1. In a zener regulator, what are the symptoms of an open zener diode?
2. If a zener regulator fails so that the zener impedance is greater than the specified value, is the output voltage more or less than it should be?
3. If you measure 0 V at the output of a zener-regulated power supply, what is the most likely fault(s)?
4. The zener diode regulator in a power supply is open. What will you observe on the output with a voltmeter if the load resistance is varied within its specified range?
SECTION 3–7 CHECKUP
168 ◆ Special-Purpose Diodes
A partial datasheet for a 7812 is shown in Figure 3–69(a) Notice that there is a range of nominal output voltages, but it is typically 12 V. The line and load regulation specify how much the output can vary about the nominal output value. For example, the typical 12 V output will change no more than 11 mV (typical) as the load current changes from 5 mA to 1. 5 A. Package configurations are shown in part (b).
1. From the datasheet, determine the maximum output voltage if the input voltage to the regulator increases to 22 V, assuming a nominal output of 12 V.
2. From the datasheet, determine how much the typical output voltage changes when the load current changes from 250 mA to 750 mA.
The LED A typical partial datasheet for a visible red LED is shown in Figure 3–70. As the datasheet shows, a forward current of 10 mA to 20 mA is used for the test data.
Electrical Characteristics (MC7812E)
(Refer to test circuit ,08C < TJ < 125 , IO = 500mA, VI =19V, CI= 0.33mF, CO=0.1 , unless otherwise specified)mF,
Parameter Symbol Conditions MC7812E
Min. Typ. Max. Unit
Output Voltage VO
TJ = +25 11.5 12 12.5
5.0mA …IO…1.0A, PO…15W V
VI = 14.5V to 27V 11.4 12 12.6
Line Regulation (Note1) Regline TJ = +25 VI = 14.5V to 30V - 10 240 VI = 16V to 22V - 3.0 120 mV
Load Regulation (Note1) Regload TJ = +25 IO = 5mA to 1.5A - 11 240 IO = 250mA to 750mA - 5.0 120 mV
Quiescent Current IQ TJ = +25 5.1 8.0 mA
Quiescent Current Change DIQ IO = 5mA to 1.0A -
-
0.1 0.5
VI = 14.5V to 30V - 0.5 1.0 mA
Output Voltage Drift (Note2) DVO/ T ID O = 5mA - -1 - mV/8C
Output Noise Voltage VN f = 10Hz to 100kHz, TA = +25 8C 76 - mV/Vo Ripple Rejection (Note2) RR f = 120Hz
VI = 15V to 25V 55 71 - dB
Dropout Voltage VDrop IO = 1A, TJ= +25 2 - V
Output Resistance (Note2) rO f = 1kHz -
- -
18 - mV
Short Circuit Current ISC VI = 35V, TA= - 230 - mA
Peak Current (Note2) IPK TJ = +258C 8C +258C 8C 8C 8C 8C 8C
- 2.2 - A
123
1 3
(a) (b) 1—input, 2—ground, 3—output
TO-220
D-PAK
▲ FIGURE 3–69
Partial datasheet and packages for a 7812 regulator. You can view an entire datasheet at www.fairchildsemi.com. Copyright Fairchild Semiconductor Corporation. Used by permission.
Optical and Electrical Characteristics
Tamb = 258C, unless otherwise specified
Red
TLHK51..
1) in one Packing Unit IVmin/IVmax… 0.5
Parameter Test condition Part Symbol Min Typ. Max Unit
Luminous intensity 1) IF = 20 mA TLHK5100 320 mcd
Dominant wavelength IF = 10 mA 626 630 639 nm
Peak wavelength IF = 10 mA 643 nm
Angle of half intensity IF = 10 mA 6 9 deg
Forward voltage IF = 20 mA 1.9 2.6 V
Reverse voltage IR = 10 mA 5 V
Junction capacitance VR = 0, f = 1 MHz
IV ld lp
VF VR
Cj 15 pF
w
▲ FIGURE 3–70
Partial datasheet and package for a typical red LED. To view a complete datasheet, go to www.vishay.com. Datasheet courtesy of Vishay Intertechnology, Inc.
3. Determine the value of the resistor shown in Figure 3–68 for limiting the LED current to 20 mA and use the next higher standard value. Also specify the power rating of the limiting resistor.
The Fuse The fuse will be in series with the primary winding of the transformer, as shown in Figure 3–68. The fuse should be calculated based on the maximum allowable primary current. Recall from your dc/ac circuits course that if the voltage is stepped down, the current is stepped up. From the specifications for the unregulated power supply, the maximum load current is 250 mA. The current required for the power-on LED indica- tor is 15 mA. So, the total secondary current is 265 mA. The primary current will be the secondary current divided by the turns ratio.
4. Calculate the primary current and use this value to select a fuse rating.
Simulation
In the development of a new circuit, it is helpful to simulate the circuit using a software program before actually building it and committing it to hardware. We will use Multisim to simulate this power supply circuit. Figure 3–71 shows the simulated regulated power supply circuit. The unregulated power supply was previously tested, so you need only to verify that the regulated output is correct. A load resistor value is chosen to draw a current equal to or greater than the specified maximum load current.
RL= 12 V
250 mA =48 V
The closest standard value is 47 V, which draws 255 mA at 12 V.
5. Determine the power rating for the load resistor.
Simulate the circuit using your Multisim or LT Spice software. Verify the operation with the virtual voltmeter.
Prototyping and Testing
Now that all the components have been selected and the circuit has been simulated, the new components are added to the power supply protoboard from Experiment 2 and the circuit is tested.
▲ FIGURE 3–71
Simulation of the regulated 12 V power supply circuit.
Printed Circuit Board
The 12 V regulated power supply prototype has been built and tested. It is now committed to a printed circuit layout, as shown in Figure 3–72. Notice that a heat sink is used with the regulator IC to increase its ability to dissipate power. With the ac line voltage and load resistor connected, the output voltage is measured.
6. Compare the printed circuit board to the schematic in Figure 3–71.
7. Calculate the power dissipated by the regulator for an output of 12 V.
XFMR
12.6 V .1
6800
120 V 60 Hz
47 V5 W
.33
▲ FIGURE 3–72
Regulated 12 V power supply on the printed circuit (PC) board.