22uH
4A
BRIDGE
10,000uF (2)
50WQ03F + 6.3V
6.3VAC
7
2
5
+
+
+
100uF
3 6.19K
1.91K
1.5K
0.47
LM2588-
ADJ
4
+
1000uF
COM
on time, I need to compensate for
temperature variations inside the clock
case. I would like to mount a power
resistor in the case and use it as a
controlled heater element to keep the
temperature at a constant 78 degrees
Fahrenheit. Could you suggest a 12-volt
circuit that would maintain this
constant case temperature?
Edward J. Wilk
■ FIGURE 5
6. 3 Volts DC, 3A
two EL34 power tubes and utilizes a
solid-state rectifier; the output of the
rectifier is 430 volts. My AC mains can
fluctuate substantially, being 121 VAC
one day and 127 VAC the next, and I
have noticed that plate and bias voltages change 10 volts at the plates and
one volt at the grid bias — depending
on the day — which can result in
several watts output variation. Is there
a simple way to either regulate the AC
supply into a power transformer or regulate the DC output from the rectifier?
Alan Mihalko
AA Sola ferroresonant transformer can regulate the AC
input — kinda. Basically, it
provides a constant output
voltage using a saturated core transformer. Unfortunately, this arrangement
“flattens” the top of the sine wave so it
more closely resembles a square wave.
Being a tube guy myself, I find that the
temperature (voltage) on the filament
has more influence on the output
power than the plate voltage.
I would change the AC filament
voltage into DC (which reduces hum),
then regulate the DC voltage using a
■ FIGURE 6
5K
1K
0.33
1K
Thermistor
2k @ 80F
LM339 10K
1K
step-up switching regulator — like the
LM2588. The EL34 (a.k.a., 6CA7) needs
about 1.5 amp filament current at 6. 3
VAC. Toss in an ECC83 (12AX7)
preamp/driver, and the total load is a
tad over three amps. For this design, I
went to the National Semiconductor
website ( www.national.com), selected
Power under WEBENCH Design Tools,
and entered the operating parameters.
What popped up on the screen was the
circuit in Figure 5.
The design also includes a Bill of
Materials (BOM) that lists parts by manufacturer number and specs. Circuit
simulation and analysis are also part of
the design features, and an optional
custom prototype kit for some designs.
If you haven’t taken advantage of this
National Semiconductor service, there’s
no better time than the present.
ALL-PURPOSE
THERMOSTAT
QI am in the process of constructing a clock using an old
master clock movement and
pendulum. To keep the clock
+12V
78L05
OUT IN
COM
0.1 1N4002
12V
1K
LM339
Heater
2N3904
General Purpose
Thermostat
AWhat you need is something
that doesn’t disturb the air
inside the cabinet so as not
to upset the swing of the
pendulum or dry out the wood. In
other words, a gentle heat with no
circulating fan. The circuit in Figure 6
has been around forever and can easily maintain the temperature to within
three degrees Fahrenheit. The power
resistor should be mounted on an
aluminum heatsink and placed at the
bottom of the case where convection
will draw the heat up the column. You’ll
have to experiment with the placement
of the thermistor — a NTC (negative
temperature coefficient) device with a
resistance of about 2K at 80°F — for
best results.
GHOST OF A
MM5369 CLOCK
QI am looking for a MM5369
chip that uses a color-burst
crystal to generate a 50-Hz
or 60-Hz timebase. I have
been told that this chip is no longer
available, but heard that you can make
a substitute using two chips and a
crystal. Have you seen this design?
Craig Kendrick Sellen
Carbondale, PA
AELM Electronics (www.elm
electronics.com) makes an
ELM440 IC that does exactly what you ask (Figure 7);
the ELM446 is a 50-Hz version. Both
eight-pin devices are based on
the 12C5xx family of devices from
Microchip Technology, Inc., and are
available in two sizes — either the
standard DIP package, or in the
smaller SOIC surface mount version.
ELM has no minimum purchase (the
14
January 2006
