On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
wrote:

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.

You are right, Rich. (And thanks for your post because
it caused me to solve a problem I didn't even know I was
about to encounter!)

http://www.st.com/stonline/books/pdf/docs/3566.pdf

See the first five pages for an accessible analysis of the
hazards of driving inductive loads with 'SCR' type devices.

See pages '6/16' and '7/16' for a clever triac circuit ST
developed to drive inductive loads without the flaws of
the earlier circuits.

Quoting them:
"This circuit has been developed by the STMicroelectronics
applications laboratory and used with success for a wide
range of equipment."


See the text at the bottom of page
'6/16' for a circuit explanation. See Figure 9 on page
'8/16' for component values.

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on. Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current