Diodes are different and useful electrical components. Diodes are used in many applications like the following.
- Converting AC power from the 60Hz line into DC power for radios, televisions, telephone answering machines, computers, and many other electronic devices.
- Converting radio frequency signals into audible signals in radios.
- Given a circuit with a diode,
- Be able to use a simplified model of a diode to predict when current flows through the diode, and when it does not.
- Be able to use information about current flowing to predict other behavior in a circuit.
Diode Properties
Diodes have the following characteristics.
- Diodes are two terminal devices like resistors and capacitors. They don't have many terminals like transistors or integrated circuits.
- In diodes current is directly related to voltage, like in a resistor. They're not like capacitors where current is related to the time derivative of voltage or inductors where the derivative of current is related to voltage.
- In diodes the current is not linearly related to voltage, like in a resistor.
- Diodes only consume power. They don't produce power like a battery. They are said to be passive devices.
- Diodes are nonlinear, two terminal, passive electrical devices.
Note the following.
- When the voltage across the diode is positive, a lot of current can flow once the voltage becomes large enough.
- When the voltage across the diode is negative, virtually no current flows.
Thinking About Diodes
Diodes are a little schizophrenic.
- Sometimes they let a lot of current flow through them,
- Sometimes they permit hardly any current flow through them.
We're going to adopt a simplified model for the diode. Instead of the actual voltage-current curve for the diode shown in the thin, lighter red, curved line below, we're going to imagine that the diode has the voltage-current curve shown in the thicker, dark red lines below.
The approximate voltage-current curve gives us one way to analyze circuits that contain diodes, and to account for their schizophrenic behavior.
- When current is flowing, this approximate model predicts no voltage across the diode. In this situation, we say that the diode is ON.
- When the voltage across the diode is negative, this approximate model predicts no current flowing through the diode. In this situation, we say that the diode is OFF.
- When the diode is ON, it has no voltage across it so it acts like a short circuit! When the diode is ON, the current through the diode is positive, and the voltage across the diode is zero.
- When the diode is OFF, current is zero, so it acts like an open circuit! When the diode is OFF, the voltage across the diode is negative, and the current through the diode is zero.
Using Diodes
Now, let's examine a simple diode circuit. Remember what we know about ideal diodes. We will assume that the diode is ideal for the sake of argument.
- When the diode is ON, it has no voltage across it so it acts like a short circuit!
- When the diode is OFF, current is zero, so it acts like an open circuit!
It's just a diode and a resistor operating on an input voltage. We would like to determine how the output voltage depends upon the input voltage. We know something about the circuit.
- When the diode is ON, the voltage across it is zero because it acts like a short circuit.
- When the diode is OFF, the current through it is zero because it acts like an open circuit.
- We have one or the other of these two situations. It can't be both ways, and it has to be one or the other. That gives us a strategy that will let use figure out what happens in circuits with diodes.
- We can assume that the diode is ON and check whether that assumption is consistent with what else we know - KCL, KVL and the diode.
- We can assume that the diode is OFF and check whether that assumption is consistent with what else we know - KCL, KVL and the diode.
- We are using the method of contradiction to solve this problem. Click here for a short note on the method of contradiction.
We've replaced the diode with a short circuit below.
Since it's now a short circuit, Vd has to be zero. Let's think this through.
- The diode is ON and the voltage across it is zero.
- The current through the diode, Id, must be postive. It can't be negative. Current through a diode can never be negative.
- The current through the diode, Id, is V_{in}/R, (use Ohm's Law) so you cannot have a negative input voltage.
- That means that our assumption that the diode is ON has to be false for negative input voltages.
- The diode is ON for V_{in} > 0.
- The diode is OFF for V_{in} < 0.
- The diode is OFF and the current through it is zero.
- The voltage across the diode, V_{d}, must be negative. It can't be positive.
- The voltage across the diode, V_{d}, is just V_{in}, (use KVL) so you cannot have a positive input voltage.
- A positive input voltage is inconsistent with the assumption the diode is OFF.
- The diode is OFF for V_{in} < 0.
- The diode is ON for V_{in} > 0.
What can we conclude here?
- If the input voltage is positive, current flows through the diode, and the output voltage is equal to the input voltage.
- If the input voltage is negative, no current flows through the diode, and the output voltage is zero.
What If The Circuit Is More Complex?
If the circuit is more complex, then we still need to remember that every diode can be ON or it could be OFF. Here's a circuit with two diodes.
There are four combinations of diode states that can occur in this circuit. Let us examine all four possibilities. Here are the four combinations with each diode replaced by either a short circuit or an open circuit, depending upon whether we assume the diode is ON or OFF.
- D1 OFF, D2 OFF
- D1 OFF, D2 ON
- D1 ON, D2 OFF
- D1 ON, D2 ON
To determine how this circuit works, you'll have to check every possibility. We will start with the first case. In this situation, we have:
- D1 OFF, D2 OFF
In this case, both diodes are OFF.
- Since both diodes are OFF, there is no current though either diode. Consequently, there is no current through the resistor and V_{out} = 0.
- If V_{out} = 0, we have enough information to compute the voltage across each diode assuming that we know the input voltages.
- We can write KVL around either of two loops, and each loop will contain just one diode.
- Around the first loop we have:
- V_{D1} = V_{1} - V_{out} = V_{1}
- Since the voltage across the diode must be negative when there is no current through the diode we must have V_{1} < 0.
- Around the second loop we have:
- V_{D2} = V_{2} - V_{out} = V_{2}
- Since the voltage across the diode must be negative when there is no current through the diode we must have V_{2} < 0.
- We conclude:
- V_{out} = 0 when V_{1} < 0 and V_{2} < 0.
- In words, the output voltage is zero when both input voltages are negative.
- D1 OFF, D2 ON
- Since D_{2} is ON, it has been replaced by a short circuit, and that makes V_{out} = V_{2}.
- If D_{2} is ON, the current must be positive, and that will occur only when V_{2} > 0.
- If V_{out}= V_{2}, we have enough information to compute the voltage across D_{1}.
- We can write KVL around the loop that contains the resistor and D_{1}. Around that loop we have:
- V_{D1} = V_{1} - V_{out} = V_{1}- V_{2}
- Since the voltage across a diode that is OFF must be negative, we have to have V_{1}< V_{2}.
- In words, when V_{2} is positive and we have V_{1}< V_{2}, the output will be V_{2}.
- D1 ON, D2 OFF
This case is exactly the same as the second case except that the two diodes are reversed. The same argument we used for the second case works here with 1s and 2s interchanged, so we conclude:
- In words, when V_{1} is positive and we have V_{2}< V_{1}, the output will be V_{1}.
- D1 ON, D2 ON
- Since both diodes are ON, both diodes have been replaced by short circuits.
- The output voltage, V_{out}, is equal to both V_{1} and V_{2}.
- The only way that can happen is if we have, V_{out} = V_{1} = V_{2}.
- In words, when both input voltages are equal, that is what the output voltage becomes.
- Given the diode circuit:\ below, and assuming that the diodes are ideal,
- When both input voltages are negative the output is zero.
- When either or both input voltages are positive, the output voltage is equal to the larger of the two input voltages.
What If I Want A Better Diode Model?
We've been operating on the assumption that the diodes all act like our ideal model which has no voltage drop in the forward direction - when current flows. The ideal model, and a theoretical voltage-current curve are shown below.
This is the model we've been working with. A better - but still not exact model - is shown below. You can see the model by clicking the small red button at the bottom right of the graph.
This, new and improved - but not perfect - model can be modelled in terms of the first model we used - the ideal diode. (It's not a perfect model of the diode because - as you can see - the two straight lines do not model the "corner" in the curve to perfection.) A circuit model that gives the better voltage current curve is shown below - within the dotted lines around the circuit model.
The diode inside the model is ideal, in the sense that it has no forward drop across it when current flows through it. The source in series with the ideal diode serves to account for the forward voltage drop - assumed constant in this model. Note that the added voltage source serves to oppose the flow of curent until the voltage applied to the diode exceeds the threshold voltage, V,. In the model above, the threshold voltage is 0.8v.
There are still better models for diodes. The diode has a nonlinear capacitance associated with it, for example. You might want a more detailed model for the diode if you were using a simulation program and you wanted the results to be as exact as possible. There are lots of other effects that could be modelled. However, that's a topic for another lesson, another day. That's it for this lesson.
However, before you leave this lesson, be assured that the model we now have, and even the ideal diode model can often be used to predict performance of circuits with diodes, and they can help you understand those circuits.
دسته بندی : رباتیک ,