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Rectifier and regulator
In this lab you will construct and analyze a full wave rectifier and a shunt voltage regulator. All component types in the example circuit are available in OrCAD – Capture libraries for simulation.
I. Introduction
1.1 The Full Wave Rectifier
The first building block in the dc power supply is the full wave rectifier. The purpose of the full wave rectifier (FWR) is to create a rectified ac output from a sinusoidal ac input signal. It does this by using the nonlinear conductivity characteristics of diodes to direct the path of the current.
Figure 1. Common four-diode bridge configuration for the FWR
Diode Currents Consider the current path in the diode bridge rectifier. In the positive half cycle of Vin, diodes D4 and D3 will conduct. During the negative half cycle, diodes D2 and D1 will conduct. As a result, the load will pass current in the same direction in each half cycle of the input.
Design Concerns
- Reverse current does not exceed the breakdown value
- Power dissipation limit P = Vd Id is not exceeded
Diode Voltages
o If we consider a simple, piece-wise linear model for the diode IV curve, the diode forward current is zero until Vbias >= Vthreshold , where Vthreshold is 0.6 V to 0.8 V. The current increases abruptly as Vbias increases further. Due to this turn-on or threshold voltage associated with the diode in forward bias, we should expect a 0.6 to 0.8 V voltage drop across each forward biased diode in the rectifier bridge. In the case of the full wave rectifier diode bridge, there are two forward biased diodes in series with the load in each half cycle of the input signal. o The maximum output voltage (across load) will be Vin - 2 V threshold , or ~ V in - 1.4 V. o Since some current does flow for voltage bias below Vthreshold and the current rise around is Vthreshold is more gradual than the piece-wise model, the actual diode performance will differ from the simple model.
- Reverse Bias o In reverse bias (and neglecting reverse voltage breakdown), the current through the diode is approximately the reverse saturation current, Io. The voltage across the load during reverse bias will be Vout = Io R load. o In specifying a diode for use in a circuit, you must take care that the limits for forward and reverse voltage and current are not exceeded.
1.2 Filtered Full Wave Rectifier
The filtered full wave rectifier is created from the FWR by adding a capacitor across the output.
Figure 2. Filtered full wave rectifier
Since T2 - T1 ~ T/2, where T is the period of the sine wave, then
Peak Current Levels Diodes in the bridge conduct only in the time period from T0 to T1. The diode current must replace the charge lost by the capacitor during its discharge. I = dQ/dT = C*dV/dT As the magnitude of the filter capacitor increases, the peak current through the diodes must increase to replace the charge in less time. Therefore it is not always best to choose the largest value of C1 available. In a dc power supply, you can rely on the stages following the FFWR to significantly improve the voltage regulation.
1.3 The Shunt Regulator
A shunt regulator may be placed between the filtered full wave rectifier and the load resistance (impedance). Its purpose is to minimize the variation in the voltage across the load, as either the input voltage or the output resistance changes.
Figure 4. Filtered FWR and shunt regulator
This regulator is called a shunt because it provides an additional path for current to flow, so that some current can bypass the load. The shunt regulator consists of a zener diode and a resistor. The zener diode has a nearly constant voltage drop when used in reverse bias. The resistor is chosen to maintain the zener in its proper working region, where it can provide regulation and not exceed a maximum power limit.
A simple model for the zener diode is a dc supply (battery) with a value of Vzo, where
Vzo is the effective zener voltage, , Vz is the rated breakdown voltage, and Rz is the effective resistance of the zener, given by the inverse of the slope of the IV curve in the working region.
Figure 5. Filtered FWR and shunt regulator with the zener diode replaced with its circuit model
In the zeners working region, Rz is small (0.1 to 50 ohm ). For voltages less than the knee voltage, Rz is very high, and for purposes of hand calculations can be considered to be an open circuit.
Figure 6. Current-voltage characteristic of a zener diode
You can show for the circuit above that
where IL is the current through the load. The 1st term in this equation is constant since it depends only on the diode zener voltage and two resistances. The 2nd and 3rd terms depend on
- Input AC voltage (Vin): Vin is a 10 Vpeak and 60 Hz sinusoidal wave. Use VSIN with the setting: VOFF = 0, VAML = 10 and FREQ = 60
- Full wave rectifier (FWR): The full wave rectifier is constructed in the form of bridge rectifier using four diodes ( D1N4004 ).
- Load resistor: 200 Ω and 500 Ω resistors are used to understand the effect of load resistor on the performance of the DC power supply. Simulation results required in your lab report:
- Output voltage for R load = 200 Ω
- Output voltage for R load = 500 Ω
All the simulations in this project are in transient mode with run time = 35ms. On the simulation results, you should indicate the maximum output voltage (Vmax), the minimum output voltage (Vmin) and the ripple voltage Vr (Vmax – Vmin).
To simulate the filtered full wave rectifier circuit as shown in Figure 2, the filter capacitor is chosen from 100 uF, 470 uF and 1000 uF. Simulation results required in your lab report:
- Output voltage for R load = 200 Ω and C 1 = 100 uF
- Output voltage for R load = 200 Ω and C 1 = 470 uF
- Output voltage for R load = 200 Ω and C 1 = 1000 uF
- Output voltage for R load = 500 Ω and C 1 = 100 uF
- Output voltage for R load = 500 Ω and C 1 = 470 uF
- Output voltage for R load = 500 Ω and C 1 = 1000 uF
Part 2:
To design and simulate a filtered full wave rectifier with a shunt regulator, the following design steps should be followed:
- To design a shunt regulator, first pick up a 3.3 volts zener diode (a particle diode, part number 1N5226) and plug it into the curve tracer. Caution: zener diode should be reverse biased. The cathode of the zener diode (the end with a black ring) should be connected to the “A” of the diode test port. Menu Settings : type – diode; Vd – 5 volts; Id
- 20 mA; Rload – .25 ohm; Pmax – 2 Watt). Choose any two points in the linear region
and use CURSOR function to display Id and Vd of the two points. Print out from the screen and it should look like that in Figure 6 except in the first quadrant.
- Calculate the effective zener resistance Rz and effective zener voltage Vzo from the equation or Rz = (Vz2 – Vz1)/(Iz2 – Iz1) and Vzo = Vz1 – Iz1*Rz.
- Calculate the value of the series resistor R (R 5 in Figure 4) for the shunt regulator with the equation
− Vin(min) is the minimum input voltage, Vin(min) = Vp – 2*0.7 – Vr, Vp is the peak input voltage or 10 volts in this lab, 0.7 volt is the voltage drop across one diode, Vr can be used as 2 volts for an estimation − Vzo and Rz are obtained in step 2 − Iz(min) is the minimum current needed for the zener diode to operate properly, for example, 5 mA is a good rating − IL (max) is the maximum load current and determined by Vo/Rmin. The output voltage of the shunt regulator is about the zener voltage used, Rmin is the minimum load resistance. In this lab, Vo ~ 3.3 volts and Rmin = 200 Ω
- Get a practical resistor with a value close but smaller than the resistance R calculated above. Use this value for R in all the following calculation and simulation.
- Calculate the capacitance required for the filter using C = Vp/(2fVr*R).
- Create a FWR and shunt regulator circuit in OrCAD-Capture as shown in Figure 4. R 5 and C 1 in Figure 4 should use the value of R and C obtained in step 4 and step 5, respectively. Dz1 is the 3.3 volts zener diode (D1N5226).
- Simulate the circuit with Rload = 200 Ω. Obtain a capture of the output voltage.
- Simulate the circuit with Rload = 500 Ω. Obtain a capture of the output voltage.
2.2 Measurements
Part 1:
- Build the hardware circuit of a full wave rectifier as shown in Figure 1. The input voltage Vin is a 10 volts peak, 60 Hz sinusoidal wave. Vin is stepped down from line voltage ( Hz and 110 Vrms) using a 15:1 turns-ratio transformer. Use 1N4004 diodes to construct your bridge rectifier. Observe the output voltage across the load resistor on the scope for
