As we know that one component of reverse current in a diode is the flow of minority carriers. These carriers exist because thermal energy keeps dislodging valence electrons from their orbits, producing free electrons and holes in the process.
The lifetime of the minority carriers is short, but while they exist, they can contribute to the reverse current.
When light energy bombards a pn junction, it can dislodge valence electrons. The more light striking the junction, the larger the reverse current in a diode.
A photodiode has been optimized for its sensitivity to light. In this diode, a window lets light pass through the package to the junction. The incoming light produces free electrons and holes. The stronger the light, the greater the number of minority carriers and the larger the reverse current.
Figure 1 shows the schematic symbol of a photodiode. The arrows represent the incoming light. Especially important, the source and the series resistor reverse-bias the photodiode. As the light becomes brighter, the reverse current increases. With typical photodiodes, the reverse current is in the tens of microamperes.
As we know that, a transistor with an open base has a small collector current consisting of thermally produced minority carriers and surface leakage. By exposing the collector junction to light, a manufacturer can produce a phototransistor, a device that has more sensitivity to light than a photodiode.
Figure 2(a) shows a transistor with an open base. As mentioned earlier, a small collector current exists in this circuit. Ignore the surface-leakage component, and concentrate on the thermally produced carriers in the collector diode.
Visualize the reverse current produced by these carriers as an ideal current source in parallel with the collector-base junction of an ideal transistor [Fig. 2(b)].
Because the base lead is open, all the reverse current is forced into the base of the transistor. The resulting collector current is:
ICEO = βdcIR
Where IR is the reverse minority carrier current. This says that the collector current is higher than the original reverse current by a factor of βdc.
The collector diode is sensitive to light as well as heat. In a phototransistor, light passes through a window and strikes the collector-base junction. As the light increases, IR increases, and so does ICEO.
Phototransistor versus Photodiode
The main difference between a phototransistor and a photodiode is the current gain βdc. The same amount of light striking both devices produces βdc times more current in a phototransistor than in a photodiode. The increased sensitivity of a phototransistor is a big advantage over that of a photodiode.
Figure 3(a) shows the schematic symbol of a phototransistor. Notice the open base. This is the usual way to operate a phototransistor. You can control the sensitivity with a variable base return resistor (Fig. 3b), but the base is usually left open to get maximum sensitivity to light.
The price paid for increased sensitivity is reduced speed. A phototransistor is more sensitive than a photodiode, but it cannot turn on and off as fast. A photodiode has typical output currents in microamperes and can switch on and off in nanoseconds. The phototransistor has typical output currents in milliamperes but switches on and off in microseconds.
An optocoupler (also called an optoisolator) combines an LED and a photodiode in a single package. Figure 4 shows an optocoupler. It has an LED on the input side and a photodiode on the output side.
The left source voltage and the series resistor set up a current through the LED. Then the light from the LED hits the photodiode, and this sets up a reverse current in the output circuit.
This reverse current produces a voltage across the output resistor. The output voltage then equals the output supply voltage minus the voltage across the resistor.
When the input voltage is varying, the amount of light is fluctuating. This means that the output voltage is varying in step with the input voltage. This is why the combination of an LED and a photodiode is called an optocoupler. The device can couple an input signal to the output circuit.
Other types of optocouplers use phototransistors, photothyristors, and other photo devices in their output circuit side.
The key advantage of an optocoupler is the electrical isolation between the input and output circuits. With an optocoupler, the only contact between the input and the output is a beam of light.
Because of this, it is possible to have an insulation resistance between the two circuits in the thousands of megohms. Isolation like this is useful in high-voltage applications in which the potentials of the two circuits may differ by several thousand volts.
Figure 5 shows an LED driving a phototransistor. This is a much more sensitive optocoupler than the LED-photodiode. The idea is straightforward.
Any changes in VS produce changes in the LED current, which changes the current through the phototransistor. In turn, this produces a changing voltage across the collector-emitter terminals. Therefore, a signal voltage is coupled from the input circuit to the output circuit.
Again, the big advantage of an optocoupler is the electrical isolation between the input and output circuits. Stated another way, the common for the input circuit is different from the common for the output circuit.
Because of this, no conductive path exists between the two circuits. This means that you can ground one of the circuits and float the other. For instance, the input circuit can be grounded to the chassis of the equipment, while the common of the output side is ungrounded.
Zero Crossing Detector with Optocoupler
The 4N24 optocoupler in Fig. 6(a) provides isolation from the power line and detects zero crossings of line voltage. The graph in Fig. 6(b) shows how the collector current is related to the LED current.
Here is how you can calculate the peak output voltage from the optocoupler: The bridge rectifier produces a full-wave current through the LED. Ignoring diode drops, the peak current through the LED is:
Figure 6(b) shows the static curves of phototransistor current versus LED current for three different optocouplers. With a 4N24 (top curve), an LED current of 10.2 mA produces a collector current of approximately 15 mA when the load resistance is zero.
In Fig. 6(a), the phototransistor current never reaches 15 mA because the phototransistor saturates at 2 mA. In other words, there is more than enough LED current to produce saturation.
Since the peak LED current is 10.2 mA, the transistor is saturated during most of the cycle. At this time, the output voltage is approximately zero, as shown in Fig. 6(c).
The zero crossings occur when the line voltage is changing polarity, from positive to negative, or vice versa. At a zero crossing, the LED current drops to zero. At this instant, the phototransistor becomes an open circuit, and the output voltage increases to approximately 20 V, as indicated in Fig. 6(c).
As you can see, the output voltage is near zero most of the cycle. At the zero crossings, it increases rapidly to 20 V and then decreases to the baseline.
A circuit like Fig. 6(a) is useful because it does not require a transformer to provide isolation from the line. The photocoupler takes care of this.
Furthermore, the circuit detects zero crossings, desirable in applications where you want to synchronize some other circuit to the frequency of the line voltage.