If they're air core, it's pretty easy: To an accuracy of about 1%, the inductance in microhenries of an air-core coil is given by the formula:
L = a**2 n**2 ----------- 9*a + 10*b
where
a is the radius of the coil in inches b is the length of the coil in inches n is the number of turns.
Note that it's easy to get the turns count too low by 1/2 or 1 turn: a hairpin counts as 1 turn. Also this formula works best for coils with many turns.
Cheers,
Phil Hobbs
There are formulas that include the wire diameter, but it doesn't matter very much--the inductance of a single wire goes as the negative logarithm of the wire diameter, which is a pretty slow dependence.
I'd use the mean diameter (average of ID and OD) but it doesn't matter too much at the accuracy level we're talking about, assuming the wire is thin. Also note that this formula works only for single-layer coils. There are formulas for multi-layer coils, but they're much more complicated. If you have multiple layers, your best bet is to use a grid-dip meter.
Cheers,
Phil Hobbs
GDO's are rare & expensive these days - if the OP doesn't already have one the only realistic option might be build one!
There are several schematics in the Heath folder at the BAMA boat anchor site, including tube, tunnel diode & transistor designs.
The twin gang air spaced tuning caps are getting scarce too! Watch out for a
70's tabletop transistor portable to salvage one from.
Err - forgot all about ebay!
I dunno...I bought one off Ebay for about $25 just lately. There are pots of them.
Cheers,
Phil Hobbs
I used to put a known capacitance across a coil and connect it to a signal generator through a 1 K resistor. I would tune the generator to find the resonant frequency, and use a simple formula to determine the inductance. This worked for single or multiple layer and any type of core.
They also trade on Heath lists.
Use this program to find the value of the coil:
I duplicated a miniature bird telemetry transmitter from an existing design, and would like to replace the 2 wound coils in my next one with available dipped through hole, or even surface mount inductors
The coils are:
4 turns #32 wire 1/8" dia.
Is this enough information that someone can give me the values I need to order as replacements?
Thanks Jim
O.K., sounds easy enough. Is the "radius of the coil in inches", the outside radius or inside? These were wound on 1/8" drills, so I guess I need to measure the ouside with calipers??
So the wire diameter doesn't figure in? Curious.
Jim
design,
available
the
On Sat, 9 Sep 2006 02:31:25 -0400, "Jim" Gave us:
No. The missing information revolves around the ferrous core material properties of the "coil" you are replacing. It's perm specs and its mass are important factors. You also would need to analyse the coil, and you need to know what frequency it is being utilized at as well to do so.
Why do you need to "replace" them? Are you attempting to miniaturize the circuit or such?
On Sat, 9 Sep 2006 02:57:08 -0400, "Jim" Gave us:
Two things. STOP top posting and set you computer's date and time correctly.
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