On Sunday, 19 July 2020 23:56:53 UTC+1, Fredxx wrote:
On 19/07/2020 21:58:31, Chris Holford wrote:
In article , Mike McLeod
writes
I only ask because I'm making up a controller for one. I don't find
the 3 heat settings provide enough choice so I'll be controlling a
small triac via a PWM chip. Although the max current draw of this
appliance is only 350mA, the triac (a Z0607) is pretty puny and I
don't want it operating at too close to its max ratings obviously. I'm
using a snubber to be on the safe side WRT to back EMF anyway.

Here's the datasheet for the triac:

https://www.st.com/resource/en/datasheet/z00607.pdf

Will that do it or is something more meaty required?
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.

Nichrome is a sufficiently cheap material there is no need to worry
about cold resistance. In fact having a wire with a high temperature
coefficient would cause hot spots.

This means that the initial current at switch-on will be
higher than the normal operating current.

For reasons above there will be no switch on surge.

For a filament lamp where the
operating temperature is in the order of 2000degrees Celsius, the cold
resistance is about a tenth of the resistance at operating temperature.

We are talking about something completely different and using a Tungsten
filament with a resistance temperature coefficient far removed from
Nichrome.

In the case of an electric blanket the operating temperature of the
element is much less than that of a lamp filament so this effect will be
less significant; still a good idea to use a generous rating for the
triac.

An 80W blanket is 0.3A and will take the same at switch on. The OP is
proposed a 0.8/0.9A TRIAC. That should be sufficient margin. Personally
I might go a little higher as more rugged ones don't cost the earth.

0.3A blankets take 0.45A peak. A 0.8A triac should be fine.

The elements may be wound, but their inductance is still tiny.


NT