# "Variable heat" electric range available anywhere?

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.
Cheers, Paul

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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.
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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 easy task).
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.
Cheers, Paul

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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.
Dave
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On Feb 14, 12:12 pm, snipped-for-privacy@cs.ubc.ca (Dave Martindale) wrote:

--
Yes, Paul has already confirmed that is the case -- the thermostat
describes it\'s operation in terms of the "on" fraction of time
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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....
Cheers, Paul
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:)
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...