1. Diode Circuits and DC Power Supply
Week of 11th August 2008

2. Questions Difference between Analog and Digital electronics
Book on Engineering Circuit Analysis by Hayt and Kemmerly - ( Can also be used to study the topics given in book by Richard Dorf )

3. DC Power Supply
© Electronic Principles, Malvino, TMH

4. Bridge rectifier Has four diodes
Load voltage is a full-wave signal with peak value equal to the secondary voltage

5. The bridge
rectifier
Vin
Vout

6. Bridge rectifier signals
The dc value of the output is the average value.
Vdc = 2VP/p
fout = 2fin
Second approximation: VP(out) = VP(in) V

7. Which full wave rectifier to use
Which full wave rectifier to use ? The two diode or the four diode one rectifier
The two diode rectifier
Need of a center tapped transformer
The four diode bridge rectifier
The full secondary voltage, from a standard transformer is available for rectification
Twice the peak voltage and twice the DC ( average) voltage

8. Bridge Circuit

9. Capacitor-input filter
Most widely used
The peak value of the rectified signal passes to the load resistor
With a large capacitor, ripple is small

10. The capacitor-input filter
Discharge VP +
Charge -
VR = I fC
Where VR is the peak-to-peak ripple voltage

11. Effects on voltage and current
Ac line regulation can have an effect
Bulk resistance of rectifiers causes a voltage drop
Resistance of transformer windings causes a voltage drop
Ac ripple increases and average dc decreases when load current increases

12. Questions – 14th August
Difference between 2 diode and 4 diode full wave rectifier ?
New students – how do we cope with the lost lectures?
Difference between analog ground and digital ground ?

14. Voltage regulator - 7805 What does the word “regulation” mean?
The job of the regulator is to hold the value of a regulated parameter at a constant value, under varying external conditions
Percentage change in value from no load to full load

15. 7805 – 5 volt regulator Output Current 500 mA Input Min Voltage
Input Max Voltage
35 Volt
Output Voltage
5 Volt
Adjustable Output No
On/Off Pin
Error Flag
Temperature Min
-40 deg C
Temperature Max
125 deg C

16. DC Power Supply PCB – Lab 3

17. A sample Printed Circuit Board (PCB)

18. Peak inverse voltage
Maximum voltage across the nonconducting diode of a rectifier circuit
Voltage must be less than diode breakdown voltage

19. Diode ratings
Half-wave rectifier with capacitor-input filter: PIV = 2VP Idiode = Idc
Full-wave rectifier with capacitor-input filter: PIV = VP Idiode = 0.5Idc
Bridge rectifier with capacitor-input filter: PIV = VP Idiode = 0.5Idc

20. Typical bridge supply problems
No output - - blown fuse, two or more diodes open, load shorted
Low output/extra ripple - - bad filter, open diode, shorted winding, overload
Full-wave signal at output - - open filter capacitor
Half-wave ripple frequency - - open diode

21. Power supply topics
Real transformers usually specify secondary voltage at a rated load current
Slow-blow fuses are often used to protect against surge current
Average diode current in a half-wave rectifier equals the dc load current
In a full-wave or bridge, the average current in any diode is half the dc load current

22. Positive clipper RS
0.7 V
Vin RL
Vout

23. Negative clipper RS
Vin RL
Vout
-0.7 V