In article ,
Bill Noble wrote:

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

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.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.


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

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn