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

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 )

DC Power Supply © Electronic Principles, Malvino, TMH

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

The bridge rectifier Vin Vout

Bridge rectifier signals The dc value of the output is the average value. Vdc = 2VP/p fout = 2fin Second approximation: VP(out) = VP(in) - 1.4 V

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

Bridge Circuit

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

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

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

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 ?

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

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

DC Power Supply PCB – Lab 3

A sample Printed Circuit Board (PCB)

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

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

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

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

Positive clipper RS 0.7 V Vin RL Vout

Negative clipper RS Vin RL Vout -0.7 V