and switching the other off. IC4 is
a FET driver; it can supply a lot of
urrent for a short time, which is what
is needed to charge the input capacitance of the FET. R3 and R4 limit the
peak current that IC4 has to supply.
The FET is rated 100 volts and 22
amps so it is well within its rating, even
though the input current will exceed
20 amps under full load, because
each one is on 50% of the time.
For the transformer, I used a 12
pin bobbin that was on hand. I tried
a smaller one but could not get
enough turns on it. I tried to wind
#16 wire but that was impossible so
I used four #27 wires in parallel.
Using parallel wires is an advantage
because the current runs on the
surface and multiple wires provide
greater surface area. The transformer
layout is Figure 4. By starting on pin 1
and finishing on pin 11, and starting
on 2 and finishing on 12, I can run
power straight across from 2 to 11
for the center-tap, leaving 1 and 12
for the transistor drain connections.
On the secondary, winding from pin
9 to 4 then 6 to 7 allows me to
jumper 4 and 6 for the center-tap,
which simplifies the layout. The
bobbin is Magnetics #PC-B3515-L1
and the core is Magnetics
#OF43515EC.
A big problem when operating
in an automotive environment is
protecting the circuit from transients
which can reach 60 volts when starting,
or when the battery is disconnected
while running. I have not tried the
transzorb approach before but if it
works as advertised, the peak voltage
should be limited to 15 volts. The
output has multiple filter caps to handle
the ripple current and to reduce the
ripple voltage. Figure 5 is the parts list.
can increase C1 for longer time.
You can substitute another MOSFET
but it must have a logic level rating.
I did not put a diode across the relay
because it turns off slowly and the
100 volt rating of the MOSFET is
unlikely to be exceeded. The Mouser
part number for the MTP10N10EL
is 863-MTP10N10ELG and the
0.33 μF cap is 581-BQ024D0334K
( www.mouser.com).
POWER TRANSFORMERS
RELAY DRIVER
QWhat I lack now is information on the physics of
constructing my own power
transformers. Do you know
of an outlet to purchase components
to make a power transformer, i.e. the
core, bobbin, and magnet wire? Also,
what is the deal on toroidal transformers? Any information on constructing
those? Notes on what gauges of wire
produce what voltages would be
helpful also.
SS
F
S
— Tyre Daniely
10T
10T
21T
21T
QI need a circuit to key a five
volt SPDT relay using the
external speaker output from
a GMRS radio. Once keyed,
it needs to remain keyed as long as a
signal is being received. Use transistor
or op-amp circuits if possible.
— Rileys Electronics
FF
S
F
6 to 12 volt xfmr, bottom view
12 June 2008
Each winding consists of 4 #27
wires in parallel.
■ FIGURE 4
AThis circuit (Figure 6) will
work as long as the input
signal is one volt peak or
more, but don’t exceed five
volts without a reverse diode to
ground at the NPN base. The turn-off
time is about 1/2 second, but you
■ FIGURE 5
SIX TO 12 VOLT CONVERTER PARTS LIST
PART DESCRIPTION MOUSER P/N
■ IC1 555 Timer, CMOS 511-TS555IN
■ IC2 4013 Dual D Flip/Flop 595-CD4013BME4
■ IC3 4081 Quad 4 Input AND 595-CD4081BE
■ IC4 MC34152 FET Driver (SMD) 863-MC34152PG
■ Q1, Q2 N-FET, 60V, 22A, TO-220 512-FQPF30N06L
■ D1 Zener, 5V, 1/4W, 1N751 512-1N751ATR
■ D2 Schottkey, Dual, 100V, 12A (SMD) 844-12CWQ10FNPBF
■ Z1 Tranzorb, 10V, 1.5 k W 625-1.5KE10CA-E3/54
■ C1 100 pF, 50V, 5%, NPO 140-50NS-101J-RC
■ C2 .1 μF, 50V, X7R 140-CC502B104K-RC
■ C3-C8 270 μF, 16V, Electrolytic 75-94SP277X0016FBP
■ C9 10 μF, 16V, Tantalum (SMD) 80-T494B106K016
■ R1 39.2K, 1/4W, 1% 271-39.2K-RC
■ R2 14.7K, 1/4W, 1% 271-14.7K-RC
■ R3, R4 5 ohms, 1/4W, 5% 291-5.1-RC
■ R5 47 ohms, 1/4W, 5% 291-47-RC
ADesigning a transformer is
a very complex undertaking.
I don’t know how to do it
even though I have built
some transformers that worked.
I will try to provide some foundation
and you can study catalogs from
Magnetics, Inc., and Micrometals, Inc.
( www.mag-inc.com and www.micro
metals.com). That being said, it is
more cost-effective to buy a power
transformer than to build one; mainly
because there is no source for small
quantities of the core or bobbin.
Most manufacturers will give you a
sample if you want to build one
anyway. I have taken a laminated core
transformer apart to rewind it, but
that is a pain that I prefer not to
repeat. Enameled magnet wire is
available from any electronics supplier
(Mouser and Allied, for example).
Whenever a current flows, a
magnetic field is produced around
the conductor. The magnetic field
increases as the current increases and,
as the magnetic field increases, it
generates a voltage in the conductor
that opposes the force voltage. This
conforms to Newton’s third law: For
26
September 2008