Amazing Devices
In The Trenches
Anti Gravity Projects
All new mini 35 kv 1.5
ma adjustable output
power supply with
instructions on making
a simple craft.
GRA1K Kit ......................... $69.95
GRA10 Assembled .......... $119.95
Green Lasers Pointers
with Coliminator
10,000 feet plus - Full 5 mw. A real
beauty!!
LAPNGR5 Ready to use...$129.95
Ion Ray Guns
Potential
concept for
the ultimate weapon of the
future. Produces force fields,
induces shocks and other weird
effects.
IOGHP1 Plans .................... $10.00
IOGHP1K Kit .................... $149.95
IOGHP10 Assembled ....... $249.95
Laser Window Bounce
Receiver and
laser illuminator
modules for
building a listening device.
LWB9 Plans complete system..$20.00
Infra Red Laser Module
CWL1K Kit ...................... $199.95
C WL10 Assembled .......... $299.95
Optical Receiver with Voice Filter
LLR4K Kit ........................ $149.95
LLR40 Assembled ........... $199.95
F
o
r
E
l
e
c
t
r
o
n
i
c
s
Electrokinetic Guns
Fires an actual
projectile using a
magnetic pulse. Advanced
project must be used with
caution. Battery powered.
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
EML3 Plans ....................... $10.00
EML3K Kit ......................... $69.95
Information Unlimited
Box 716, Amherst, NH 03031 USA
Orders: 1-800-221-1705
Fax: 1-603-672-5406
Email: riannini@metro2000.net
Catalog $2.00
Circle #106 on the Reader Service Card.
86
Many inexpensive temperature
sensors have an accuracy of ±2
degrees. A reading of 100 degrees
may actually be anywhere from 98 to
102 degrees. Obviously, this is not
very good for determining if a baboon
has a fever.
Therefore, a special thermistor
will be needed. Understanding the
difference between accuracy and resolution specifications is critically
important in engineering.
Note: It is not unheard of for
an engineer to identify "impossi-ble" specifications at a design
review while having a solution
already in hand. This solution may
or may not be presented at that
meeting.
However, sometime soon, that
engineer will "realize" that such a
solution exists and provide it to the
surprise and delight of all
involved. This saves the company
time and money. It also improves
the engineer's reputation.
Theatrics and good timing are not
unethical.
Hard and Soft
Specifications
How long a battery will last in a
heart pacemaker is a hard specification. How long a battery will last in a
boombox is fairly soft. It's surprising
how often engineers fail to consider
this aspect. Not all specifications are
created equal. Making sure the multimeter measures the correct AC
voltage at 60 Hz is important
because that's the frequency that is
most commonly measured. If the
multimeter is off by 5% at 250,000
Hz, it's not as important. Most users
would rather have a meter that
measures 60 Hz accurately and
250,000 Hz with 5% error than a
meter that was off by 2.5% across
the frequency range.
Obviously, the most important —
or hard — specifications are those
pertaining to personal safety. It is
critical that these specifications
always be conservative. What is the
AC leakage current? Is the fuse
rating correct? (Do you know how to
properly choose a fuse?) The two
greatest personal hazards electrical
engineers need to consider are
electric shock and fire hazard. They
should always be considered in every
design — even in battery-operated
products.
Suppose you have to design a
simple linear power supply — just a
transformer, rectifiers, and a filter
capacitor. It’s easy, right? It should
be, but it doesn't seem to be. You
need 18 volts DC at 750 mA, so you
choose a 24 VAC transformer that
provides 2 amps, to be conservative.
The filter capacitor should be conservatively rated. Let's add 50% over the
24 volts and use a 35 volt filter
capacitor. That seems reasonable,
right?
Wrong — possibly dangerously
wrong! The transformer is rated in
RMS voltage and under full load (2
amps). Multiply the 24 volts by
1.414 to get the peak voltage. This
comes to 33. 9 volts. Add about 10%
to compensate for the loaded
voltage drop and the unloaded
peak voltage out of the transformer
is 37. 3 volts. The filter capacitor is
really a bomb waiting to explode
(and they do!). The absolute
minimum voltage for a filter
capacitor is twice the transformer
voltage. In this case, that's 50 volts.
To be conservative, 63 volts or
more is needed.
The rectifiers can be 1 amp
types, right? After all, we're only
drawing 750 mA. Again, wrong. The
transformer is rated at 2 amps and
can provide considerably more for a
short time. At power-up, the
capacitor acts like a short circuit
until it gets charged up, so the inrush
current will be over 2 amps. The
larger the capacitor, the worse this
problem is.
Admittedly, rectifier diodes are
pretty rugged and can take substantial inrush overloads, but this is still
not a conservative design choice. (An
inrush-limiting resistor can be a
simple and effective solution.)
SEPTEMBER 2004
