Cambridge Thermionic type LS- 3.
L2: 45 turns #30 wire on
3/8” form with slug (Cambridge
Thermionic type LS- 3); 100 pF in
L3: 36 inches of #30 wire tapped
12 inches from crystal end, wound
on 3/8” form with slug (Cambridge
Thermionic type LS- 3).
AThe Cambridge Thermionic
coil forms are obsolete and
I could not find data, but I
suspect that the number of
turns would be more than will fit on
the form. I found CWS Bytemark, a
company that supplies magnetic
components to the amateur and
experimenter market ( www.byte
mark.com). Their minimum order is
$25 but that is better than the $100
minimum order of most suppliers.
Bytemark has a ferrite pot core that is
good in this frequency range but it is
not tunable. I think the L- 57 shielded
coil form with tuning slug is better
suited and it has a bobbin that will
make winding easier. The coil form
has six terminals to allow a center-tapped primary and secondary. The
L57-3 is rated for 10 kHz to 500 kHz
and will produce 204 microhenries
(µH) with 100 turns.
With a tuning capacitance of
1,000 pF, the required inductance is
633 microhenries (0.633 mH). Using
the relation: Turns = √((desired
L)/(90%*µH/100T)), I come up with
the number of turns = 167. The 90%
factor is just to provide the tuning
range. The slug will tune the coil 2:1
so there should be no problem
Re: 60 volt, 13 amp supply,
December 2008, page 24. I absolutely
love your response on this switching
supply. You made it so simple that I
could understand nearly all of it and
I looked up the rest! I would like
you to be more specific about the
Magnetics, Inc., manual for ferrite
cores. I’ve been cruising around
their website and I’m not sure
which one to download ... or is it
something I have to purchase?
— Chuck Larson
reaching 200 kHz.
For the coil L1, you will want a
secondary of 20*167/25 = 134 turns
center-tapped. That is 301 turns total;
let us see if #30 wire will fit: I
calculate the winding area of the
bobbin to be 0.03 sq in and from
tables find that #30 wire uses 7,000
turns per sq in; 0.03*7000 = 210
turns max, so it won’t fit. Try #34
wire which is 16,000 turns/sq in *
0.03 = 480 turns max. That will fit.
Keep in mind that the tables are for
machine wound coils so hand-winding will result in less turns fitting
in the max dimension. Coil L2 has no
secondary so 167 turns #30 will fit
okay or you can use 167 turns #34
wire. Coil L3 wants a tap 1/3 up the
winding so it will be 167 turns with a
tap at 56 turns.
For 175 kHz, you could increase
the capacitance to 1,304 pF and use
the same coils or increase the
inductance to 827 µH to use 1,000
pF. There should be enough range in
the slug tuning to reach both frequencies with the same coil, but I
don’t know that 167 turns will do it.
The number of turns for 827 µH
(reduced 10% for tuning) is 212
turns. L1 will have a secondary of
170 turns center-tapped; that will be
tight using #34 wire but could fit.
Coil L2 has 212 turns; not a problem,
and L3 wants a tap at 71 turns.
You may be interested in my
setup for winding coils. I am using a
portable drill that had a dead battery
so I drive it with a power supply (it
needs to supply at least one amp). A
dowel taped to the chuck operates a
Response: Go to this site:
and choose either the uncut or
sectioned versions. There is no cost.
Re: Blinking Lamp Schematic,
December 2008, page 26. Hi – I am
almost a complete novice. I enjoy
reading your magazine and trying to
follow along. I was reading the
“Blinking Lamp schematic”
discussion and have a follow-up
question. The schematic shows C1
in parallel with R1 and R2. Is that
correct? Shouldn’t C1 be in series
QUESTIONS & ANSWERS
microswitch and a three digit counter.
Figure 5 is a photo and Figure 6 is
the schematic of the counter.
SOFT START AC
QI would like to build a soft
start circuit for a standard
light bulb (100W).
What circuit options do I have?
— Dean Larsen
AIncandescent light bulbs
are passé; I would rather
answer a question about
fluorescent or LED lights!
However, a soft start will extend the
life of a light bulb as will lower
voltage. I don’t recommend a PWM
solution because that will waste 20 or
25 watts and the light bulb is already
wasting 99.9 watts. A thermistor in
series with the light bulb will give a
soft start and also reduce the voltage
slightly, and extend the life
tremendously. The 100 watt bulb is
about 10 ohms at room temperature,
so the inrush current is 12 amps peak
with no thermistor. Mouser part
number 871-B57153S330M is 33
ohms at room temperature and about
one ohm with one amp of current
flowing through it. The inrush current
is reduced to 120/43 = 2.8 amps
peak and the operating voltage is
reduced about one volt. That is not
enough that you will notice a
difference in the light output but the
life will be extended. NV
with R1 and R2? And shouldn’t C1
be connected to ground?
— Russ Hintze
Response: My connection of C1
is a little bit unorthodox but the + 12
volt supply is virtual ground, so it
works the same as if C1 were
grounded. However, noise on the
power supply will cause timing
problems, so this only works if the
power supply is clean.
Re: How to Measure Impedance,
December 2008, page 25. I believe
May 2009 35