A review of how to read resistors and of what the equation and schematic looked like was discussed today.
Vs = Batter Voltage
Vf = LED Voltage
I = Current of LED
In general students have been using a 9V battery for Vs. Where it gets a little more difficult is when students needed to select their LED's.
The data sheet on the package was a little difficult to read but we eventually found the maximum forward voltage for the LED's that the students selected.
I modified the original equation a little since LED1 and LED2's voltages should be the same we can just simply multiply by 2:
$\large R_1=\frac{V_s-2V_f}{I}$
So if I have a red LED's whose maximum voltage is 2.3V I can simply set up my problem like so.
First I define my variables:
$\large R_1=\frac{V_s-2V_f}{I}$ - $V_s=9$
- $V_f=2.3$
- I=20mA or since we need amps I= .02A
$\large R_1=\frac{9-2(2.3)}{.02}$
$\large R_1=\frac{9-4.6}{.02}$
Note: We must follow the order of operations here!
$\large R_1=\frac{4.4}{.02}$
Note: This looks daunting but since the denominator is in hundredths lets multiply the denominator and numerator by 100!
$\large R_1=\frac{4.4 * 100}{.02 * 100}$
Note: Now just simply move the decimal two places to the right in both the numerator and denominator!
$\large R_1=\frac{440}{2}$
Note: Ahhh much better!
$R_1=220 \Omega$
Now we have found our resistor value! We will need to see the chart below to determine the color bands of our resistor.
Since we need a resistor that is $220 \Omega$ we know that the first number relates to the first band of our resistor which 2 = red. Next we look at the second number and this is our second band of our resistor which is 2 which is red.
Recall that the third band of our resistor is reserved for our multiplier.
22 multiplied by what will give us 220?
I hope you said 10!
If you look at your chart a multiplier of 10 gives us brown for our third color.
Therefore the resistor in this example would be: Red Red Brown!
Color | First Band | Second Band | Multiplier | Tolerance |
Black | 0 | 0 | 1 | |
Brown | 1 | 1 | 10 | |
Red | 2 | 2 | 100 | |
Orange | 3 | 3 | 1000 | |
Yellow | 4 | 4 | 10000 | |
Green | 5 | 5 | 100000 | |
Blue | 6 | 6 | 1000000 | |
Violet | 7 | 7 | 10000000 | |
Gray | 8 | 8 | 100000000 | |
White | 9 | 9 | 1000000000 | |
Gold | ±5% | |||
Silver | ±10% |