On Sun, 2007-04-08 at 12:50 -1000, Ken Brown wrote:
> This is completely off topic, except that it is in a general sense about
> the production or RFI or the lack thereof. If you are one of those
> people whose blood pressure rises when you see a question that is not
> specifically about a Ten-Tec product on this reflector, please delete
> this message immediately to avoid any hazard to your health.
>
> I have an electric stove in my home, and have had them in many homes I
> have lived in. I have never heard any RF noise that I could attribute to
> the heat controls on an electric stove. I have also never experienced a
> failure in an electric stove other than at the connections to the
> heating elements. If they used electromechanical relays I would expect
> to have seen a failure at that point by now. If they use SCRs or other
> solid state controls I would have expected to hear some RFI from one by
> now. How do they do it?
>
> DE N6KB
>
Some have two sections to the element and so have three or four
connections, most often three. With 240 volts and neutral supplied to
the stove the switch can connect one section to 120, or even two
sections in series to 120, the one or two sections to 240 first in
series and then in parallel to get several variations in heat. Some
include a bimetallic thermostat sensing the temperature of the bottom of
the pan, those can get temperamental.
I think some vary the duty cycle with a bimetallic sensor in the switch
probably switching at several second or even minute intervals rather
than having the RFI complication of part cycle switching. And if the
long time switching is always done at a zero crossing the RFI is
automatically minimized. ICs for that have existed for a long time.
Point is the time constant of the surface element and pot is long enough
that fast switching isn't necessary.
There are probably other schemes too. I've not been into a new high end
stove. My last one was a recluse from storage and after 36 years of
little use, it went to the salvage yard last fall. Cost almost as much
to dispose of as I paid for it.
I'm sure I've hard RF clicks from oven thermostat operation. And
probably from range surface elements being turned on or off since my mom
had an electric stove when I was at home in the 1950s and first
licensed.
Somewhere there is probably a wikipedia on range surface element
controls but I've not looked for it.
73, Jerry, K0CQ
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