On Fri, 08 Jan 2010 07:28:05 -0700, Dave wrote:

On 1/7/2010 7:45 PM wrote:
For the top puzzle, try solving the circuit with the numbers being
ohms resistance, and then possibly solve for power?

The total equivalent resistance of the circuit is 4.1433 ohms, = 2.89
amps X 12 volts = 34.7 watts.

I think that solves the top one.


The bottom one is a bit more challenging -
It likely has something to do with the different forward voltages
between red and yellow LEDs., and the way they are connected - but the
units used remain the first part of the puzzle. The top puzzle is
obviously ohms, and solved for watts.

Ok, I think you're on to something. 4.1433 ohms @ 12V is 34.75491 watts
which looks like it could be what I need but I don't understand the math
for how you came up with 4.1433 ohms.

The formula I found for total ohms in a series circuit is to add up all
ohms in the circuit. For parallel circuits its 1/RT = 1/R1 + 1/R2 so
1/2.8398 + 1/5.6534 = 8.5472/16.35980892 which is something like 5.2245
ohms. The total for a combo circuit is to add the parallel value to the
series values and I come up with a total of 19.9873. Nothing even close
to your 4.1433. So I'm clearly not understanding how resistance math is
supposed to work.

By the way, I believe I was wrong on the LED puzzle. I think the result
should be something around 4 instead of -112.


For parallel circuits 1/r1 + 1/r2 +1/r3 = 1/RT
Solve the parallel portions of series strings first. Then add the
series strings, then solve the remaining/resulting parallel string.

There are also online parallel resistance calculators that make the
job dead simple - if you remember the rules.