Calc. # turns to get coil resistence

Just measure out 157.9 feet and wind it up. Ignore the number of turns. That's a lot easier. Better yet, measure out 12 ohms of wire and then wind that up.

But my wire tables show #28 annealed copper as 0.0649 ohms per foot. Are you using something else?

Why make a copper resistor?

John

"John Larkin"

.... Phil

What difference does that make?

John

ps - I didn't answer his question.

pps - why do you think body parts are insults?

Not necessarily. I had to do a very similar calc recently. I went with taking a length and then winding the coil with it.

"John Larkin"

.... Phil

I think PA may be jelous that he missed out on taking a length. (o)

Not possible.

Unless, of course, you're installing the turns on the inner surface of the cylinder. The ID is 20 mm but what is the OD? 22 mm? 220 mm?

--- Sounds like a trick question since, without knowing the _OD_ of the bobbin, it's impossible to calculate the number of turns.

However, just for grins, assuming the bobbin has an OD of 1", here's how to do it:

First, bare 28AWG copper wire has a nominal diameter of 0.0126" and a resistance of 0.06533 ohms per foot

Second, #28 with a single formvar coating has a nominal diameter of

As others have noted, the easy way out would be to determine the length of wire needed to equal 12 ohms and just wind that length on the bobbin.

Since the resistance is 0.06533 ohms per foot, then to get 12 ohms you'd divide 12 ohms by 0.06533 ohms per foot and wind up with 183.68 feet.

To do it the way you asked, though, you'd add the diameter of the wire to the OD of the bobbin to get the mean diameter of the winding:

Dm = Db + Dw = 1.0" + 0.0137" = 1.0137"

Then, since the circumference of a circle is equal to pi times its diameter, the mean length of turn is:

L = piD = 3.14 * 1.0137" = 3.183"

The resistance of the wire is 0.06533 ohms per foot, which is

Making the resistance per turn:

Finally, since your target resistance is 12 ohms and we know the resistance per turn, we can say:

Since the diameter of the wire is 0.0137", that would also be the pitch of the winding, making the length of the winding:

L = Dw N = 0.0137" * 692.44 turns ~ 9.5"

Since 1" = 2.54cm, 9.5" would be 241.3mm, so the turns wouldn't fit as a single layerer on the 100mm long bobbin.

I haven't double-checked my math, so there may be an error in there somewhere, since all I was really interested in was showing you the procedure. :-)

--- JF

You like to use that body part as an insult. And the corresponding female gadget. How charmingly Victiorian of you; or maybe you're just messed up about sex.

Still, it's better than the poop fetish.

John

Unless you carefully layer wind it, the solution won't be very exact.

John

So John L. gave you the length. To get the number of turns you can get close by assuming the wire is square in cross-section. You can then get then number of turns on the first layer, by simple geometery. Then do the second layer. The radius of each circle increases with each layer.

George H.

According to the Australian electronics newsgroup, he was thrown out of college for having sex with his male teacher.

The calculated number of turns won't match the actual number of turns.

Who? Larkin ?:-) ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

Sorry, by "ID" I meant the OD of the unwound bobbin shaft. The OD of the final coil would depend on the turns number.

Thank you for the general math procedure which is what I was after.

Since another respondant inquired, the reason I need the fixed resistance is to limit current without having to use an external power resistor.

And, no, it's not homework. Wish I was still that young.

Glenn Syborn

For DC resistance, only the total wire length matters, not the number of turns.

If you want inductance there are other formulas. Also, "solenoid tables" already calculated for various sizes.

"John Larkin" "Phil Allison"

F****it.

.... Phil