Version: 13. September 2004
23
[ ] C124
22nF
223
A3
[ ] Dr10
47µH
SMCC
C3/4
[ ] Dr11
47µH
SMCC
D4
The Spectrol 75H’ s are trimming potentiometers in metal casings.
[ ] P3
10K Spectrol 75H
B5
[ ] P4
10K Spectrol 75H
C5
Now we encounter two news parts: transformers on Amidon toroid ferrite
cores. As this is an error prone area, we will present a little more text on
them.
Little toroid coil school
As our American QRP friends, we often use high Q toroids from Amidon.
Basically iron carbonyl rings are for narrow band applications and ferrites
for broad band applications. On the CD you will find the Mini RK program by
Wilfried, DL5SWB. With this small, but helpful program, it is child’s play to
calculate the necessary turns for a given inductance, or the other way
around, the inductance for a known number of turns.
Winding toroids always provokes fear in beginners. Unjust! In you start
without prejudice and keep the basic rules in mind, nothing can go wrong.
Important: one wire through the ring is one turn. For practice begin by
making Tr7, as we will need this shortly.
Cut off some 25 cm of 0.3 mm lacquered copper wire (AWG 28), and take
the core in one hand. Put one end of the wire through the hole. That was
the first turn. But STOP!!!
Look at your work, and consider how you put the wire through the ring.
There are two possibilities. Either back-to-front, or front-to-back. From an
RF point of view, this doesn‘t matter. but for the later mounting, this is
very important, as it decides the placement on the pc board. You should
chose the direction that feels best for you.
If you started back-to-front, the next turn must be clockwise, to get the
right geometry for Tr7. If you start front-to-back, you must wind counter
clockwise.
This rule is only for Tr7. Developers have their oddities too. Wayne, the
constructor of the K2, always turns opposite to DK1HE. Worse: For DK1HE
the optimum layout of the pc board is so important, that he changes the
winding geometry for each coil in the Speaky. A quick test winding will
always make it possible to see, what the developer meant to be done.
Now put on the necessary number of turns distributed well over the ring. To
avoid error, count the turns on the INSIDE of the ring. The coil in the
picture has 8 turns. Well distributed means that the turns are spread over
some 270 degrees of the ring circumference. That is the optimal range for
toroid coils. When you think of this while winding, you will never need to
adjust the turns afterward, even though it is possible to some degree. Turns
should never cross, but should be single layer for each coil (primary or
secondary for transformers).
Take care on winding, to pull the wire snugly on the core. This is no pro-
blem on iron carbonyl rings as they are round and smooth. But on ferrites,
the sharp edges might damage the isolation on the wire.
Cut the free ends, but not too short, and solder the ends. How this is done
is a little under discussion.
The lacquer on the wire used
is solderable, this means
that it melts at soldering
temperature. Up to about
0.8 mm wire the heat capa-
city of a standard soldering
iron suffices for this, even
for burning off the lacquer.
To do this, touch the wire as
close to the core as possible
with the soldering iron, and
put on liberal amounts of solder. You should get a real drop. After a short
while the lacquer should melt, and smoke should rise. Keep your nose away,