to help tweak it.
Another circuit —
which does simulate — is
Figure 4. This circuit is
intended to produce 100V
at 1 mA output. The
regulation is not very good
and the frequency changes
with voltage, but otherwise
it is similar to Figure 3.
by J. A. Stratton by G.P. Harnwell by Michael Faraday
E. P. Dutton
QI have found an old iron magnet hat originally was part of a
crank telephone from the
1920s to 1950s. It is a
horseshoe shape; each
leg is 1/2 x 3/4 inches and
the length is five inches.
It has lost almost all
its magnetism and I want to
re-magnetize it back to full strength.
Also, I am looking for three
books on the subject of magnetism
(see Table 1). All the books were
published before 1960 so may be
hard to find, but I need them to own
■ FIGURE 4.
Io = 25 mA.
Lmin = (Vi-Vsat)*Ton/Ip = (3-
.1)*18u/.065 = 800 uH.
Choose L = 1000 uH
Rsc = Vb/Ip = .4/.065 = 6 ohms
where Vb = Q1 base to emitter
Output filter cap (Co) =
Io*Ton/Vr = .025*18u/.1 = 4. 5 µF
where Vr = ripple voltage.
night light limits the current so that a
one amp diode (like the 1N4007)
can be used. The 100 µF capacitor is
charged to 160 VDC and when the
switch is closed, the current is limited
only by the circuit resistance — about
The turns in the coil are 417, so
the ampere-turns = 41,700 which is
more than enough. The capacitor
value is not critical — the larger the
better — and has low ESR (like
The coil is made from a spool of
Belden #20 magnet wire (Mouser
part 566-8050). The spool becomes
the bobbin for the coil. The hole in
the spool is one inch in diameter
which will accept one leg of the
horseshoe magnet. You wouldn’t
have to wind it except the start end is
buried, so you have to unwind the
I recommend that you
temporarily wind the wire on another
spool (or rolled oats box) in order to
avoid a mess. Drill a 1/16 hole near
the bottom of the winding for the
start, and two above the winding to
anchor the end wire.
To use the magnetizer, put one
leg of the magnet through the hole in
the bobbin, put the keeper in place,
plug in the power, and when the light
is dark, close the switch. One
application is all it takes; multiple
applications will not increase the
magnetization. BTW, I think the
keeper should be the same cross-section as the magnet (1/2 x 3/4).
A local hardware store will have
I tried to simulate the circuit, but
after six hours I stopped it and the
simulation had only got to five volts
output. If you build it and have
problems, let me know. I may be able
■ FIGURE 5.
AAll those books are available from Amazon.com; Faraday’s book can be purchased
from the National Book Network at
(800) 462-6420; price is $175 for the
three volume set. The same book
from Amazon is more, but used is
available for less.
My solution to your magnetizer
problem is based on info that I got
from the World Wide Web that at
least 20,000 ampere-turns is needed
to do a good job. The circuit
(Figure 5) will do double that. If you
have an isolation
recommend you use it.
connection to the wall
socket will work. Just
be careful because 120
VAC can be lethal. The
QMy wife picked up a lighted sign on a stand that is fashioned after a stop light signal: three 110V AC
bulbs in a red, green, and yellow
housing. While I could design a
simple relay driven circuit to step
through the lights, I’d like something
more eloquent. Could you suggest a
circuit that does a “soft” bright/dim