[Boatanchors] Re: How much does Henry weigh?

J. Forster jfor at quik.com
Thu Jan 5 00:25:20 EST 2006


StephenTetorka at cs.com wrote:

> Hi Ken:
>
> Yes, things like thinking while typing become more challenging with age...as
> my notes shows.
>
> Kindly excuse me.
>
> I need an idea of the amp rating...it is apparently ?5 amps...with my guess
> being that the "?" is a 2...thus I arrive at 0.25 amps.

Separately, you wrote the thing has a DC R = 170 ohms, so at .25 A, the dissipation
is I * I * R = 10.6 W...  not unreasonable.

> Soooo might a 15 Hy @ 0.25 amp choke weigh as much as 18 pounds or should it
> be more like 1.8 pounds, say?

More like 18 pounds, because with enough turns to get 15 Hy @ .25 A, you'd almost
certainly have to have an air gap in the core to avoid saturation ( H= N*I )

> Being a former specialty wire sales engineering...I'd have a darn better idea
> if I could see the wire - but - this unit is completely enclosed.

The DCR tells a lot. Here are some rough guesses guesses:  You said the thing was 6
x 5 x 4. From the dimensions you give, the core apertures are roughly 1" x 3" with
a stack height of 2". This means the center leg is about 2" x 2" and the average
turn is 3" x 3" = 12" = 1' per turn. Convenient.

Conventional design is roughly 500 circular mills per amp. From wire tables:

http://www.interfacebus.com/Reference_Cable_AWG_Sizes.html

so it's roughly #29 or #28 AWG...  choose #28 at 66 ohms/1000', so 170 ohms = 2600'
= 2600 turns

#28 has a OD of roughly 12.6 mils which is roughly 80 TPI. With some allowance for
the ends you get roughly 200 T/layer.
So, that means about 13 layers.

If you allow for 0.005 paper between layers, you get roughly 0.020" / layer for a
total winding depth of 0.26"

That's a little shallow...  I'd expect it to me something more like .75". If you
changed the wire to #26, it'd likely come out close to right.

Not a bad guess, huh??

As to the inductance, that's a crap shoot because of the air gap.

> I put this question out there as I've read about folks ballparking
> transformer number based upon weight when no other data was to be had.

Yes...  40 Watts/lb at 60 Hz.

> Or to put it another way on this transformer question - what might be the
> range of inductance for power transformer windings with 115 primary and say 800
> vct secondary and some filament taps too?  And could they be used as chokes as
> I recall some folks saying.
>
> Tnx,
> Steve

You have to measure or reverse engineer it as above. Typically on a 2 winding
transformer, the pri and sec each occupy about half of the window. There are sites
listing the inductance per turn for various E-I cores. L goes with N * N

As to choke use, essentially no. W/o an air gap the thing will saturate with little
DC flowing through it.

BTW, it would be VERY dangerous to use the filament leads as choke terminals. The
thing will act as an open circuited current transformer, and produce very high
voltages.

FWIW,
-John



More information about the Boatanchors mailing list