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Basic form and application circuit of a typical optocoupler. Figure 2 shows the basic form of an optocoupler, together with a very simple application circuit. Here, when SW1 is open, no current flows in the LED, so no light falls on the face of Q1; Q1 passes virtually zero collector current under this condition, so zero voltage is developed across output resistor R2.

Major points to note about the Figure 2 optocoupler are that its output current is controlled by its input current, that a control circuit connected to its input can be electrically fully isolated from the output circuit, and that — since the input controls the output via a purely optical link — potential differences of hundreds of volts can safely exist between the input and output circuits.

Optocouplers can also be used to replace low-power relays and pulse transformers in many applications. Figures 3 and 4 show two other types of optocoupler. The device shown in Figure 3 is known as a slotted optocoupler, and has a slot molded into the package between the LED light source and the phototransistor light sensor. Here, light can normally pass from the LED to Q1 without significant attenuation by the slot.

The optocoupling can, however, be completely blocked by placing an opaque object in the slot. Slotted optocoupler device. Reflective optocoupler. The device shown in Figure 4 is known as a reflective optocoupler. Here, the LED and Q1 are optically screened from each other within the package, and both face outwards towards a common point from the package. The construction is such that an optocoupled link can be set up by a reflective object such as metallic paint or tape, or even smoke particles sited a short distance outside the package, in line with both the LED and Q1.

The reflective optocoupler can thus be used in applications such as tape-position detection, engine-shaft revolution counting or speed measurement, or smoke or fog detection, etc. One of the most important parameters of an optocoupler device is its optocoupling efficiency and, to maximize this parameter, the LED and the phototransistor which usually operate in the infrared range are always closely matched spectrally.

The most convenient way of specifying optocoupling efficiency is to quote the output-to-input current transfer ratio CTR of the device, i. In practice, CTR may be expressed as a simple figure such as 0. It should be noted that, because of variations in LED radiation efficiency and phototransistor current gains, the actual CTR values of individual optocouplers may vary significantly from the typical value.

This is the maximum permissible DC potential that can be allowed to exist between the input and output circuits. Typical values vary from V to 4kV. This is the maximum allowable DC voltage that can be applied across the output transistor. Typical values vary from 20V to 80V. IF MAX. Typical values vary from 40mA to mA. This is the typical maximum signal frequency that can be usefully passed through the optocoupler when the device is operated in its normal mode. Typical values vary from 20kHz to kHz, depending on the type of device construction.

Typical simple a and Darlington b isolating optocouplers. Optocouplers are produced by several manufacturers and are available in a variety of forms and styles. Simple optocouplers are widely available in six basic forms, which are illustrated in Figures 6 to 8.

Four of these Figures 6 and 7 are isolating optocouplers, and the remaining two are the slotted optocoupler Figure 8 a and the reflective optocoupler Figure 8 b. The table of Figure 9 lists the typical parameter values of these six devices. The simple isolating optocoupler Figure 6 a uses a single phototransistor output stage and is usually housed in a six-pin package, with the base terminal of the phototransistor externally available.

The phototransistor can, however, be converted to a photodiode by shorting the base pin 6 and emitter pin 4 terminals together; under this condition the CTR value falls to about 0. Typical dual a and quad b isolating optocouplers. The Darlington optocoupler Figure 6 b is also housed in a six-pin package and has its phototransistor base externally available. The dual and quad optocouplers of Figure 7 use single-transistor output stages in which the base terminal is not externally available.

Note in all four isolating devices that the input pins are on one side of the package, and the output pins are on the other. This construction gives the maximum possible values of isolating voltage. Also note in the multichannel devices of Figure 7 that, although these devices have isolating voltages of 1. Typical slotted a and reflective b optocouplers. Isolating voltage values are not specified for the slotted and reflective optocoupler devices of Figure 8.

Finally, the reflective optocoupler of Figure 8 b uses a Darlington output stage and has a useful bandwidth of only 20kHz. Even so, the device has a typical minimum CTR value of only 0. Typical parameter values of the Figures 6 to 8 devices. The corresponding input-output waveforms are shown in Figure 6 f : 4.

Such a circuit of half-wave rectifier is shown in Figure 5 g. The op-amp output turns out to be positive only during the positive half cycle of the input waveform, which eventually makes the diode D1 forward biased and the positive half cycle waveform appears across the output terminal [ 30 ]. In contrast, during the negative half cycle, the op-amp output becomes negative, turning OFF diode D1.

This will not allow the negative portion of input to appear across the output. The overall outcome is impeccable half-wave rectification, as epitomized in Figure 6 g. Such a full-wave rectifier circuit is shown in Figure 5 h , which is actually the combination of two half-wave rectifier circuits.

This is achieved using two different op-amps A1, A2 and diodes D1, D2. The op-amp A1 output turns out to be positive only during the positive half cycle of the input waveform. The voltage at the inverting terminal of A2 appears at the output, which is equal to the input, due to the virtual ground at the two input terminals of op-amp A2. Consider now the negative cycle of the input waveform. This causes output voltage to be equal to the negative of the input voltage at A1 and thus positive.

Therefore, outputs during two half cycles are same and full-wave-rectified output voltage is obtained as shown in Figure 6 h. If such a circuit is implemented using op-amp, a diode is used at the output of op-amp, to remove off some portion of the input signal. This type of circuit confiscates negative peak of the input signal. The value of reference voltage Vref decides the clipping portion. The positive input gets grounded initially when the Vref is zero. Whenever input Vin is positive, the output will follow the input Vin, since the op-amp output becomes positive, turning ON the diode.

The op-amp output will go negative during negative half cycle of the input. In this case, the final output Vo will follow the negative half cycle of the input till the input voltage is greater than the reference voltage only. Hence the portion of the negative half cycle will be clipped for which input voltage becomes less than the reference voltage. In order to adjust the clipping level, Vref needs to be adjusted as per requirement.

In this case, both AC and DC input voltages need to be applied and output of the clamper is a net result of AC and DC input voltages applied to the inverting terminal, and a potentiometer is connected to noninverting input terminals to vary Vref. The output voltage Vo will also be positive, which turns ON the diode D1 completing the feedback loop.

The op-amp then works as a voltage follower, since capacitor C1 blocks DC voltage and Vo becomes equal to Vref. Now, consider the voltage Vin at the inverting input. During its negative half cycle, capacitor C1 will charge to the negative peak value of the voltage VP since diode D1 starts conducting. The peak voltage VP across the capacitor acquired during the negative half cycle is retained because diode D1 turns OFF during the positive half cycle of Vin [ 40 ]. The input and output waveforms are shown in Figure 6 j.

This type of comparator is called as noninverting comparator. Thus, Vo varies from one level to another level whenever input increases or decreases with respect to Vref as shown in Figures 6 k and 6 l. This circuit detects the positive peak of the input signal.

The capacitor C1 gets charged to the peak value Vpp of the input voltage, whenever diode D1 becomes forward biased during every positive half cycle of the input. Thus, the capacitor retains the peak value even as the waveform drops to zero.

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