6.Experiments

5.3. HALF-WAVE RECTIFIER

203

AC source voltage is speci¯ed as 8.485 instead of 6 volts because SPICE understands AC voltage in terms of peak value only. A 6 volt RMS sine-wave voltage is actually 8.485 volts peak. In simulations where the distinction between RMS and peak value isn't relevant, I will not bother with an RMS-to-peak conversion like this. To be truthful, the distinction is not terribly important in this simulation, but I discuss it here for your edi¯cation.

204	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

5.4Full-wave center-tap recti¯er

PARTS AND MATERIALS

²Low-voltage AC power supply (6 volt output)

²Two 1N4001 rectifying diodes (Radio Shack catalog # 276-1101)

²Small "hobby" motor, permanent-magnet type (Radio Shack catalog # 273-223 or equivalent)

²Audio detector with headphones

²0.1 ¹F capacitor

²One toggle switch, SPST ("Single-Pole, Single-Throw")

It is essential for this experiment that the low-voltage AC power supply be equipped with a center tap. A transformer with a non-tapped secondary winding simply will not work for this circuit.

The diodes need not be exact model 1N4001 units. Any of the "1N400X" series of rectifying diodes are suitable for the task, and they are quite easy to obtain.

CROSS-REFERENCES

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

LEARNING OBJECTIVES

²Design of a center-tap recti¯er circuit

²Measuring "ripple" voltage with a voltmeter

SCHEMATIC DIAGRAM
Diode
AC	Mtr (motor)
power
supply
Diode
ILLUSTRATION

5.4. FULL-WAVE CENTER-TAP RECTIFIER

205

Low-voltage

AC power supply

12

6 6

Terminal

strip

Motor

INSTRUCTIONS

This recti¯er circuit is called full-wave because it makes use of the entire waveform, both positive and negative half-cycles, of the AC source voltage in powering the DC load. As a result, there is less "ripple" voltage seen at the load. The RMS (Root-Mean-Square) value of the recti¯er's output is also greater for this circuit than for the half-wave recti¯er.

Use a voltmeter to measure both the DC and AC voltage delivered to the motor. You should notice the advantages of the full-wave recti¯er immediately by the greater DC and lower AC indications as compared to the last experiment.

An experimental advantage of this circuit is the ease of which it may be "de-converted" to a half-wave recti¯er: simply disconnect the short jumper wire connecting the two diodes' cathode ends together on the terminal strip. Better yet, for quick comparison between half and full-wave recti¯cation, you may add a switch in the circuit to open and close this connection at will:

Mtr

Switch

(close for full-wave operation)

206	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

Low-voltage

AC power supply

		12
Switch	6	6

With the ability to quickly switch between halfand full-wave recti¯cation, you may easily perform qualitative comparisons between the two di®erent operating modes. Use the audio signal detector to "listen" to the ripple voltage present between the motor terminals for half-wave and full-wave recti¯cation modes, noting both the intensity and the quality of the tone. Remember to use a coupling capacitor in series with the detector so that it only receives the AC "ripple" voltage and not DC voltage:

5.4. FULL-WAVE CENTER-TAP RECTIFIER

207

Capacitor

0.1 μF

headphones

Sensitivity	plug

Test "probes"

COMPUTER SIMULATION

Schematic with SPICE node numbers:

1	D1	2	2
V1			Rload
0			0
V2
3		2
	D2

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

Fullwave center-tap rectifier v1 1 0 sin(0 8.485 60 0 0) v2 0 3 sin(0 8.485 60 0 0) rload 2 0 10k

d1 1 2 mod1

208	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

d2 3 2 mod1

.model mod1 d

.tran .5m 25m

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

.end

5.5. FULL-WAVE BRIDGE RECTIFIER

209

5.5Full-wave bridge recti¯er

PARTS AND MATERIALS

²Low-voltage AC power supply (6 volt output)

²Four 1N4001 rectifying diodes (Radio Shack catalog # 276-1101)

²Small "hobby" motor, permanent-magnet type (Radio Shack catalog # 273-223 or equivalent)

CROSS-REFERENCES

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

LEARNING OBJECTIVES

²Design of a bridge recti¯er circuit

²Advantages and disadvantages of the bridge recti¯er circuit, compared to the center-tap circuit

SCHEMATIC DIAGRAM

AC

power supply

Mtr (motor)

ILLUSTRATION

Low-voltage

AC power supply

12

6 6

Terminal

strip

Motor

210	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

INSTRUCTIONS

This circuit provides full-wave recti¯cation without the necessity of a center-tapped transformer. In applications where a center-tapped, or split-phase, source is unavailable, this is the only practical method of full-wave recti¯cation.

In addition to requiring more diodes than the center-tap circuit, the full-wave bridge su®ers a slight performance disadvantage as well: the additional voltage drop caused by current having to go through two diodes in each half-cycle rather than through only one. With a low-voltage source such as the one you're using (6 volts RMS), this disadvantage is easily measured. Compare the DC voltage reading across the motor terminals with the reading obtained from the last experiment, given the same AC power supply and the same motor.

COMPUTER SIMULATION

Schematic with SPICE node numbers:

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

5.6. RECTIFIER/FILTER CIRCUIT

211

5.6Recti¯er/¯lter circuit

PARTS AND MATERIALS

²Low-voltage AC power supply

²Bridge recti¯er pack (Radio Shack catalog # 276-1185 or equivalent)

²Electrolytic capacitor, 1000 ¹F, at least 25 WVDC (Radio Shack catalog # 272-1047 or equivalent)

²Four "banana" jack style binding posts, or other terminal hardware, for connection to potentiometer circuit (Radio Shack catalog # 274-662 or equivalent)

²Metal box

²12-volt light bulb, 25 watt

²Lamp socket

A bridge recti¯er "pack" is highly recommended over constructing a bridge recti¯er circuit from individual diodes, because such "packs" are made to bolt onto a metal heat sink. A metal box is recommended over a plastic box for its ability to function as a heat sink for the recti¯er.

A larger capacitor value is ¯ne to use in this experiment, so long as its working voltage is high enough. To be safe, choose a capacitor with a working voltage rating at least twice the RMS AC voltage output of the low-voltage AC power supply.

High-wattage 12-volt lamps may be purchased from recreational vehicle (RV) and boating supply stores. Common sizes are 25 watt and 50 watt. This lamp will be used as a "heavy" load for the power supply.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 2, chapter 8: "Filters"

LEARNING OBJECTIVES

²Capacitive ¯lter function in an AC/DC power supply

²Importance of heat sinks for power semiconductors

SCHEMATIC DIAGRAM
		Rectifier
AC	-	+
in		DC
		out

212	CHAPTER 5. DISCRETE SEMICONDUCTOR CIRCUITS

ILLUSTRATION

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

INSTRUCTIONS

This experiment involves constructing a recti¯er and ¯lter circuit for attachment to the lowvoltage AC power supply constructed earlier. With this device, you will have a source of low-voltage, DC power suitable as a replacement for a battery in battery-powered experiments. If you would like to make this device its own, self-contained 120VAC/DC power supply, you may add all the componentry of the low-voltage AC supply to the "AC in" side of this circuit: a transformer, power cord, and plug. Even if you don't choose to do this, I recommend using a metal box larger than necessary to provide room for additional voltage regulation circuitry you might choose to add to this project later.

The bridge recti¯er unit should be rated for a current at least as high as the transformer's secondary winding is rated for, and for a voltage at least twice as high as the RMS voltage of the transformer's output (this allows for peak voltage, plus an additional safety margin). The Radio Shack recti¯er speci¯ed in the parts list is rated for 25 amps and 50 volts, more than enough for the output of the low-voltage AC power supply speci¯ed in the AC experiments chapter.

Recti¯er units of this size are often equipped with "quick-disconnect" terminals. Complementary "quick-disconnect" lugs are sold that crimp onto the bare ends of wire. This is the preferred method of terminal connection. You may solder wires directly to the lugs of the recti¯er, but I recommend against direct soldering to any semiconductor component for two reasons: possible heat damage during soldering, and di±culty of replacing the component in the event of failure.

Semiconductor devices are more prone to failure than most of the components covered in these experiments thus far, and so if you have any intent of making a circuit permanent, you should build it to be maintained. "Maintainable construction" involves, among other things, making all delicate components replaceable. It also means making "test points" accessible to meter probes throughout the circuit, so that troubleshooting may be executed with a minimum of inconvenience. Terminal strips inherently provide test points for taking voltage measurements, and they also allow for easy disconnection of wires without sacri¯cing connection durability.