copper current capacity ?

[Dominic-Luc Webb]

I have made a filter for a classic light dimmer, not having a local
source for what I need; two lower pics at:

http://www.megspace.com/science/stp/alum/dimmer/dimmer.html

It uses 1.4 mm diameter transformer wire @ 58 turns wrapped
around a piece of 15 mm diameter wooden dowel with a 6 mm
diameter steel rod driven through the length, as seen in
the pic. This was covered with heat shrink. I do not have
convenient way to measure the Henries, but I cannot detect
any radio frequency interference at any dimming level and
operation is quiet. I am wondering how to rate the current
carrying capacity. There are two values listed on relevant
websites (i.e., www.powerstream.com/Wire_Size.htm) indicating
very different capacities for power transmission or chassis
wiring. Any recommendations on how I should rate this?

I could note that typically these are recommended to
be 20-100 uH @ 4.5 amp. I would like to be able to
operate this a bit higher, like 10 amps (I plan to
switch the triac to a similarly configed 25 amp version)

I am curious about criticisms of this construction overall,
of course. While we are on the subject, any neat tricks
to measure L?


Dominic

 

[John Popelish]

Dominic-Luc Webb wrote:

I have made a filter for a classic light dimmer, not having a local
source for what I need; two lower pics at:

http://www.megspace.com/science/stp/alum/dimmer/dimmer.html

It uses 1.4 mm diameter transformer wire @ 58 turns wrapped
around a piece of 15 mm diameter wooden dowel with a 6 mm
diameter steel rod driven through the length, as seen in
the pic. This was covered with heat shrink. I do not have
convenient way to measure the Henries, but I cannot detect
any radio frequency interference at any dimming level and
operation is quiet. I am wondering how to rate the current
carrying capacity. There are two values listed on relevant
websites (i.e., www.powerstream.com/Wire_Size.htm) indicating
very different capacities for power transmission or chassis
wiring. Any recommendations on how I should rate this?

I could note that typically these are recommended to
be 20-100 uH @ 4.5 amp. I would like to be able to
operate this a bit higher, like 10 amps (I plan to
switch the triac to a similarly configed 25 amp version)

I am curious about criticisms of this construction overall,
of course. While we are on the subject, any neat tricks
to measure L?

Dominic

The smallest wire gauge usable for a given current depends on two
factors. One is voltage drop. You do not want to waste more than a
minor fraction or your voltage getting the current wherever it is
going. The second factor is temperature rise and risk of damage or
fire. For wound arrangements, the second factor far outweighs the
first almost every time, since the heat is concentrated and the escape
of heat is inhibited. So ampacity tables that are made for wiring (1
to 3 strands run between points) are not at all conservative enough
for wound components. And specific limits are a lot harder to
generalize, since there is such variety in volume to surface ratio and
material temperature capability. Your design has a strike against it
by including a flammable material (wood) under the heat source
(wire). If I were you I would either purchase a current rated
inductor for this job such as the 5626 to 5628:
http://rocky.digikey.com/WebLib/JW%20Miller/Web%20Data/5600%20Series.pdf

This type of low cost inductor is manufactured by many vendors. They
are wound on an epoxy coated ferrite drum core. These cores are also
available if you want to wind a custom version with much more
effectiveness per amount of copper and heat generated compared to your
design.

If you must make your own inductors, you need to do some testing by
putting a thermocouple under some insulation at the middle of your
coil and measure the temperature at full current and see if the
temperature rise is low enough. For materials like wood, I would
think that should stay below 100c. You could also look at the data
sheet above to get an idea what wire size is used for variously rated
coils. For instance, in the above sheet, .082 inch wire (2.1mm) wire
is used to wind the 14 amp 100 uHy version, while .065 inch (1.7mm)
wire is used for the 250 uHy 9 amp one.

--
John Popelish