Q&A
five amps ( 60 VA) running
through the same 0.1
ohms, the loss is now 0.5
volts — 11. 5 volts to the 150k
load ( 57. 5 VA). Quite an
improvement. Applying +
9V
the same analysis to 36
volts (a.k.a., 42 volts) gives
us 35. 98 volts ( 59.9 VA) to 68k
the load.
The first 42-volt systems will be hybrids: a mixture of 12 volts and 42
volts (Figure 1). The 42-
volt system is actually
based on a 36-volt battery,
but it's called 42 volts
because it takes 42 volts to charge a
36-volt battery. This is much like
today's systems, which operate at 14
volts but use a 12-volt battery. The
alternator will output 42 volts for
charging the 36-volt battery. A 14-volt
battery charger, based on DC/DC
technology, will keep the 12-volt
battery fresh.
For the moment, these controls
will be separate (42-volt headlamps
are a ways off), but eventually, they'll
merge into a single 42-volt bus. The
reason for choosing 42 volts over a
higher voltage is the 50-volt barrier,
which is considered the highest voltage that is safe to work around without special precautions.
As for the immediate advantages:
10k 1k
150k
1uF
470
R1 10k
25k
LP339 4.7M
2N3906
4.7k
10M
LM385-2.5
2N3904
microamps, which is just
slightly more than the self-discharge current of the
same battery sitting on a
shelf. To calibrate the monitor, connect it to a 7.2-volt
source (see insert) and
slowly adjust R1 until the
LED starts to blink.
Low-Battery
Indicator
Iron Keeps its
Cool
Figure 2
Adjust to 7.2 volts
+
9V
10k
LED
Calibrator
CMC Power Systems
www.cmcpower.com/html/electrici
ty/ why42volt.asp
Canadian Driver
www.canadiandriver.com/articles/jk/
at_010515.htm
SAE
www.sae.org/42volt
Q. Most of us in the shop
work with an 80 W solder iron and would like to
control its temperature in
order to prolong the tip and iron life
by preventing overheating. Can you
please provide a simple yet effective
diagram for a solder iron heat control
device with a 120 VAC socket? This
would also make a great project for
beginners in electronics!
D. Zillbermann
via Internet
Low-Battery
Indicator
•Wiring harnesses will be thinner and
lighter in weight.
•Connectors will have less loss.
•Window motors will be smaller and
more efficient.
•Electric fans will replace pulley-driven radiator fans.
Q. I have an electric guitar that
uses active pickups powered by a
nine-volt battery. I'm looking for a
simple circuit that would live inside
the guitar and tell me at a glance if
the battery is still good. I imagine an
LED that turns yellow when the voltage drops to five or six volts, otherwise the LED does not light. Do you
have any suggestions?
David Walker
via Internet
A. If you're talking about turning
down the heat when the soldering
iron is in the holder, then turning it
back up when it's removed, the circuit
in Figure 3 is what you want. This
design simply inserts a diode into the
AC line to reduce the average voltage
to the iron. The thermal switch (
available from All Electronics, 800-826-
5432, www.allelectronics.com --
catalog THSW- 70) is positioned so
that it opens when in proximity to the
soldering iron with the iron in its holder. The heat from the soldering iron
causes the switch to open and reduce
the AC Out voltage. When the iron is
For more details and features,
check out the following websites:
Assembly Magazine
www.assemblymag.com/CDA/Arti
cleInformation/coverstory/BNPCover
StoryItem/0,6490,98697, 00.html
Auto Speed
www.autospeed.co.nz/cms/A_0319/
article.html
FEBRUARY 2004
A. How about a blinking LED? The circuit
in Figure 2 monitors the
voltage of the nine-volt
battery, and flashes the
LED every two seconds
when the voltage falls
below 7.2 volts (the voltage most manufacturers
consider the end of battery life). The circuit
draws a mere 120
Figure 3
9
