

wrote:

On Sun, 07 Nov 2010 21:28:49 -0500, "Michael A. Terrell"
wrote:

wrote:

On Sun, 07 Nov 2010 03:20:13 -0500, "Michael A. Terrell"
wrote:

wrote:

On Sat, 06 Nov 2010 11:45:19 -0700, Rich Grise
? wrote:

?Wild_Bill wrote:
??
?? In the DIY article I saw, there was a step-down transformer utilized, and
?? numerous comments about how some dimmers won't last long when attempting
?? to drive (the primary winding of) a transformer, instead of driving a
?? resistive load of incandescent lamp.
??
?I'm an electronics guy by experience and training, and I'd be terribly
?reluctant to use SCR control on the primary of a transformer. I'm "only" a
?tech, so I can't quote numbers; it's just sort of a gut feeling that the
?inductive reactance could cause a phase shift and upset the firing sequence
?of the SCRs.
?
?Of course, I'd be happy to hear of anyone else's experience with this kind
?of setup.
?
?Thanks,
?Rich
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.

The transformer core can saturate from the DC bias caused by cheap
dimmers.
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)

You can use a handful of old filament transformers & switches to get
a lot more choices from a Buck/Boost configuration. 240V variacs are
usually cheap, but have to be derated becasue you are using them at half
voltage.
My old buck-boost is one I built from a heavy fillament transformer
many many years ago, that has followed me around like a well trained
puppy.

As for the 240 volt variacs, they are good for the nameplate AMPERAGE
on 120, which reduces the power (va or wattage) rating significantly.

Yes, but small ones can be dirt cheap. Most people seem to think
they can handle double the current when one is used at half the
nameplate voltage, and can't be convinced otherwise till smoke pours out
of their recently departed variac.

And I see my fat-finger syndrome spelt voltage wrong again. Thought I
better correct it before someone else jumped on it.

--
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