My ham radio 110-13.8 V power supply interacts with my electric
frying pan. No matter how far away in the house I plug them in, there
is a harmonic that causes the power supply to make a loud hum.

I must choose between rag chewing and food chewing. Help. What gizmo
can I purchase to filter out the resonance between them? Actually, it
only happens when the pan is switched to "low", not on "high".

What these symptoms suggest to me is that the "low" setting of the pan
is probably switching in either a half-wave rectifier diode, or some
sort of triac-based switcher... the former seems more likely. This
would cause the pan's load on the powerline mains to be very
asymmetrical... it'd draw full current during one phase of the cycle
and none during the opposite phase. This will have the effect of
causing the mains voltage at all of the other outlets on the circuit
to have a significant DC offset voltage. This will induce a current
into the primary winding of other transformers, possibly saturating
the core (a bigger problem with toroid transformers than with EI-core,
I believe). The symptoms are usually buzzing and humming, and
sometimes excessive heating in the windings and cores of affected
transformers.

Triac-based switchers, such as are found in many cheap light dimmers,
tend to cause a slightly different set of problems. They don't
generate a DC offset, but they do generate a lot of odd-order
harmonics. They can cause transformer buzzing and the "singing" of
light bulb filaments.

There are a number of possible solutions, none of which are terribly
wonderful:

- Replace the electric fry-pan with a different model that doesn't
cause the problem. For example, a model which has multiple heating
elements, and switches them into different combinations of series
and parallel and disconnected states, can provide multiple power
levels without having to use a rectifier or triac - it'll always
present a well-behaved resistive load to the mains and won't cause
the problem.

- Put a high-amperage isolation transformer between the frypan and
the mains. This will prevent the DC offset from travelling back
onto the mains. The isolation transformer may hum or buzz when the
pan is on its low-power setting, but the problem at your power
supply should be greatly reduced or eliminated.

- Put the isolation transformer between your ham supply and the
mains. The isolation transformer may be less prone to hum or buzz
than the power supply's transformer.

- Try a ferroresonant constant-output-voltage transformer (Sola makes
these). This will ensure that your power supply receives a fairly
consistent AC waveform. [Unfortunately, these ferroresonant
transformers are themselves somewhat problematic... they run hot,
hum, and their output isn't a terribly good sinusoid even in the
best of times.]

- Change to a different type of power supply. A ham-rated
switching-type supply might be much less affected by this problem.

- Unplug your ham supply from the mains when the fry-pan is in use,
and operate your station from a large battery during those times.
Recharge the battery from the supply when the fry-pan is not in use.

- To mis-quote the old saying, "Be cooking with gas!"

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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