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Posted to uk.d-i-y
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Is an electric blanket an inductive load?
wrote:
On Monday, 20 July 2020 09:03:05 UTC+1, Chris Green wrote:
tabbypurr wrote:
On Sunday, 19 July 2020 23:17:07 UTC+1, Mike McLeod wrote:
On Sun, 19 Jul 2020 21:58:31 +0100, Chris Holford
wrote:
Although the inductance is probably not enough to worry about, the cold
resistance of the element will be less than the resistance at operating
temperature. This means that the initial current at switch-on will be
higher than the normal operating current.
Yup, I was aware of that and measured the current draw under warmed-up
running conditions.
what you need to know is cold resistance.
As I've said above the cold resistance will be very close to the hot
resistance, it's not being heated up to thousands of degrees like an
incandescent bulb. Anyway the element will be made of resistance wire
which will have a very small change of resistance with temperature.
Yup. But from the triac's POV, cold R is what matters. Apply 330v to that
R to work out the peak i the triac sees - it will not be 0.35A for a 0.35A
blanket.
But cold resistance = hot resistance to within a percent or so.
You're not going to size the triac that close to its maximum ratings.
It could even be that hot resistance is lower than cold (there are
resistance wires with negative temperature coefficients), in that case
it would be the hot resistance that mattered! :-)
Agreed about the mutiplying factor for peak current versus RMS current
(which is what the rating shows of course).
--
Chris Green
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