Diode Bridges. Book page

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Belinda King
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2 Rectification The process of converting an ac supply into dc is called rectification The device that carries this out is called a rectifier Half wave rectifier only half of the current is used each cycle Full wave rectifier both halves of the AC cycle are used requires a more complex circuit Diodes are semiconductors that allow current to go in one direction only

Semiconductors Semiconductors are made from Group 4 silicon and germanium doped (covered with) impurity atoms from group 3 or 5 to modulate its electrical properties p type or n type Group 4

3 Semiconductors Semiconductors are made from Group 4 silicon and germanium doped (covered with) impurity atoms from group 3 or 5 to modulate its electrical properties p type or n type Group 4 has 4 valence electrons Doping with Group 5 elements produces electron rich layers of n type semiconductors Doping with Group 3 elements produces electron deficit layers of p type semiconductors

Semiconductors N - type 5 valence electrons Form covalent bonds with 4 valence e s of silicon 1 free electron left for conduction P - type 3 valence electrons Net result is a hole as not enough

4 Semiconductors N - type 5 valence electrons Form covalent bonds with 4 valence e s of silicon 1 free electron left for conduction P - type 3 valence electrons Net result is a hole as not enough electrons are present to form covalent bonds An electron from a n- type semiconductor can move into the hole Electrons can move from hole to hole and create a PD These wafers can be placed on top of each other to form layers

layer Negative charge builds up Positive charge builds up This will stop e-movemen A depletion layer forms Internal

6 Current flow Semiconductors allow the current to flow in both directions If a p type is placed next to a n- type current flows easily in one direction, but almost none in the opposite direction This is called a P N junction diode Depletion layer Negative charge builds up Positive charge builds up This will stop e-movemen A depletion layer forms Internal electric field opposes further diffusion holes electrons Formation and Properties of Junction Diode - Physics - YouTube [720p].mp4

7 Wider when connected to an emf - + PD creates an electric field that adds up to that in the depletion layer Increase in R in the p-n junction Diode is reverse biased Only a very small current can flow through diode due to the electric field This is called leakage current

Reverse terminals of source emf External field Electric field of depletion + - - Electron flow Depletion layers become narrower PD creates a field opposite to that in

8 Reverse terminals of source emf External field Electric field of depletion Electron flow Depletion layers become narrower PD creates a field opposite to that in the layer and reduces the net electric field R in the p-n junction decreases Diode is forward biased cgrahamphysics.com 201 Depletion layer is removed and current can flow

9 Connecting an ac power supply Junction diode Load resistor Cathode ray oscilloscope Records current wave Over time Diode forward biased conducts from x y Reverse biased Leakage current y - z Only half of available input cgrahamphysics.com is used, 2016 the other half is wasted

10 Diode Bridge In order to have most available power, 2 or 4 diodes are used in a circuit This is called a diode bridge The magnitude of current through the resistor changes, but the direction of current flow is the same through the resistor for each half cycle of AC current + - _ +

11 Half wave rectification Forward biased diode Basic circuit: x Load resistor will only have PD across it for half a cycle A small forward PD is needed to start conduction y Input: ac X positive: current through diode X negative: diode is reversed biased, no current Although 1 direction only, current is not constant Output: dc

12 Increasing current and PD To increase a constant value of current or PD use a resistor and capacitor connected in parallel between a diode and the load 1 st half cycle capacitor charges up PD approaches peak of transformer output emf When the current is zero in 2 nd half cycle, the capacitor discharges through the resistor The rate is determined by the time constant of the circuit RC = τ R = circuit resistance, C = circuit capacitance

13 Rate of discharge RC = τ If τ >> time for a half cycle, Q released and PD will be small When the diode conducts again in the next half cycle, the charge stored in the capacitor will be topped up A discharge charge cycle is created with PD across capacitor varying much less Reservoir capacitor and small variation in output creates a ripple voltage A large time constant τ means a good smooth curve, but needs a more expensive capacitor

14 Full wave rectification Use of two diodes and center tap on transformer Need of a resistor capacitor pair The tap is ½ length of transformer coil Y X > 0 D 1 will conduct, which is connected to half of the secondary coil Y always at zero point ½ time x is positive and z will be negative During the other half polarities reverse ½ cycle later x < 0, z > 0 D 2 will conduct During both half cycles current is supplied to the capacitor This is a full wave rectification

15 Disadvantage To achieve peak PD, twice as many turns are required on secondary coil compared to half wave arrangement Use a diode bridge to overcome this feature Function Of A Bridge Rectifier.flv

16 Diode Bridge A full coil is used and supplies current 4 diodes are needed X > 0 Junction D 1 &D 4 > 0, Z < 0 D 1 conducts A is positive D 3 conducts B is negative Capacitor charges Current is supplied to the rest of circuit Polarity switches, X < 0, Z > 0 Conducting now D 2 &D 4 Charge delivered to capacitor is unchanged

Diode Characteristics and Applications

Diode Characteristics and Applications Topics covered in this presentation: Diode Characteristics Diode Clamp Protecting Against Back-EMF Half-Wave Rectifier The Zener Diode 1 of 18 Diode Characteristics