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Electronic Instrumentation Experiment 6: Diodes * Part A: Diode I-V Characteristics * Part B: Rectifiers •Part C: PN Junction Voltage Limitation •Part D: Zener Diode Voltage Regulator Diodes D1 ANODE CATHODE DIODE A diode can be considered to be an electrical one-way valve. They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide … Diodes In effect, diodes act like a flapper valve • Note: this is the simplest possible model of a diode Diodes For the flapper valve, a small positive pressure is required to open. Likewise, for a diode, a small positive voltage is required to turn it on. This voltage is like the voltage required to power some electrical device. It is used up turning the device on so the voltages at the two ends of the diode will differ. • The voltage required to turn on a diode is typically around 0.6-0.8 volt for a standard silicon diode and a few volts for a light emitting diode (LED) Diodes D1 D1N4002 VAMPL = 10V V1 R1 FREQ = 1k 1k 0 10 volt sinusoidal voltage source Connect to a resistive load through a diode • This combination is called a half-wave rectifier Diodes VAMPL = 10V V1 Sinusoidal Voltage FREQ = 1k 10V 5V 0V -5V -10V 0s 0.5ms 1.0ms 1.5ms V(D1:1) Time 2.0ms 2.5ms 3.0ms Diodes VAMPL = 10V D1 V V1 D1N4002 V R1 FREQ = 1k 1k Half-wave rectifier 0 10V 5V 0V -5V -10V 0s 0.5ms V(D1:1) 1.0ms 1.5ms V(D1:2) Time 2.0ms 2.5ms 3.0ms At the junction, free electrons from the N-type material fill holes from the Ptype material. This creates an insulating layer in the middle of the diode called the depletion zone. Part A: Diode i-v Characteristics Diode V-I Characteristic For ideal diode, current flows only one way Real diode is close to ideal Ideal Diode Diode Characteristics A very large current can flow when the diode is forward biased. For power diodes, currents of a few amps can flow with bias voltages of 0.6 to 1.5V. Note that the textbook generally uses 0.6V as the standard value, but 0.7V is more typical for the devices we will use in class. Reverse breakdown voltages can be as low as 50V and as large as 1000V. Reverse saturation currents Is are around 1nA. Diode Characteristics The iD-vD relationship (excluding breakdown) can be written simply as: vD iD I s exp 1 nVT Note that for vD less than zero, the exponential term vanishes and the current iD is roughly equal to the saturation current. For vD greater than zero, the current increases exponentially. Diode Characteristics Recall that the i-v relationship for a resistor is given by Ohm’s Law: v=Ri If we plot this expression, we obtain i v The slope of the straight line is given by the resistance R i-v Characteristics PSpice can be used to obtain such plots 10mA 5mA V-I Characteristic of a 500 Ohm Resistor 0A -5mA -10mA -6.0V I(R1) -4.0V -2.0V 0V 2.0V 4.0V V(R1:1) - V(R1:2) R1 500 V1 15V R2 1k 0 6.0V R1 V 5Vdc V 1k V2 D2 D1N4148 i-v Characteristics 0 19m 16m 12m vD iD I s exp 1 nVT 8m 4m iD 0 -16V -14V I(D2) (7e-9)*(exp( -12V -10V -8V -6V -4V -2V 0V V(D2:1)/(.05107))-1) V(D2:1) Both the simulated current vs. voltage and the characteristic equation for the diode are plotted 2V i-v Characteristics In this experiment, you are asked to find the parameters for the equation vD iD I s exp 1 nVT That is, you need to find the constants in this equation so that it matches what PSpice determines. Note that VT=25mV, so you need to find n and Is i-v Characteristics The i-v characteristic can be checked by building the circuit and measuring the same two voltages shown on the diode circuit. From these voltages and the value of the resistance, both the current through the diode and the voltage across the diode can be determined. Part B: Rectifiers Rectifiers As noted above, the main purpose of diodes is to limit the flow of current to one direction. Since current will flow in only one direction, even for a sinusoidal voltage source, all voltages across resistors will have the same sign. Thus, a voltage which alternately takes positive and negative values is converted into a voltage that is either just positive or just negative. Rectifiers If a time-varying voltage is only positive or only negative all of the time, then it will have a DC offset, even if the original voltage had no offset. Thus, by rectifying a sinusoidal signal and then filtering out the remaining timevarying signal, we obtain a DC voltage from an AC source. Diodes – Recall from Previous Slide D1 D1N4002 VAMPL = 10V V1 R1 FREQ = 1k 1k 0 10 volt sinusoidal voltage source Connect to a resistive load through a diode • This combination is called a half-wave rectifier Diodes VAMPL = 10V V1 Sinusoidal Voltage FREQ = 1k 10V 5V 0V -5V -10V 0s 0.5ms 1.0ms 1.5ms V(D1:1) Time 2.0ms 2.5ms 3.0ms Diodes VAMPL = 10V D1 V V1 D1N4002 V R1 FREQ = 1k 1k Half-wave rectifier 0 10V 5V 0V -5V -10V 0s 0.5ms V(D1:1) 1.0ms 1.5ms V(D1:2) Time 2.0ms 2.5ms 3.0ms Diodes Note that the resulting voltage is only positive and a little smaller than the original voltage, since a small voltage (around 0.7V) is required to turn on the diode. 0.7V 10V 5V 0V -5V -10V 0s 0.5ms V(D1:1) 1.0ms 1.5ms V(D1:2) Time 2.0ms 2.5ms 3.0ms Diodes Filtering can be performed by adding a capacitor across the load resistor D1 D1N4148 V2 R1 1k 0 C1 47uF Do you recognize this RC combination as a low pass filter? You will see how this looks both with PSpice and experimentally Diodes The rectifier we have just seen is called a half-wave rectifier since it only uses half of the sinusoidal voltage A full-wave rectifier uses both the positive and negative half cycles of the sinusoid Full-Wave Rectifier Shown are the original voltage, the rectified voltage and the smoothed voltage Capacitor Discharging Full Wave Rectifier R3 50 V D1N4148 D5 D6 R4 V2 VOFF = 0 VAMPL = 10 FREQ = 1k 10k V- D7 V+ D8 D1N4148 0 Note path of current when source is positive Full Wave Rectifier 10V 1.4V (2 diodes) 5V 0V -5V -10V 110.0ms V(D5:2) 110.5ms 111.0ms 111.5ms V(R4:2,D7:1) Time 112.0ms 112.5ms 113.0ms Full Wave Rectifier With Smoothing R1 50 D1N4148 D1 D3 R2 V1 VOFF = 0 VAMPL = 10 FREQ = 1k 10k C1 D4 0.1uF D1N4148 0 D2 Full Wave Rectifier With Smoothing Smoothed Voltage 10V 5V 0V -5V -10V 110.0ms V(R1:2) 110.5ms V(R2:2,D1:1) 111.0ms 111.5ms V(R4:2,D7:1) Time 112.0ms 112.5ms 113.0ms Part C: PN Junction Voltage Limitation Voltage Limitation In many applications, we need to protect our circuits so that large voltages are not applied to their inputs We can keep voltages below 0.7V by placing two diodes across the load R1 A B 1k V1 D1 D1N4148 0 D2 D1N4148 Voltage Limitation When the source voltage is smaller than 0.7V, the voltage across the diodes will be equal to the source When the source voltage is larger than 0.7V, the voltage across the diodes will be 0.7V The sinusoidal source will be badly distorted into almost a square wave, but the voltage will not be allowed to exceed 0.7 V You will observe this with both PSpice and experimentally Voltage Limitation R1 A B 1k V1 D1 D1N4148 D2 D1N4148 0 Case 1: The magnitude of the diode voltage is less than 0.7 V (turn on voltage) R1 1k 100mVdc V1 0 Diodes act like open circuits Voltage Limitation R1 A B 1k V1 D1 D1N4148 D2 D1N4148 0 Case 2: The magnitude of the diode voltage is greater than 0.7 V (turn on voltage) Diodes act like voltage sources Voltage Limitation R1 1k 10Vdc V1 V2 0.7Vdc 0 Case 2: The current drawn by the diode is given by the resistor current V 10 0.7 I 9.3mA R 1000 Voltage Limitation R1 1k V V3 V VOFF = 0 VAMPL = 10 FREQ = 1k D1 D2 D1N4148 D1N4148 0 10V 5V (1.2420m,718.277m) 0V -5V -10V 0s 0.5ms V(R1:1) 1.0ms 1.5ms V(R1:2) Time 2.0ms 2.5ms 3.0ms Input Protection Circuits More than one diode can be connected in series to increase the range of permitted voltages Part D: Zener Diodes Zener Diodes Up to this point, we have not taken full advantage of the reverse biased part of the diode characteristic. Ideal Zener Diode I -VZ V Zener Diodes For the 1N4148 diode, the breakdown voltage is very large. If we can build a different type of diode with this voltage in a useful range (a few volts to a few hundred volts), we can use such devices to regulate voltages. This type of diode is called a Zener diode because of how the device is made. Zener Diodes R1 A B 1k V1 D1 1V D1N750 0 You will again find the i-v characteristic with PSpice and experimentally. Such circuits can be used in combination with the rectifier and filtering to obtain a well regulated DC voltage. Zener Diodes Knee Current Note that, for a real Zener diode, a finite current (called the knee current) is required to get into the region of voltage regulation VZ is the Zener Voltage Zener Diodes R1 1k V V D1 V1 VOFF = 0 VAMPL = 10 FREQ = 1k D1N750 Note the voltage limitation for both positive and negative source voltages 0 10V 5V 0V -5V -10V 0s 0.5ms V(R1:1) 1.0ms 1.5ms V(D1:2) Time 2.0ms 2.5ms 3.0ms Zener Diode Voltage Regulation 8.0V R5 50 V D1N4148 D9 D10 D1N4148 V3 VOFF = 0 VAMPL = 10 FREQ = 1k 4.0V D12 D1N4148 D1N4148 0V 0 C4 Note stable voltage 1mF R7 10 -4.0V D14 D1N750 R8 -8.0V 110.0ms 10k V- V+ V(D10:1) 110.5ms 111.0ms 111.5ms V(R8:1,R8:2) Time Full Wave Rectifier