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[answered] Lab 7: Optocoupler Objectives 1. To use an ohmmeter to dete

Here are the 2 labs for this week, Thank you once again!!!! eet250_u5_lab7 gots with the Lab.7 document, and lab 2 goes with the multisim file p11-51 I will need it in the form that is required, which is in a lab report. Thank you!

Lab 7: Optocoupler










4. To use an ohmmeter to determine the condition of the optoisolator.


To observe the operation of an optocoupler.


To determine the maximum frequency response of the optocoupler.


To use optocoupler as an isolated current transducer Introduction


An optoisolator is a hybrid integrated circuit that contains an LED on one side and a


photodetector on the other. The entire IC is hermetically sealed and light-tight. It is called


an optoisolator because it is used to electrically isolate a low-voltage circuit from a highvoltage circuit or one low-voltage circuit from another. A low-voltage control circuit is


used to energize the LED. Light from the LED is then optically coupled to the detector,


which is used to control a secondary circuit. For this reason, optoisolators are also called


optocouplers or optical coupling devices. Optocouplers isolate sensitive circuits, such as


computer chips, from the hazards associated with controlling high-voltage and high


power circuits. They offer the advantages of separating high-voltage circuits from low


voltage circuits, blocking power circuit voltage spikes from returning to the control


circuit, and preventing ground-loop problems in data and control systems. They are


capable of safely isolating 5V computer circuits from hundreds of volts in the output


circuit. Optoisolators are also relatively inexpensive, take up little space, and greatly


simplify control circuitry.


In this lab exercise, the operation of the optoisolator using a transistor as the detector will


be observed.




When light from the LED is off, the phototransistor will not be on, causing the resistance


between the emitter and collector to be extremely high. Figure 6-1


Step 1


Using Figure 6-2 as a reference, place an ohmmeter between the emitter and collector


with the LED off. The negative lead should be on the emitter, the positive lead on the




Rce (LED off ) = ?............


When the light from the LED is on, the phototransistor will be on and create a low


resistance between the emitter and collector.


Step 2


Use a DC voltage source and a resistor to forward bias the LED of the optocoupler as


shown in figure 6-2. Change the DC voltage source and measure the resistance of the


collector-emitter of the output transistor and fill in the following.




Rce(VDC=5V)=.............. Rce(VDC=2V)=............... Figure 6-2


Step 3


Draw the circuit in Figure 6-3 in Multisim. Set the function generator for a 5V peak


1000-Hz square wave. This will provide a 5V forward-biased signal for the LED of the


optoisolator. Step 4


Connect channel 1 of the oscilloscope to the output of the function generator, and channel


2 of the oscilloscope to the output of the optoisolator. Turn on and adjust the power


supply for 15V. Figure 6-3 Adjust the oscilloscope to display the input and output signals of the optoisolator. In the


space provided below, draw the two signals displayed on the oscilloscope. On the


drawing, indicate the on or off condition of both the LED and the phototransistor of the


optoisolator. LED




Condition Optoisolator




Output Step 5


Increase the frequency of the function generator to up to 10kHz. Compare the input


signal to the output signal. Does the phototransistor of the optoisolator react faster to the


on-to-off or off -to-on transition of the LED?


A. on-to-off


B. off -to-on Because there is a delayed reaction of the optoisolator, it has a maximum frequency


rating when a square wave signal is applied to its input. Beyond the maximum frequency,


the output signal cannot reach a good on or off level before the input signal is switched.


Assume the input signal, which is a square wave, is applied. An acceptable output signal


should reach at least 75 percent of its maximum signal.


Step 6


Increase the frequency of the square wave applied to the optoisolator until the output


becomes unacceptable. Record this value.


Maximum operation frequency: ?............... Hz The ratio of the forward current flow in the LED and the collector current of the


phototransistor is called the current transfer ratio (CTR). It is usually expressed as a


percentage and can be calculated by using the formula:


CTR= (Ic/If ) x 100


where If is the forward current through the diode and Ic is the collector current of the




Step 7


Using the circuit in Figure 6-4, turn on power and record:






Calculate the current transfer ratio for the circuit. CTR Figure 6-4 Step 8


Construct the circuit in Figure 6-4. Connect a voltmeter across the output and ground.


Turn power on to the circuit. Figure 6-4


Press and release the push button. Note that the output of the optoisolator circuit is 15V


when the push button is open and 0V when the push button is closed. (A closed push


button will turn on the LED, which in turn turns on the phototransistor. The voltage


across the phototransistor drops to 0V.)


Design Problem


How can the circuit be modified so that the output of the optoisolator circuit will be 0V


when the push button is open and 15V when the push button is closed? Use the same


components. Step 9


Using optocoupler as an isolated current transducer to measure current in high voltage


AC circuits and interface it to a low voltage electronic circuit is shown in figure 6-5. In


this circuit the high voltage AC circuit is optically isolated from the low voltage


electronic measurement circuit. Draw the circuit shown in figure 6-5 in Multisim and


simulate it. Make lamps in parallel and connect to the AC source one by one and measure


the output voltage of the optocoupler using a voltmeter. Fill out the following table: Table 6-1


AC circuit current Optotransistor output


voltage No lamp connected


One lamp connected


Two lamps connected


Three lamps connected


What is the purpose of diode D1 in the circuit? Figure 6-5 Step 10


Observe the input and output voltage of the optocoupler using oscilloscope and draw


below. Voltage across R3: Voltage across collector-emitter of optotransistor: Experiment Questions


1. T/F: An optocoupler is also referred to as an optoisolator.


2. When the LED in the optocoupler is turned on, the optodetector is turned.


A. on


B. off


3. When the optodetector is turned on, its resistance is.


A. low


B. high


4. If the current that turns on the LED of an optoisolator is 12 mA, and causes 3 mA to


flow through the photodetector, the CTR is ?........


5. An acceptable square wave at the output of an optocoupler is ?.............. of its


maximum signal.


A. 25%


B. 50%


C. 75%


D. 100%


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