On Sep 3, 2011, at 3:34 AM, Peter Heckert wrote:
Am 02.09.2011 22:04, schrieb Horace Heffner:
The above makes no sense to me. Resistor wattage ratings are
merely the maximum wattage that can be put though them without the
expectation they will be destroyed. The power ratings are not used
to compute the power or current through them. If you put 600 W
through a 300 W resistor you can expect it to soon burn out.
The printed values are the recommended safe operation conditions,
not the maximum rating.
"Resistors Wattage Rating and is defined as the amount of heat that a
resistive element can dissipate for an indefinite period of time
without degrading its performance."
http://www.electronics-tutorials.ws/resistor/res_7.html
"Practical resistors are also specified as having a maximum power
rating which must exceed the anticipated power dissipation of that
resistor in a particular circuit"
http://en.wikipedia.org/wiki/Resistor
I have had lots of experience burning out resistors. 8^)
With this type of heater resistors, the maximum rating depends
mostly on cooling and can be multiply exceeded with good cooling.
The above is generally true. However, we are talking about band
heaters and immersion heaters. They are designed for operation in
the manner Rossi is using them, though probably *not* with thick
insulation wrapped around the band heaters. Also, such things are
made for heating water, not boiling water.
The problem is, then the voltage must been increased above 230V. P=
U^2/R. For double power the voltage must be 1.41*230V.
My point was that the drawing posted showed two 300 W resistors.
Rossi put 1200 W through his device in at least one test, AND his
controllers output 220 V max. This indicates he has a device with
multiple band heaters, or lower resistances and higher power ratings.
Perhaps he has a higher wattage 220 V immersion heater inside. Who
knows?
However, if I compare this resistor with the heater coil inside my
hairdryer, then the coil in my hairdryer is much smaller and it has
1200W. So, with good cooling, the resistor could deliver about 4
times the wattage. Maybe the lifetime could decrease, but it should
be possible.
This does not seem relevant to me because the resistors are being
used as designed, at no more than 220 V, but with the exception of
heavy insulation on the outside.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
