[DMCForum] Re: Ohm's Law

["checksix3" <jetjock11@xxxxxxxx>]

>But I bet Gary could set us straight.<

Are being a dumbass *and* a wiseass Walter? ;)

For those who know this stuff, my apologies for reducing it to a 
tedious level.

Rich, you're not a dumbass for asking electrical questions...the 
cemetaries are full of people who didn't understand electricity.

Martin is right, in this case stop thinking of the car itself as 
being at fault for blowing fuses. Fuses blow because the load 
downstream of them exceeds the fuse's rating. This load can be normal 
or it can be a short, if either one causes current flow greater than 
the fuse's rating the fuse will open to protect the load and wiring. 
(A short for sure will cause excessive current to flow).

Btw, you can tell what kind of fault opened a typical AG fuse by 
looking at it's element. If it's blasted apart it was a short, if the 
element is intact but open it was mild overcurrent.

Voltage is pressure, current is flow. Think of PSI and GPM in a 
liquid system. If you flow too many amps thru a conductor (or any 
part of a circuit, including fuses) that is too small the electrons 
start bumping into each other because the "pipes" they're flowing 
thru aren't big enough. Rubbing things together causes heat and 
electrons are no different. It's this hysteresis energy that causes 
electrical fires and fuses take advantage of it by using low temp 
melting alloys (such as bismith) in their elements.

Your cigarette lighter is a resistive device, it makes heat by 
deliberately running current through a resistance low enough to make 
it hot. (Think of it as a "partial"  short circuit). It works exactly 
like your toaster at home. The lighter circuit's fuse (and wiring) 
need to be rated high enough to handle this current without blowing. 
As such, it's plenty high enough to handle the other loads you 
described so it's not the car's "fault" per say.

If the fuses in your adaptors are blowing the problem will still be 
downstream of those fuses, ie; the devices the adaptor is powering. 
Maybe not far downstream but downstream nevertheless. Now if you're 
saying the adaptors are blowing the lighter socket's fuse then you 
have a problem with the adaptor or it's shorting the car's lighter 
socket. At any rate, when multiple fuses are in a circuit the smaller 
rated fuse will always open first. 

Installing a resistor will only serve to limit the current flow 
because you're increasing total circuit resistance. Theoretically 
this could stop the fuse from blowing but it will also mask the real 
problem and your devices may not operate because they will be unable 
to draw the current they require. (You would also have to have 
different resistors for different device loads, not an elegant 
solution).

In short (pun intended), as I understand your problem the car is not 
at fault. As Walt said, it could be at fault only if the voltage was 
screwy. The car's lighter circuit doesn't "put out" any current per 
say, it's set up to have a certain capacity (called ampacity to be 
technically correct). A fuse is there to protect the circuit, it's 
rating needs to be slightly lower than the circuit's ability to 
handle current based on the the physical size and resistance of it's 
elements (wiring, connectors, etc). Exceed that ampacity and you blow 
the fuse, thus protecting against hysteresis and the resulting 
meltdown. Fuses must be rated for circuit protection while the 
circuit must be rated for the applied load. This is something many 
people don't understand when they choose a fuse based only on the 
load. The result can be wiring that burns up but saves the fuse.

Since the power source (in this case the battery and charging 
circuit) is capable of providing massive amounts of current to run 
things like headlights, fans, stater, ect, fuses are needed to 
prevent any circuit from drawing "all it wants" if something shorts
or the load is too big for the wiring. Unfortunately (due to the D's 
small wiring and poor connections) the car is basically a giant 
toaster. 

>From all this you can see that fuses should always be connected as 
close to the power source as possible, something very poorly executed 
in the D. I have to disagree the engineering was done right for it's 
time, the fact is the car's electrical system is very poorly 
engineered and whomever did it should be slapped and have to turn in 
their credentials. For example, the use of multiple conductors in 
parallel for power wiring is an astonishing screwup that in essence 
defeats any overcurrent protection located upstream of it. (Then 
again, the car as it comes has none which is why you should replace 
that wiring from the alternator to the relay compartment and install 
a fusible link).

There are several fuses that should be added to the car as well as 
other modifications to the electrical system but that's a topic for 
another time. Lets just say that until you do them you'd better carry 
a fire bottle at all times, preferably Halon 1301.

Anyway, I'm over simplifiying this a bit but you get the idea. The 
lighter circuit has a limit as to how much it can provide, that's not 
quite the same as thinking about how much it can "put out". It 
doesn't put out anything until you connect a load. (Although I can 
see how you might think of it this way). The voltage is waiting there 
for you to connect something, it has the "potential" to make current 
(and lots of it) only when the circuit is complete, be it a dead 
short or some sort of load. The overcurrent protection (in this case 
fusing) simply limits how much current will flow.

In the end it all comes down to resistance. In a DC circuit (assuming 
a constant voltage) the current flow will be dependent on the 
resistance of the load (for simplicity I'm ignoring inductive loads 
like motors, relays, etc). You have no control over the voltage and 
you have no control over the design ampacity of a particular circuit, 
all you have control over is the resistance, ie; the load (what you 
connect to the circuit).

Sorry for the length...are we bored (or confused) yet?

Btw Walt, attenuation is...well, look it up. It applies mainly to RF 
circuits. Impedence refers to AC circuits, of which RF is one type.

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