Phys 334 Lab 3 Diodes and Rectification July 9, 2012 1 Rectified differentiator In this section we will be working with the rectified differentiator circuit shown in Figure 1. The circuit should be driven with a square wave at around 10 khz. Note that you should measure the output of this and later circuits with a 10X scope probe. These probes increase the oscilloscope input impedance by a factor of 10. This helps to reduce the load on the circuit under test which may otherwise change the operation of the circuit. When using 10X probes it is easiest to set the Probe softkey (available under the CH1 or CH2 menu) to X10 otherwise your voltages will appear 10 times larger than expected. In 560 pf 1N914 1 k 2.2 k Figure 1: Rectified differentiator. 1
Task 1 : 1. Draw the input and output waveforms and explain what is happening. 2. Remove the 2.2 kω load resistor. Explain the output, taking into account the equivalent circuit including the oscilloscope input impedance. 3. Your oscilloscope trigger settings include a switch to set the triggering to either rising or falling edge. Can you imagine how this might work? /1 2 Diode clamp In this section we will be working with a diode clamp circuit of the basic design shown in Figure 2. Variations on this type of circuit are often used to protect the inputs to sensitive circuits from high voltages. As real diodes do not present exactly zero impedance in the on condition, you may observe the output voltage during clamping to not be perfectly flat. The dynamic resistance of the diode can be estimated by driving the clamp circuit with a triangle wave. The output should show a flattened triangle. The height of the upper piece from the point where it begins to turn downward to the tip gives V across the diode. The difference between the tip of the output wave and the tip of the input wave gives V across the 1 kω resistor, and hence, the current in the diode. In V clamp 1 k 1N914 Figure 2: Diode clamp circuit. 2
Task 2 : 1. Construct a diode clamp circuit using a +5 V clamp voltage from your power supply and a sine wave input. Sketch and explain the output you observe. 2. Estimate the dynamic resistance of the diode in your circuit. 3. Replace the direct +5 V source with a similar source derived from a 15 V supply using a potential divider with a 1 kω and 2 kω resistor. Explain any difference in the output. 4. Add a 15 µf or similar capacitor in parallel with your clamp supply (i.e. between V clamp and ground, ensure you put it the correct way around). Explain any improvement in the output. /3 3 Half-wave rectifier In this section we will be looking at the half wave rectifier circuit shown in Figure 3. We will be driving the circuit with our 6.3 V transformer. Use any two taps on the transformer. Note that transformers are usually quite conservatively rated so don t be surprised if you peak output voltage is greater than 6.3 2 V. Be sure not to ground the center tap of the transformer, that would short out one of the coils and could damage it. Note that if you short out the transformer some way you will blow the fuse and the light will go out. It is advisable to check your diodes before using them. This can be done with the diode test feature of your DMM. A working diode should read about 0.6 V when connected around the right way. Note that the line on the outside of the diode represents the line or cathode end of the diode in the circuit symbol. Our oscilloscopes have the ability to be triggered based on the AC line voltage. You may find this the more convenient means of operation in this exercise. Task 3 : Construct the circuit shown in Figure 3. Sketch and explain the circuit output, noting the voltage drop across the diode. /1 3
1N914 110 V 2.2 k Figure 3: Half-wave rectifier circuit. 4 Full-wave bridge rectifier We will now be extending our half-wave rectifier to make use of both halves of the AC cycle. This is done with the full-wave bridge rectifier circuit shown in Figure 4 which we have discussed in lectures. Important note: the ground terminals for both channels of the oscilloscope are connected together internally. This means you can t simultaneously connect the oscilloscope ground to one side of the transformer secondary and the bridge rectifier ground. 110 V + put 4X 1N4004 Figure 4: Full-wave bridge rectifier circuit. 4
Task 4 : 1. Explain what would happen if you ignored the important note above. 2. Assemble the circuit shown in Figure 4, at first without the filter capacitor and with a load resistor of 2.2 kω. Sketch and explain the output, in particular note the maximum voltage and the flat regions near zero volts. 3. Connect a 15 µf or similar capacitor across the output, observing correct polarity. Sketch the output and note the ripple voltage. 4. Try a few different load resistors and note the corresponding ripple voltage. Plot this and compare with theoretical approximation, V ripple = V max /2fR L C. Do not use a load resistor smaller than about 500 Ω. /3 5 Regulated power supply In this exercise we will be testing the adjustable voltage regulator. This device is a 3-terminal integrated circuit which uses feedback to try and generate a constant DC output voltage independent of load. We will discuss feedback later in the course, but for now we can treat the device as a black box. The typical circuit for a regulated linear supply is shown in Figure 5. As this circuit has a few more components than earlier exercises, it is advisable make a quick sketch of your breadboard layout before assembling the circuit to ensure you have enough space. You may find it easiest to organize the rectifier and regulator separately. This is useful for troubleshooting, for example you may want to test the regulator part separatey with a DC power supply. The output voltage adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the adjust terminal of the regulator. The ratio of resistances determines the output voltage. Typically a fixed value of 240 Ω is used for R1. You will need to consult the device data sheet to identify the pins on the particular device you have and to find the equation for determining the set voltage. 5
110 V 4X 1N4004 + In 470 µf LM317 Adj R1 put 1 µf R2 Figure 5: Regulated DC power supply circuit. Task 5 : 1. Construct the circuit shown in Figure 5, selecting resistors for an output of 2.5 V. 2. Verify the output and try to observe any ripple. Again, do not use a load resistor smaller than about 500 Ω. /2 6