-18-
Use the preceding circuit but replace the lamp (L4) with the white LED
(D6, “+” on top). Turn on the slide switch (S1); the LED lights.
Project 6
Light Bulbs of the Future
Compare the LED current (measured on the meter) to the current with the
lamp (you can also try it with the meter on the 1mA setting instead of the 1A
setting). How do they compare?
Would you rather use incandescent light bulbs or LEDs to light your home?
Notice that white LED has a “+” polarity marking, but the lamp does not.
What do you think would happen if you flipped the LED or lamp around in
this circuit? (Try it.)
LEDs are much more efficient than incandescent light bulbs and last
longer. LEDs are also more expensive, but their cost has been declining,
so LEDs are increasingly being used for home lighting.
Project 7
Ohm’s Law
Build the circuit, set the meter (M5) to the 5V setting, and turn on the slide
switch (S1). The lamp (L4) lights and the meter measures the voltage.
You can swap the location of the lamp with the 3-snap wire or slide switch in
this circuit, then measure the voltage across each of those parts and calculate
their resistance using Ohm’s law. What do you think their resistance will be?
5V
Measurements from this circuit and the project 5 circuit can be used
to measure the lamp resistance using Ohm’s Law.
1. Measure the voltage using this circuit.
2. Measure the current using the project 5 circuit (remove the 3-snap
wire, connect the meter where the 3-snap was, and set the meter to
the 1A setting).
3. Calculate the lamp resistance using Ohm’s Law:
The lamp resistance is usually 15-30 ohms, when used at 4.5V. The
other parts in the circuit (switch, meter on 1A scale, blue snap wires,
and batteries) also have resistance but these are much smaller.
Note:
Your actual results may vary. Your M5 meter is a simple
meter; don’t expect it to be as accurate as normal electronic test in-
struments.
You can also calculate the power of the lamp: using: Power = Voltage
x Current.
It should be about 1 watt. Compare this to incandescent light bulbs
in your home, which are usually about 40-100 watts.
Voltage
Current
Resistance =
LEDs are like one-way, low-current meters. LEDs have a “turn-on” voltage
threshold (about 3V for your white LED) that must be exceeded to turn them
on, then quickly get bright. LEDs can be made to product light in different
colors.
Answers are at
www.snapcircuits.net/scstem1
.
Your
calculation:
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