Download LED Biasing Fundamentals Resistor Bias

Running Head: Resistor Bias of LEDs
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LED Biasing Fundamentals
Resistor Bias of LEDs
In this lab you will create a constant current source using a voltage divider biased bipolar
transistor and compare to a resistor biased circuit. For each circuit three different loads
consisting of a red, green and blue LED will be used.
In this section we will look at a simple resistor bias and the effect on current in a circuit when an
LED is replaced with another LED of different colour.
Niagara College Photonics Engineering Technology
No sector of business or industry remains untouched by photonics, the science of generating and
harnessing light. The Photonics Engineering Technology program at Niagara College
encompasses optics, lasers, electro-optics, spectroscopy and electronics technology.
Resistor Bias of LEDs
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Document content developed by:
Alexander McGlashan
Niagara College of Applied Arts and Technology
School of Technology, Department of Photonics
Resistor Bias of LEDs
Purpose
In this lab you will create a constant current source using a voltage divider biased bipolar
transistor and compare to a resistor biased circuit. For each circuit three different loads
consisting of a red, green and blue LED will be used. In this section we will look at a simple
resistor bias and the effect on current in a circuit when an LED is replaced with another LED of
different colour.
Multisim Simulation - Resistor Biased LED
1. Open the Multisim file: Resistor Biased LED
LED1 is a red LED
LED2 is a green LED
LED3 is a blue LED
2. If a red x appears next to the myDAQ DMM instrument as shown, double click the x to
activate the instrument.
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Resistor Bias of LEDs
The DMM should now appear as shown:
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Resistor Bias of LEDs
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3. Create a table in your log book and Excel as shown:
LED Forward
Voltage Drop
LED Current
LED 1
LED 2
LED 3
4. Connect LED1 in series with R1 and the 15V supply on the myDAQ as shown:
5. Measure the voltage across and the current through the LED and record the
measurements in your tables.
Resistor Bias of LEDs
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Resistor Bias of LEDs
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6. Repeat the measurements for LED2 and LED3. Your table should appear similar to:
LED Forward
Voltage Drop (V)
LED Current
(mA)
LED 1
1.80
13.20
LED 2
2.10
12.91
LED 3
3.41
11.60
7. Graph your results. Note that as you changed the LED (each with a different forward
voltage drop) the current through the circuit changed.
Resistor Bias of LEDs
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16.00
14.00
12.00
10.00
LED
Current
8.00
(mA)
Series1
6.00
4.00
2.00
0.00
0.00
1.00
2.00
3.00
4.00
LED Voltage (V)
8. Create a second red LED and connect it in series with LED1 and the resistor as shown.
Measure and record the current.
Resistor Bias of LEDs
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9. Try adding a third red LED and then a fourth. Record the current in each case and
generate a table of data and create a line graph showing the results.
LED Current
(mA)
One LED
13.20
Two LEDs
Three
LEDs
11.41
Four LEDs
7.89
9.63
14.00
12.00
10.00
8.00
LED
Current
(mA)
6.00
Series1
4.00
2.00
0.00
One LED
Two LEDs Three LEDs Four LEDs
10. If these were real LEDs, what would happen to the brightness of the LEDs as you
continue to add more LEDs in series?
11. Comment on the stability of current in regards to changing load (the LEDs) conditions.
(Is the current stable with a change in load?)
Resistor Bias of LEDs
myDAQ Resistor Biased LED
Parts Required:
1 – 1kΩ resistor
4 – Red LEDs
1 – Green LED
1 – Blue LED
1. Repeat the experiment as described in the simulation using the myDAQ. Compare the
measured results to your simulated results. Explain any discrepancies.
2. Is the current stable as you change the colour of the LED? Is the current stable as you
add additional LEDs to the circuit? Explain your answer.
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