QUESTIONS & ANSWERS
gate has 0.9 volts hysteresis, according
to the datasheet so I computed the
R-C values using the relation: dV/dt =
I/C. In this case, the voltage on the
capacitor will ramp up and down in
0.25 seconds ( 4 Hz period).
deflection plates of a
6AR8A.
— Craig Kendrick
Sellen
dV = 0.9V
d T = 0.125 sec
Set I = 0.1 mA = 3V/R3, therefore R3
= 30K (3V is 1/2 the supply voltage).
C = Id T/dV = 0.1 mA*0.125S/0.9V =
.0139 mF; 15 µF is a standard and
close enough. I chose a tantalum cap
for low leakage and temperature
stability.
I chose the 6. 2 volt zener
because it has a sharp knee and will
hold a constant voltage over a wide
current range. The five volt zener has
high internal resistance and is not as
stable. For improved temperature
stability, you can put a silicon diode
in series with the zener. The two
temperature coefficients
approximately cancel.
■ FIGURE 5
MAGNETIC QUESTIONS
Q1. In regards to N & V March ‘ 10, page 30: What size wire do I use to wind 82. 5 or 87.2 turns on the
core of the RS 273-104? Can I round
it off to the nearest turn?
2. Using the same RS 273-104
core, I need 2 IF transformers for 455
kHz. Can I do this on this core and
how many turns will it need? IF
transformers are getting hard to find
and costly.
3. Again, using the same 273-104
core, I need a ratio detector or
discriminator transformer. The driving
tube will be 6BN11 or 6BW11. The
secondary center tap in the middle
and the two opposite ends go to the
A1. Wire size does not matter, as long as it fits in the
opening. Keeping in
mind that the inductance
varies as the square of
the number of turns, you
may want to round up or
down. Without
measuring the
inductance, it is only
approximate anyway; #16 wire or
smaller will fit.
2. I rather doubt that this core
will be good at 455 kHz and there is
no data on it. You will be better off
to salvage a transformer from an AM
radio.
3. I don’t know how the 6AR8A
discriminator works — that is up to
you — but the transformer is shown
in Figure 7. R1 simulates the 6BN11
plate resistance, and R2 and R3 load
the transformer at resonance to
widen the bandwidth. I did not
simulate the resistance of the coil; I
don’t think it will be significant. There
are three identical coils of 360 uH
each. Each one has 45 turns of $20
wire or smaller (#16 wire would take
up all the opening with no room for
insulation). I assumed that the tuning
cap would be 365 pF variable as
found in most AM radios.
searched the past several years’
worth of issues but haven’t found
it. Either I haven’t found the right
issue or it was someplace else. Any
ideas about the issue or how to
build one?
I have an old “tube type” unit
that doesn’t work — even after I
replaced the 67 1/2 volt batteries
with a bench supply. I’ve got a
nice metal box, display meter,
switches, etc., and a GM tube in a
housing. I’d like to make something
of this thing.
— Duane Haines
GEIGER COUNTER
AThe article that you remember was in March ‘05 (page 36) and May ‘05 (page 50). You can easily
search back issues on the Nuts &
Volts website ( www.nutsvolts.com)
and click on the digital issue search
( www.nutsvolts.com/search-digital.
php). If you want to repair the Geiger
counter, first check the tubes. If you
QMany months ago, I saw an article in Nuts & Volts (I think) about building a Geiger counter. I’ve
FLASHING BRAKE LIGHT PARTS LIST
PART DESCRIPTION
Q1 PNP, 100V, 25A
Q2 NPN DARLINGTON, 100V, 5A
IC1 QUAD NAND SCHMITT
D1 6.2V ZENER, 1W
C1 15 µF, 20%, 10V
R1 10 OHMS, 10W, 5%
R2, R4 3.3K, 1/4W, 5%
R3 22K, 1/4W, 5%
HEATSINK 50 DEG C/30W (1.7 DEG C/W)
■ FIGURE 6
PKG MOUSER PART #
TO-247 511-TIP36C
TO-220 512-KSD560
DIP- 14 595-CD4093BE
DO- 41 512-1N4735A
RADIAL 74-199D16V15
AXIAL 71-RH10-10
AXIAL 291-3.3K-RC
AXIAL 291-22K-RC
558-HS172
■ FIGURE 7
June 2010 23