Thanks for describing this in greater detail. I'm seeing more of
these new electronic thermostats used in electrically heated homes and
so I was curious what impact, if any, they might have on power quality
(those nasty third harmonics et al.). I have three in my own home
controlling my in-floor radiant heat and have been quite pleased with
The "third harmonic" thing is the result of chopping a DC supply, and
not the same as a chopped AC supply. I'm sure there is some 3rd-
harmonic content, but with a chopped sine the theoretical waveform
won't be the "all odd harmonics in 1/N magnitude" of the chopped DC.
OTTOMH I don't recall the characteristics of the transform for the
chopped sinusoidal case and was/am too lazy to get up and look for it
(and definitely too lazy to work it out :) ), but it's different--just
how different was what I was hemming and hawing about. It is, of
course, dependent on the phase angle as well as the discontinuity
changes characeristics as the chopping point moves through the cycle.
Actually, as I think about it, while the zero-switching is
advantageous from the standpoint of switching small currents, it is
the steepest gradient of voltage change w/ time, so in fact, the worst
from the standpoint of generating harmonics. But, the fact that it
isn't a square wave means it isn't the odd-harmonics only case.
Thanks to you and Dave for this. I know we've veered way off topic
and much of this is so technical it can be a little hard to digest
unless you happen to have some background in this area, but you've
both done a pretty good job of explaining it in a way those of us less
knowledgeable, like myself, might understand (and that can't be an
One of the things that continues to amaze me about this forum (and
others like it) is the amount of knowledge out there and, moreover,
the willingness to help others. I've certainly benefited from this
myself, many times over, both in practical, everyday matters and some
of these more theoretical concerns as well.
I'll bet that if you look at the output waveform on an oscilloscope,
you'll find that the thermostat is either on or off at any given point
in time, and that it cycles between on and off every few seconds in
order to modulate the heat. The switching could happen at random times
during the AC cycle if a mechanical relay is used, or it might be at
zero-crossings if an electronic relay is used.
But a heater has enough thermal inertia that there's no point in
switching the current 120 times per second, like a lamp dimmer does, and
switching only every few seconds reduces any electrical interference and
avoids creating non-sinusoidal current waveforms.
OK. Brace yourself, because I'm going to confirm what I understand
you to have said and the results could be a tad ugly. :-0
This electronic thermostat is simply cycling power on and off more
frequently than its mechanical counterpart, and any power disturbances
are rather trivial and occur only at the time the load is dropped and
then again as it is subsequently picked back up. It's really no
different from a conventional bimetal thermostat in this regard,
except that these minor transients [insert appropriate terminology
here] happen perhaps every 5 to 10 seconds, as opposed to once every
three to four minutes.
Hmm... no screaming, no gnashing of teeth, no hair pulling... I'm
taking that as a good sign....
Basically true -- only real difference is use of solid state switching
and the zero-crossing switching, but it still is basically "just a
switch". These both have advantages in reliability, but in reality
the frequency of control is probably overkill for the application but
it doesn't cost any more once go to the electronics anyway, so why
not? is one way to look at it. Of course, if they use the fancy words
and "high technology" to justify a high initial cost, that's another
thing, but it didn't seem all that far out of line...
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