Basic electrical tools and compenents 1/12/11

Today we were introduced to the multimeter. We first set the meter to read resistance so we could check for continuity. If their was a low resistance between the two probes we had continuity. After checking several points from our circuit we soldered yesterday we checked our power supplies which we had made yesterday as well. If there was continuity, there was something wrong and it shouldn't be plugged in. If we were reading an open circuit or high resistance we were safe to plug it in. Once we plugged in our supply we read the output voltage. About 5.1 volts was average for our 5V regulated supply. Next we created our first circuit. We connected a 467 ohm resistor and LED light in series and connected it to our supply. After we added two more circuits in parrallel with the first also one resistor and LED but with 1K ohms and 10K ohms respectively. The results shown below confirm our calculations and the lower the resistance the brighter the light.

Lowest resistance on top.

So if the lower the resistance the brighter the LED, then why not put the LED directly to the power source? Wouldn't this give us the brightest possible output?

The answer, NO. If the LED does not have any resistance to impede the current, it will burn out. From the data sheet, the voltage drop across our LED will range from about 1.85-2.5V. Our power supply is giving a constant 5.1V. This means that the voltage across our resistor will be anywhere from 2.5-3.15V. Also from the data sheet, the suggested max current for our LED is 30mA. So after rearranging Ohm's law V=IR to V/I=R. 3.15V/0.030A = 105 Ohms for our high and 2.5V/0.030A = 83 Ohms for our low end. Here is the result with an 83 Ohm resistor.

Very bright!

The voltage measured across the resistor was just about 3V. V/R=I so 3V/83Ohms = 0.036A or 36mA which is a tad bit over the data sheet. I didn't let the LED stay on for too long.

Next we were introduced to a magical resistor AKA potentiometer AKA pot. This new component can vary its resistance with the turn of the knob. I measured this pot to be a 2K Ohm pot which means its resistance can anywhere between 0 and 2000 Ohms. The picture below shows the circuit with the pot in series with a 467 Ohm resistor in series with the LED with the pot at its highest resistance. As you can see, the LED is very dim.

 Here is the same circuit, but with the pot set to its lowest resistance. Much brighter!