Amidon Associates coil system

It was once difficult to obtain coil forms to make your own project inductors. Amidon Associates Inc. makes a series of slug-tuned inductor forms that can be used to make any-value coil that you are likely to need. Figure 4-2A shows an Amidon form, and Fig. 4-2B shows an exploded view.

L-S7 coa FORM - irpul assembly

SHIELD CAN : Pbi«i copper.

Prorata maximum eleciralatk shielding —

CUP CORE : Same material as tuning slug. Provides maximum magnet* shietfng

Materál icfectwj foe opcfaitkt« twjwency.

- BOBBIN : Supports winding SIX PIN BASE r

Moulted piatoc, pitted canker pins.

4-2 Shielded slug-tuned variable inductor.

Table 4-1 gives the type numbers, frequency ranges (in megaHertz) f and other specifications for the coil forms made by Amidon. Three si2es of coil form are offered. The L-33s are 0.31" square and 0.40" high, the L-43s are 0.44" square and OSO" high, and the L-57s are O^e* square and O^O" high. The last number {e,g,( -J, -$f -10) in each type number indicates the type of material, which in turn translates to the operating frequency range (see Table 4-1), Now, see how the coil forms are used.

Table 4-1. Amidon coil form specifications

Part

Frequency

■■¡amber

range (MHz)

j4l value

Ratio

^nai

L-33-1

0,30-1.0

76

1.7:1

80

L-33-2

i.00-10

68

1.5:1

90

L-33-3

0,01-0.5

80

1.8:1

70

1^33-6

10-50

60

1.5:1

100

L-33-I0

25-100

54

1.4:1

120

L-33-17

50-200

48

1.3:1

130

L-43-1

0.30-1,00

115

l.t>:l

110

L-43-3

1.00-10

98

1.6:1

120

L-43-3

0.01-0.5

133

1-8:1

m

L-43-6

10-50

85

1.4:1

130

L-43-10

25-100

72

1.3:1

150

L-43-17

50-200

50

1.2:1

200

U57-1

0.30-1.00

175

3:1

*

L-57-2

1.00-10

125

2:1

*

L-57-3

0.01-05

204

& J

*

L-57 6

10-50

115

2:1

#

L-57-10

25-100

100

2:1

*

L-57-17

50-200

67

1.5:1

*

Determine the required inductance from Eq. (4-3). For an experiment to see how this coil system works, I decided to build a 15-MHz WWV converter tliat reduced the WWV radio station frequency to an 80- to 75-m ham-band frequency. Thus, I needed a circuit that would tune 15 MHz. It is generally a good idea to have a high capacitance-todnductance ratio in order to maintain a high Q factor I selected a 56-pF NPO capacitor for the tuned circuit because it is in the right range, and a dozen or so were in my junk box. According to Eq. (4-3), therefore, I needed a 2-^H inductor.

To calculate the number of turns (N) required to make any specifier inductance, use the following equation:

where

L = inductance m microhenrys {(jdH)

The ,41 factor is a function of the properties of the core materials and is found in Table 4-1; the units are microhenrys per 100 turns (|jiH/100 turns), in my case, I selected an L-57-6, which covers the correct frequency range and has an value of 115 jjlH/100 turns. According to Eq. (4-4), therefore* 1 need 14 turns of wire.

The coil is wound from no. 26 to no. 32 wire. Ideally, Litz wire is used, but that is both hard to find and difficult to solder. For most projects ordinary enamel-coated magnet wire will suffice. A razor knife (such as X-acto) and soldering iron tip can be used to remove the enamel from the ends of the wire. Because the forms are small, I recommend using the no. 32 size.

Winding the coil can be a bit tricky if your vision needs augmentation as much as mine. But, using tweezer, needlenose pliers, and a magnifying glass on a stand made it relatively easy. Figure 4-3 shows the method for winding a coil with a tapped winding. Anchor one end of the wire with solder oti one of the end posts and use this as the reference point. In my case, 1 wanted a 3-turn tap on the 14-turn coil, so 1 wound 3 turns then looped the wire around the center post. After this point was soldered* the rest of the coil was wound and then anchored at the remaining end post. A dab of glue or clear fingernail polish will keep the coil windings from moving.

Construction of a cuslom coil using Fjg. 4-2.

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