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BTW, I forgot to mention something.... if you are electrically challenged (or even if you aren't) there's a very cool piece of shareware out there called Edison 4 (google it for their website) that you may find useful. While on the surface it appears to be a kid's program, it is actually quite powerful, allows you to build circuits on a virtual workbench as standalone components with virtual wires from terminal to terminal, or use a breadboard. If you'd rather simply deal with schematic illustrations you can do that as well with their schematic editor, which includes tons of common components in their pre-built toolbars which are expandable in the paid version. Once you've built a circuit you can install meters to test voltage drops and currnt flows, so when you power up your virtual circuit you can see how everything works. Component properties can be edited to match real-life parts you may be dealing with in your shop. I know many of you have sophisticated pieces of software to do this kind of work, but for those of us who only need something like this once in awhile, this is a pretty cool solution. Run it as shareware and you are restricted from saving your circuits... pay the $39 they ask for it and it becomes fully featured. It's especially useful for anyone who can't keep forward and reverse bias straight... install your virtual LED backwards and it won't light up <g>. FWIW.
<marv>
PS... they also produce software that integrates with this that allows you to produce circuit board patterns directly from your schematics. I don't know if they do SM components, although given that technology's increasing popularity, even for the hobbyist, it's entirely likely that they do.
"""
The typical connection to a fuse or circuit breaker (you can use this circuit
with them as well, if you have a hard time seeing which breaker has tripped)
is from the positive side of the power bus to one end of the fuse (or cb). The
other end of the fuse, of course, goes on to the load. The LED and
resistor connect directly to the 2 fuse terminals. The anode side of the LED
(typically the long lead) attaches to the bus side of the fuse. The LED
cathode (short lead) connects to one end of a 1/4 watt 1K ohm resistor, the
other end of the resistor connects to the load side of the fuse.
Here's how it works... let's assume this circuit is for a single 100 watt
landing light and the circuit is protected by a 10 amp fuse. When you turn on
the light virtually all of the voltage is dropped across the light bulb
because of its very low resistance (maybe 1.5 ohms... 12v / 1.5 ohms = 8
amps). At this point there's just about a direct short across the
LED/resistor part of the circuit in the form of the fuse (something like .1
ohm) so the current takes the path of least resistance... just about all of
it flows through the fuse, and next to none through the LED and resistor. So
the landing light is dropping 11.99 volts, and there's maybe a 8-9 millivolt
drop across the LED/resistor, so there's nowhere near enough current there to
light the LED. Somewhere along the line we were working on our fuse panel and
accidentally put a 5 amp fuse in the landing light circuit. We go to turn on
the landing light and bango!, we blow the fuse. (100 watts / 12 volts = 8
amps... the fuse gives up the ghost at 5, maybe a little more.) The current
path is now through the LED and resistor instead of the fuse, so virtually all
of the voltage is dropped by them and there's maybe 15 millivolts left over
for the landing light, which is nowhere near enough for it to light up. The
LED, however, is merrily glowing away to let you know that the fuse has
failed. Pretty cool, eh? There is a catch. If the fuse is blown or removed
_and_ the circuit is powered down, (ie, the switch to the landing light is
off) the LED will not glow. The path through the load must be complete for
current to flow through the LED and resistor. As soon as you flip that
switch, though, the LED will light up telling you to chech that fuse and life
is good once again.
"""
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