Download Muddiest Points Week 5

Muddiest Points – Week 5
1. Sign / Polarity Convention: See Node Voltage templates
2. What method to use and when: See the template of methods I’ve been giving you in class….
Consider what you are given in a problem, and what you want to obtain. Use the templates to
plan how to solve the problem even before you start the math (or if you get stuck).
Basically: Your workhorse methods are –
Kirchoff (KVL/KCL) – solve for branch current
Node Voltage – solve for node voltages
Voltage & Current Dividers – Use if the geometry of the circuit permits, easiest of the methods.
Which is more efficient? Dividers are easiest/most efficient when you can use them readily
(geometry of the circuit). KVl/KCL and NodeV are pretty much equally efficient. KVL/KCL is used
as a concept to derive almost all other methods.
More efficient ways? Your multisim simulator is actually doing these method(s) to solve circuit
equations. These really are the workhorse methods for ECE.
3. Lab 2 – See solution uploaded to website
The "muddiest point" for me this week was from the lab. I understood what the voltage/current graphs
meant and I thought I understood that each LED turns on at about 20mA <<This is correct>>, which
corresponds to a different voltage for each color <<Yes!>>. When it came to checking the currents
through the LED in the pre-drawn circuits, however, I could not understand why we were using 0A as the
turn-on current <<Use 5 mA as the turn on, anything less is ‘off’>> and 20mA as the maximum burn-out
current << LED spec sheet, or the curves max out at about 20 mA>>. I also couldn't figure out why we
were considering the LED to be a voltage source that draws current yet has no resistance or why we
could ignore the other resistor in parallel with the LED when we wanted all current to go through the
LED.
<< You can read the LED resistance from the linear part of the curve (R=V/I, the slope of the line). It is a
small resistance, so we can neglect it in most circuits. And, because R is small (let’s say 0), it does not
impede the current, current can pass right on through like a short circuit. But, the linear part of the
curves, instead of starting at V=0 like a resistor does, start at about VF=1.5V. So, it acts like a small
resistor in series with a voltage source.
<<We aren’t actually neglecting the parallel resistor. It has a fixed amount of current going through it ,
because of the voltage source created by the LED. It is a question of how much total current is in the
circuit, and if it is greater than the current going through that resistor, we will get some through the LED
to turn it on.