Data Sheet

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90. Measuring Current
Build the circuit shown, turn on the switch (62) and you will see the
heart LED (69) is on. If you want to turn on the star LED (70), you just
need to press the press switch (61) or hold the magnet (7) near the
reed switch (83).
If you wanted to know how much current is owing in different parts of
this circuit, you could use an Ammeter. An Ammeter is an instrument
used to measure the Amps of current owing in a circuit. If you hooked
up an ammeter to the current path of the heart LED (69), you would
see about 27mA (pronounced 27 milli-Amps, which represents 27
thousandths of an Amp) of current owing when the switch (62) is ON.
If you hooked up an ammeter to the current path of the star LED (70),
you would see about 57mA of current owing when you press the press
switch (61). Can you gure out why more current ows through the
star LED (70)?
91. Calculating Internal Resistance of the LEDs
Build the circuit shown on the left, turn on the switch (62) and the reed
switch (83) and you will see the heart LED (69) is on. Now press the press
switch (61) and the star LED (70) will be turned on too. The LEDs in this
project are in parallel just like they were in project #90 so we would get
the same ammeter measurements as discussed in project #90.
You may have gured out by now that the reason more current
ows through the star LED (70) is because it has a lower internal
resistance. Recall from project #59 that the actual LEDs inside the
heart LED (69) and star LED (70) require a “turn on” voltage to light.
This is approximately 1.8V for red light, but white light requires about
2.5V. So the internal resistor in the heart LED (69) will see about 2.7V
(since the batteries provide about 4.5V, minus the 1.8V drop across
the red LED), and from the previous project the heart LED (69) sees
27mA of current, and thus based on Ohm’s Law (R=V/I) the internal
resistance of the heart LED (69) is about R=2.7/0.027 = 100W. The
internal resistor in the star LED (70) will see about 2V, and from the
previous project the star LED (70) sees 57mA of current, and thus
based on Ohm’s Law the internal resistance of the star LED (70) is
about 2/0.057 = 35W.
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