On Feb 14, 12:12 pm, (Dave Martindale) wrote:
Paul M. Eldridge writes:

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
their performance.

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.

--

Yes, Paul has already confirmed that is the case -- the thermostat
describes it's operation in terms of the "on" fraction of time
required to maintain the setpoint. It does seem to use the technology
in its promotion as being a little more than it really is (surprise,
surprise... ).

I'll respond to your other related response to my previous posting
here as well to (hopefully) bring this to a close in one location.

I agree w/ your definition of undistorted, certainly from the utility
supply side definition once I understand your intent. And also, as
you correctly point out, the effect of the switching transient goes
away very quickly and I, in fact, even though I had specifically
looked at the product brochure of Paul's thermostat had let his
question lead me down a (mostly) false path of considering the
switching transient only instead of the overall time-average effect.

So, now again mostly for Paul, I'll amplify this slightly and again
hopefully further clarify rather than confuse/obfuscate.

So, while it is true there will be wave distortion and all it's myriad
effects _at the moment of switching_, it's also true that for these
thermostats the fastest they operate is on a 15-20 sec cycle time so
that the switching transient lasts for only 2/[60*(15 to 20)] ~ 0.2%
of the time. Consequently, in the larger scheme of things, even for
the output there's effectively no distortion, and certainly not for
any primarily resistive load. It would take something _very_
sensitive to input power or radiated distortion for it to be a
problem.

So, overall, we're in agreement and thanks for pointing out where I
kinda' let myself go off into the weeds thinking of the details as
opposed to the bigger picture...