primary which will require a larger
core. That means doubling the
primary turns and now the window is
getting tight. I probably can’t double
the secondary in order to use the
two diode, full-wave rectifier circuit
and will have to use the four diode
bridge rectifier.
The multiple filter caps are
necessary to meet the ripple current
requirements of the circuit and
reduce the ESR (equivalent series
resistance) of the filter. The TL594 is
an old device but still good. It
provides push-pull output with a
dead time to allow one transistor to
turn off before the other turns on.
The oscillator runs at 200 kHz and is
divided by 2 for 100 kHz drive to the
transformer. The parts list is Figure 2.
■ FIGURE 3
ta show ya how long ago!) But for
the life of me I can’t remember the
procedure to measure the speaker Z.
Any ideas on how the home
hobbyist can do this?
■ FIGURE 1
insertion in the printed circuit board
(if you use a printed circuit board).
Put insulating tape between the
primary and secondary for safety.
Now on to the circuit. I will use
push-pull (see Figure 1) because
otherwise there will be DC in the
PWM SUPPLY PARTS LIST
HOW TO MEASURE
IMPEDANCE
QWay back when (early
‘70s), I worked for a small
electronics firm that did
government work, sub and
aircraft intercoms, and aircraft missile
power supplies. I got the project of
measuring and cataloging a pallet
load of speakers, about 50 of various
sizes and all unmarked. We had three
engineers and one senior lab tech; all
scratched their heads and walked
away mumbling. After many phone
calls, I got the answer. To measure
the Z of the speakers, I used an audio
signal generator and a VTVM. (Goes
— Bob J (long time subscriber)
AWhen a resistor is placed in
series with an inductor and
the resistor is equal to the
inductive reactance, the
voltage across the inductor is 0.707 of
the applied voltage. But the speaker
has a large resistive component or
else there is no sound. Since the
speaker is resistive, the voltage divider
in Figure 3 can be used. Rs is the
known series resistance; Rx is the
speaker impedance. The current
through the speaker is: (Vin-Vout)/Rs
and the resistance Rx = Vout/I =
(Vout Rs)/(Vin-Vout).
■ FIGURE 2
PART
R6
D2
C1, C3, C5, C6
C4
C2
C10, C11, C12, C13, C14
C15, C16, C17, C18
R1
R2, R5
R3
R4
R7, R8
D3, D4
D5, D6, D7, D8
Q1
D1
Q2, Q3
TRANSFORMER
DESCRIPTION
Inrush current limiter
Bridge, 25A, 200V
2,700 μF, 200V electrolytic
0.1 μF, 50V, 10% ceramic
1,000 pF, 50V, NPO
4,700 μF, 25V electrolytic
680 μF, 200V electrolytic
8.2K, 3W, 5%
15K, 1/4W, 1%
2.4K, 1/4W, 1%
6.2K, 1/4W, 1%
3.6K, 10W, 5%
12V zener, 1W
Dual 30A, 45V, TO-220
2N3904, general-purpose NPN
Red LED, T-1 3/4
NMOS, 500V, 40A
See text
MOUSER P/N
527-CL30
512-GBPC2502
5985-380-200V272
81-RPER71H104K2P1A03
81-RPE5C1H102J2P1A03
647-UHE1E472MHD
647-UPW2A681MHD
594-5093NW8K200J
271-15K-RC
271-2.4K-RC
271-6.2K-RC
588-20J-3K5E
78-1N4742A-TAP
747-DSSK60-0045A
610-2N3904
859-LTL-4213
844-IRFPS40N50LPBF
8. 4 VOLT REGULATOR
QI recently acquired a
Heathkit Utility
Solid-State Voltmeter,
Model IM- 17 that
requires two batteries. One is a
1.5V C cell and the other is
8.4V mercury; a little smaller
around than a C cell and larger
than an AA. I would like to
replace the 8.4V with a 9V
battery because they are cheap,
easy to find, and will fit into the
8.4V battery holder. I need an
8.4V voltage regulator circuit to
use with the 9V battery. I know
I can use the 9V battery without
a regulator but the result will be
short battery life, excessive zero
December 2008 25
