Download LED Biasing Fundamentals Transistor Bias

Running Head: Transistor Bias of LEDs
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LED Biasing Fundamentals
Transistor 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 current stability through an LED using a voltage divider biased
transistor circuit as a current source.
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Transistor 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
Transistor 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 current stability through an LED using a voltage divider biased
transistor circuit as a current source.
Multisim Simulation – Transistor Constant Current Source
1. Open the Multisim file: 06 Transistor 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.
The DMM should now appear as shown:
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Transistor Bias of LEDs
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3. Create the following table in your log book and Excel:
LED Forward
Voltage Drop (V)
LED Current
(mA)
LED 1
LED 2
LED 3
4. Measure the voltage drop across and the current through LED1. Repeat the measurements
replacing LED1 with LED2, and again using LED3. Record and plot your results.
Transistor Bias of LEDs
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Transistor Bias of LEDs
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5. Your results should be similar to:
LED 1
LED 2
LED 3
LED Forward
LED Current
Voltage Drop (V) (mA)
1.81
14.02
2.10
14.02
3.41
14.00
16.00
14.00
12.00
10.00
LED
Current 8.00
(mA)
6.00
LED Current (mA)
4.00
2.00
0.00
0.00
1.00
2.00
3.00
4.00
LED Voltage (V)
6. Create a copy of LED1 and connect the new LED in series with LED1 as shown.
Measure and record the current. Keep adding red LEDs in series as shown until the
current begins to drop significantly. Create a table and plot your results as shown.
Transistor Bias of LEDs
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Transistor Bias of LEDs
One LED
Two LEDs
Three LEDs
Four LEDs
Five LEDs
Six LEDs
Seven LEDs
LED Current
(mA)
14.00
13.97
13.97
13.95
13.92
13.89
8.50
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16.00
14.00
12.00
10.00
LED
Current 8.00
(mA)
6.00
Series1
4.00
2.00
0.00
One
LED
Two Three Four Five
Six Seven
LEDs LEDs LEDs LEDs LEDs LEDs
7. As more LEDs are added the total voltage drop across the LEDs continues to increase
(each time you add an LED the saturation current of the circuit decreases). Eventually
the designed LED current of 14mA is greater than the saturation current of the circuit
resulting in the LED current to drop.
𝐼𝐶(𝑆𝐴𝑇) = (𝑉𝐶𝐶 − 𝑉𝐿𝐸𝐷(𝑇𝑜𝑡𝑎𝑙) )/𝑅4
Add a column to your last table. Assuming a voltage drop of 1.80V for each red LED
calculate the saturation current for each case (as you add LEDs). Compare that to the
LED current.
8. Compare the current stability of the transistor circuit to the resistor biased circuit that you
tested in the previous section.
Transistor Bias of LEDs
myDAQ Transistor Biased LED
Parts Required:
1 – 1kΩ resistor
Lots… – 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. As you added LEDs in series did the brightness of the LEDs change (until you exceeded
the saturation current)?
3. Assuming a blue LED has a voltage drop of 3.40V, calculate how many blue LEDs you
could put in series and still maintain a constant current of approximately 14mA. Create a
simulation to confirm your result.
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