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re: cheap choke design
12 mar 2002
n. thornton  wrote:

>> 13      0.358    11 a                  66.5   61.5
 
>a 5f rise with 11a...

i wouldn't put much store in that number, which i mainly measured
for core loss alarmists. the garage door was open that morning, and
the day was heating up. the coil was on a ring stand in free air
about 4' above the floor. i measured the room temp near the floor.

>at 100a you have 9x the current... a temp rise of 81x 5f is a fire risk...

the resistance of 8 12.5' #12 wires in parallel is 1.588x12.5/1000/8
= 0.00248 ohms at 20 c. they would dissipate about 25 watts at 100 a. 
the coil bodies are about 2" in diameter and 10" long with 0.436 ft^2
of surface. in still air, dt = 25x3.41/(0.436x1.5) = 130 f, roughly.

both coils are mounted in a 12"x12"x4" steel box with 3 in^2 of air
vents at the top and the bottom, but the coathanger core protrudes
from each coil form 6" above the top of the box, with all 50 pieces
spread out like a tree to improve heat dissipation.

>especially if you use 105c flammable poison fume producing insulation.

the poison fumes will probably stay in the garage, which is equipped
with a fire extinguisher. 

>and thats only at the rated power, we havent even considered safety
>margins. even enamel won't survive that temp rise.

"blah, blah, blah... fire risk!!!... blah, blah, blah..."

>also you will be dropping something like 0.358v x 9. = 3v ish.

who cares? incandescent lights will last longer.

>a pair of back to back diodes would only drop about 1v max, not be a fire
>risk, and save you more on your power bill.

we've discussed this before. problems with lightning and surges, greater
(not lesser) power dissipation and heat-sinking, and so on. digi-key lists 
some $5 50 amp diodes with a 1.2 v drop at 20 amps...

>and you wouldn't get any inductive kick when something switched off.

probably no big deal, given the low-impedance surroundings.

>a small relay driving a 100a bistable relay would consume 0.5 to 1w
>while the higher price supply was on, and a pulse of 5-25w at each
>switchover.

didn't see those in home depot or the digi-key catalog. sounds like they
would be fancier and even more expensive than grainger's $342 pair of
less-fancy relays. 

nick




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