Download Physics 427 Lab # 8

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Simple power supplies
Physics 427
Lab # 8
1) Transformer
In this lab, you will use a transformer to generate a roughly 12 Volt (rms) AC signal from
which you will obtain various approximations to a constant DC voltage. While the
transformer steps down the 110 Volt line voltage, the output can supply large currents!
Be sure to wire and check your circuit before plugging the transformer in and be sure
that all three output wires from the transformer are plugged into terminal posts on your
protoboards. The secondary side of the transformer is “center tapped”; we will use one
side and the center tap — the other wire should just be plugged into a terminal post
which is not wired to anything:
Figure 1 A transformer with center tap
One advantage of using a transformer (beyond the obvious reduction in voltage and,
thus, danger) is that while the primary voltage oscillates relative to ground potential, the
secondary can “float” to any necessary level (within the limits of insulation used inside
the transformer). This is a useful feature for the measurements you will make on the
“diode bridge” circuits used below. Note that when you measure a voltage signal using
the oscilloscope, you are grounding a point in the circuit. You need to think before
doing this: you may alter the functioning of the circuit significantly and you could also
cause large currents to flow through circuit elements thus generating a characteristic
odor and smoke!
Use CH 1 of the oscilloscope to measure the output waveform of the transformer. Note
the frequency and amplitude. Measure this same signal using the Fluke DMM (digital
multimeter). Is the reading consistent with the observed waveform? (Write this down
on separate sheets of notepaper, to turn in.)
2) Half wave rectifier
A simple series connected diode which blocks half the AC waveform leaves you with a
finite DC or average level (with huge ripple). Use the oscilloscope to measure the
output waveform of the circuit shown below. Can you compute the average, or DC,
voltage? Is what you see consistent with part (1) and with the diode curves you
measured in a previous lab? Use RL = 1k. Sketch what you observe.
Figure 2 A half-wave rectifier using a 1N4004 diode.
3) Full wave rectifier
The diode “bridge” circuit shown below directs current always in the same direction
through the load resistor. Construct the circuit (without the capacitor Cf) and observe
the waveform. What is the average voltage? Confirm your expectation by switching the
oscilloscope input to the AC-coupled setting. Draw a sketch of what you observe.
Figure 3 A full-wave rectifier using four 1N4004 diodes, and an electrolytic filtering
capacitor. We will use an integrated bridge circuit to replace the four diodes.
4) Full wave rectifier with filtering
To smooth the output and better approximate a constant voltage, place a capacitor
across the output as shown in Fig. 3. What is the relevant quantity which determines
how “constant” the voltage is? Which resistance sets the characteristic time of this lowpass filter? Do you want a small or a large capacitor? Why? Try Cf = 0.2μF, then a
25μF electrolytic capacitor (or find capacitors of similar size). Be sure to observe the
polarity on the electrolytic capacitor. What resistance combines with the capacitance to
set the characteristic time of this low-pass filter? In each case, measure the ripple
voltage (peak-to-peak fluctuation). How does the DC voltage vary with load – i.e.,
characterize the “voltage regulation” by trying a different load resistor? (Calculate the
power before you make the substitution to be sure the resistor can handle the power)
5) Integrated circuit regulator
One way to make a DC supply for real circuits is to build a rudimentary DC supply such
as the one in below and then use an integrated circuit (IC) “voltage regulator” to
stabilize it. Construct the circuit shown in Fig. 4 using an LM7805 which is a 5 Volt
supply regulator (spec sheets attached). The diagram on the first spec sheet looks at
the device from the labeled side. We use this IC as a “black box” and just empirically
note the quality of performance. How does the DC voltage vary with load?
Figure 4 A full-wave rectifier with four 1N4004 diodes, an electrolytic filtering capacitor,
and an LM7805 regulator chip. We will use an integrated bridge to replace the diodes.