Diode Guide

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EECS 100 Spring 2004 Muthuswamy, Bharathwaj Diodes: Experiment Guide Components required for this lab: 1. 2. 3. 4. 5. I. Diodes Overview Diodes are mostly used in practice for emitting light (as Light Emitting Diodes, LEDs) or controlling voltages in various circuits. The best way to think about diodes is to first understand what happens with an ideal diode and then to extend it to the practical case. An ideal diode has an infinite resistance when the voltage across it is less than its “thresh
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  EECS 100 Spring 2004 Muthuswamy, Bharathwaj  1 Diodes: Experiment Guide   Components required for this lab:1. 1N4148 diode (x 1)2. 1k resistor (x 1)3. 1M resistor (x 1)4. 22u capacitor (x 1)5. 10M resistor (x 1)I. Diodes Overview Diodes are mostly used in practice for emitting light (as Light Emitting Diodes, LEDs) or controlling voltages in various circuits. The best way to think about diodes is to firstunderstand what happens with an ideal diode and then to extend it to the practical case. An ideal diode has an infinite resistance when the voltage across it is less than its“threshold voltage” (or v threshold ) and zero resistance when the voltage is greater than thethreshold. The threshold voltage is just a characteristic of each individual diode i.e. every1N4148 diode should have the same threshold voltage (around 0.6 volts) whereas an LEDmay have a different threshold voltage. This threshold voltage concept comes from the factthat a diode is just a  pn junction. Don’t feel bad if you haven’t studied pn junctions before;it is not required for this lab. The I-V graph for an ideal diode looks like: Figure 1. Ideal Diode I-V Curve and Symbol   In the above graph, the threshold voltage (i.e. the voltage when the slope of the linechanges from 0 to ∞ ) is at 0. This will not be the case for the real diodes we use in lab.For the diodes we will use in this lab, all threshold voltages will be positive (Zener diodeshave a low reverse threshold – you will deal with them later). We will see shortly that thebehavior of diodes is actually somewhat like a switch, and so there are some easy ways toanalyze circuits with diodes in them.  EECS 100 Spring 2004 Muthuswamy, Bharathwaj  2 II. Diode I-V characteristics The I-V graph for a non-ideal diode is shown in figure 2, along with an ideal approximationto accommodate the non-zero threshold voltage. The diode will be easier to understand if we compare the diode to another two terminal device we know (and love) the resistor. Figure 2. Non-Ideal Diode I-V Curve and an approximation to the non-ideal diodeFigure 3. The resistor vs. the diode From figure 3, we see that both diodes and resistors are two terminal devices. However,their I-V characteristics are very different. An equation that models the I-V characteristic of a non-ideal diode is shown below. threshold  Dthreshold  DvvS  D vvif  vvif  e I i th D <=≥= 0   If v D is greater than v threshold , then the diode is said to be forward-biased or it is said to be inthe forward-biased region. If not, the diode is said to be operating in reverse-bias. Also, inthe equation above:  EECS 100 Spring 2004 Muthuswamy, Bharathwaj  3 ã I S is a constant called the reverse bias saturation current and is approximately equalto 1 x 10 -11 A ã V th is a constant called the thermal voltage ( this is different from the thresholdvoltage ) and is approximately equal to 26 mV at room temperature.So, what makes a diode hard to deal with? The diode equation above is very hard to solvein practice because it is non-linear. For instance, let us try and solve for the voltage acrossthe resistor (Vload) in figure 4 if Vin = 3 V and R=1k:Vload = i(1k)The current through the resistor is the same as the current flowing through the diode.However, we first have to figure out if the diode is on (current is flowing through it) or off (nocurrent flows through the diode). You can’t readily tell since you don’t know the voltageacross the diode. If you did, you could compare it to the threshold voltage. Usually, youdon’t know the voltage across a diode. Thus, there are no hard and fast rules for determining whether a diode is on or off. A standard method is to use the ideal diodemodel first to figure out which diodes in a circuit are on and which are off. Then, if necessary, you solve for the exact value of the current through the diode. Let us assumethe diode is on. Then, the current through the diode is: th D vvS  e I i =  and v D is 3 – Vload (KVL). Thus, we have to solve the following equation: mV Vload  e xk Vload  26)3(11 ).101( 1 −− =  The above equation is a recursive non-linear  1 equation. Mathematical techniques for solving the above equation are beyond the scope of this class. I solved the equation aboveusing my calculator and obtained:Vload ≈ 2.497 voltsSolving non-linear equations in general is very difficult. You can imagine what wouldhappen if we have multiple diodes in our circuit. Hence, the ideal model shown in figures 1and 2 is very helpful. You usually use the model in figure 1. The approximation in figure 2is used if we need to take into account the threshold voltage. The circuit models for figures1 and 2 are shown below. Make sure you understand them. If you have any questions,ask your TA before the lab starts. 1 It is recursive because the unknown variable is on both sides of the equation. It is non-linear because the function inthe equation is not a straight line.  EECS 100 Spring 2004 Muthuswamy, Bharathwaj  4   Figure 4. Ideal diode model without threshold voltageFigure 5. Ideal diode model with threshold voltage One more property of the diode - looking at figures 4 and 5, if you think about the diodesymbol as an arrow - you can infer that current can flow through the diode only in thedirection of the arrow. Let us apply these two models and study the very practical diodecircuit shown in figure 6 – the half-wave rectifier.
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