6.Experiments

5.6. RECTIFIER/FILTER CIRCUIT

213

Bolt the recti¯er unit to the inside of the metal box. The box's surface area will act as a radiator, keeping the recti¯er unit cool as it passes high currents. Any metal radiator surface designed to lower the operating temperature of an electronic component is called a heat sink. Semiconductor devices in general are prone to damage from overheating, so providing a path for heat transfer from the device(s) to the ambient air is very important when the circuit in question may handle large amounts of power.

A capacitor is included in the circuit to act as a ¯lter to reduce ripple voltage. Make sure that you connect the capacitor properly across the DC output terminals of the recti¯er, so that the polarities match. Being an electrolytic capacitor, it is sensitive to damage by polarity reversal. In this circuit especially, where the internal resistance of the transformer and recti¯er are low and the short-circuit current consequently is high, the potential for damage is great. Warning: a failed capacitor in this circuit will likely explode with alarming force!

After the recti¯er/¯lter circuit is built, connect it to the low-voltage AC power supply like this:

Low-voltage

AC power supply

12

6 6

	Rectifier			Capacitor
	-	AC
AC	AC	+	-		DC
in			-		out

Measure the AC voltage output by the low-voltage power supply. Your meter should indicate approximately 6 volts if the circuit is connected as shown. This voltage measurement is the RMS voltage of the AC power supply.

Now, switch your multimeter to the DC voltage function and measure the DC voltage output by the recti¯er/¯lter circuit. It should read substantially higher than the RMS voltage of the AC input measured before. The ¯ltering action of the capacitor provides a DC output voltage equal to the peak AC voltage, hence the greater voltage indication:

214	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

Full-wave, rectified DC voltage

Time

Full-wave, rectified DC voltage, with filtering

Time

Measure the AC ripple voltage magnitude with a digital voltmeter set to AC volts (or AC millivolts). You should notice a much smaller ripple voltage in this circuit than what was measured in any of the un¯ltered recti¯er circuits previously built. Feel free to use your audio detector to "listen" to the AC ripple voltage output by the recti¯er/¯lter unit. As usual, connect a small "coupling" capacitor in series with the detector so that it does not respond to the DC voltage, but only the AC ripple. Very little sound should be heard.

After taking unloaded AC ripple voltage measurements, connect the 25 watt light bulb to the output of the recti¯er/¯lter circuit like this:

Low-voltage

AC power supply

12

6 6

	Rectifier			Capacitor
	-	AC
AC	AC	+	-		DC
in			-		out

Re-measure the ripple voltage present between the recti¯er/¯lter unit's "DC out" terminals. With a heavy load, the ¯lter capacitor becomes discharged between recti¯ed voltage peaks, resulting in greater ripple than before:

5.6. RECTIFIER/FILTER CIRCUIT

215

Full-wave, filtered DC voltage under heavy load

Time

If less ripple is desired under heavy-load conditions, a larger capacitor may be used, or a more complex ¯lter circuit may be built using two capacitors and an inductor:

DC out

If you choose to build such a ¯lter circuit, be sure to use an iron-core inductor for maximum inductance, and one with thick enough wire to safely handle the full rated current of power supply. Inductors used for the purpose of ¯ltering are sometimes referred to as chokes, because they "choke" AC ripple voltage from getting to the load. If a suitable choke cannot be obtained, the secondary winding of a step-down power transformer like the type used to step 120 volts AC down to 12 or 6 volts AC in the low-voltage power supply may be used. Leave the primary (120 volt) winding open:

Leave these wires disconnected!

DC out

COMPUTER SIMULATION

Schematic with SPICE node numbers:

1
					D	1	1	D	2
		3								2					2
V1
							0			C1					Rload
0					D3			D4
															3

216 CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

Netlist (make a text ¯le containing the following text, verbatim):

Fullwave bridge rectifier v1 1 0 sin(0 8.485 60 0 0) rload 2 3 10k

c1 2 3 1000u ic=0

d1 3 1 mod1

d2 1 2 mod1

d3 3 0 mod1

d4 0 2 mod1

.model mod1 d

.tran .5m 25m

.plot tran v(1,0) v(2,3)

.end

You may decrease the value of Rload in the simulation from 10 k- to some lower value to explore the e®ects of loading on ripple voltage. As it is with a 10 k- load resistor, the ripple is undetectable

on the waveform plotted by SPICE.

5.7. VOLTAGE REGULATOR

217

5.7Voltage regulator

PARTS AND MATERIALS

²Four 6 volt batteries

²Zener diode, 12 volt { type 1N4742 (Radio Shack catalog # 276-563 or equivalent)

²One 10 k- resistor

Any low-voltage zener diode is appropriate for this experiment. The 1N4742 model listed here (zener voltage = 12 volts) is but one suggestion. Whatever diode model you choose, I highly recommend one with a zener voltage rating greater than the voltage of a single battery, for maximum learning experience. It is important that you see how a zener diode functions when exposed to a voltage less than its breakdown rating.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 3: "Diodes and Recti¯ers"

LEARNING OBJECTIVES

² Zener diode function

SCHEMATIC DIAGRAM

10 kΩ

Zener

diode

ILLUSTRATION

-

+

218	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

INSTRUCTIONS

Build this simple circuit, being sure to connect the diode in "reverse-bias" fashion (cathode positive and anode negative), and measure the voltage across the diode with one battery as a power source. Record this voltage drop for future reference. Also, measure and record the voltage drop across the 10 k- resistor.

Modify the circuit by connecting two 6-volt batteries in series, for 12 volts total power source voltage. Re-measure the diode's voltage drop, as well as the resistor's voltage drop, with a voltmeter:

V A

V A

OFF

A COM

-

+

-

+

Connect three, then four 6-volt batteries together in series, forming an 18 volt and 24 volt power source, respectively. Measure and record the diode's and resistor's voltage drops for each new power supply voltage. What do you notice about the diode's voltage drop for these four di®erent source voltages? Do you see how the diode voltage never exceeds a level of 12 volts? What do you notice about the resistor's voltage drop for these four di®erent source voltage levels?

Zener diodes are frequently used as voltage regulating devices, because they act to clamp the voltage drop across themselves at a predetermined level. Whatever excess voltage is supplied by the power source becomes dropped across the series resistor. However, it is important to note that a zener diode cannot make up for a de¯ciency in source voltage. For instance, this 12-volt zener diode does not drop 12 volts when the power source is only 6 volts strong. It is helpful to think of a zener diode as a voltage limiter: establishing a maximum voltage drop, but not a minimum voltage drop.

COMPUTER SIMULATION

5.7. VOLTAGE REGULATOR					219
Schematic with SPICE node numbers:
1	10 kΩ				2

										Zener
										diode
	0								0
	Netlist (make a text ¯le containing the following text, verbatim):
Zener diode
v1	1			0
r1	1			2		10k
d1	0			2		mod1
.model						mod1 d bv=12
.dc v1						18 18 1
.print						dc v(2,0)
.end

A zener diode may be simulated in SPICE with a normal diode, the reverse breakdown parameter (bv=12) set to the desired zener breakdown voltage.

220	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

5.8Transistor as a switch

PARTS AND MATERIALS

²Two 6-volt batteries

²One NPN transistor { models 2N2222 or 2N3403 recommended (Radio Shack catalog # 2761617 is a package of ¯fteen NPN transistors ideal for this and other experiments)

²One 100 k- resistor

²One 560 - resistor

²One light-emitting diode (Radio Shack catalog # 276-026 or equivalent)

Resistor values are not critical for this experiment. Neither is the particular light emitting diode (LED) selected.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 4: "Bipolar Junction Transistors"

LEARNING OBJECTIVES

² Current ampli¯cation of a bipolar junction transistor

SCHEMATIC DIAGRAM

560 Ω

6 V

6 V				100 kΩ

Q1

ILLUSTRATION

5.8. TRANSISTOR AS A SWITCH

221

CBE

- -

++

INSTRUCTIONS

The red wire shown in the diagram (the one terminating in an arrowhead, connected to one end of the 100 k- resistor) is intended to remain loose, so that you may touch it momentarily to other points in the circuit.

If you touch the end of the loose wire to any point in the circuit more positive than it, such as the positive side of the DC power source, the LED should light up. It takes 20 mA to fully illuminate a standard LED, so this behavior should strike you as interesting, because the 100 k- resistor to which the loose wire is attached restricts current through it to a far lesser value than 20 mA. At most, a total voltage of 12 volts across a 100 k- resistance yields a current of only 0.12 mA, or 120 ¹A! The connection made by your touching the wire to a positive point in the circuit conducts far less current than 1 mA, yet through the amplifying action of the transistor, is able to control a much greater current through the LED.

Try using an ammeter to connect the loose wire to the positive side of the power source, like this:

222	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

- -

++

V A

V A

OFF

A COM

CBE

You may have to select the most sensitive current range on the meter to measure this small °ow. After measuring this controlling current, try measuring the LED's current (the controlled current) and compare magnitudes. Don't be surprised if you ¯nd a ratio in excess of 200 (the controlled current 200 times as great as the controlling current)!

As you can see, the transistor is acting as a kind of electrically-controlled switch, switching current on and o® to the LED at the command of a much smaller current signal conducted through its base terminal.

To further illustrate just how miniscule the controlling current is, remove the loose wire from the circuit and try "bridging" the unconnected end of the 100 k- resistor to the power source's positive pole with two ¯ngers of one hand. You may need to wet the ends of those ¯ngers to maximize conductivity: