T. Ment wrote:
On Wed, 26 Jun 2019 19:39:53 -0400, Paul wrote:

If you want exact solutions for the circuit behavior,
you could try some sort of Newton-Raphson iterative
method.

I just want to learn enough so I don't burn the house down. I googled
and found questions similar to mine:

https://electronics.stackexchange.co...-the-same-time



First, let me assure you, that if where those two LED
legs plug in, if you short those two together, the
180 ohm resistor limits the current. By shorting
the legs, the 180 ohm resistor sees a power of
V^2/R or 5*5/180 = 0.138W or less than a quarter
watt (0.250W). Usually, an engineer selects a resistor
suited to being "shorted out by a customer".

There are a few things on a computer which are
actually dangerous. The SPKR 1x4, where that plugs
in, one of the wires is tied to +5V, and if you
short the red wire from SPKR to chassis, the
wire smokes. This usually happens when someone
closes the side on the computer, the red or orange
"SPKR" wire happens to get pinched, and the sharp
metal casing slices the insulation so that the wire
touches ground.

That's about the largest exposure I know of. The
USB and Firewire ports probably use Polyfuses.

Also, the cooling fan +12V bus, is not fuse protected.
If you have an issue with a fan, the copper trace in
the motherboard will burn out (without a fire issue),
and you lose all cooling fans as a result. I suspect
some mental genius does this on purpose, and uses
an undersized copper track as a zero dollar cost
safety precaution.

You can, of course, use wire adapters and run a lead from
a Molex to the fans, and get power for them that way.
It doesn't mean the computer is "finished" - it just
means you will spend hours poring over catalogs looking
for the right wire set(s) to bring the fan power back :-)
This is some kind of punishment, I would guess.

I have a few bags of connectors here, so that if
this happened to me, I have all the parts I need
to fix it :-) That doesn't say I would appreciate
having to do that though. I've never lost a fan track,
and I've taken my share of chances with goofy
wiring jobs too. It's not like I'm a boy scout on
the wiring :-)

*******

So no, for a fan job, the odds are good there isn't
a fire risk. The mobo resistor is on the +5V side,
and ground is ground. By adding your additional
resistor, the LED is going to get dimmer. Even if
you screw up and short the pins on the LED, that
should not harm it.

Paul