Transistor Optocoupler

LED forward voltage and trigger current
This tells you how you need to power your input LED to ensure it turns on and provides the desired switching behavior. In transistor optocouplers designed to be switched with a square wave or PWM signal, the peak forward current required to trigger the switch depends on the pulse width of the signal in the ON state. Shorter pulses require larger peak signal current to force triggering.

Output-to-input current ratio
This tells you the current transfer between each end of the optocoupler. Note that this is dependent on the absolute maximum collector-emitter voltage for a phototransistor transistor optocoupler.

Forward voltage VS. forward current curve
This specification has the same meaning as that for a standard LED, but it should not be confused with the trigger current.

Temperature variations
These specifications are quite important for power systems as they can reach high temperature during operation.

Safety ratings and IEC/UL certification
If you’re designing for a power system or for data transfer in a high voltage environment near AC mains, IEC 60747-5-2 is one important standard to watch for to ensure high transient voltages can be withstood. You need to follow the safety and insulation guidelines to ensure you’re compliant with safety standards.

Data rate or switch speed
Components that are intended for use in data networks will normally specify a maximum data rate, although a switching speed or frequency could also be specified.

Input / Output Specifications
Transistor optocouplers are often specified by their input and output circuit configurations. For example, input specs consist of information about the light source, such as an LED's forward current, power dissipation, or wavelength. Output specs often include similar information about the device's detector. One specification common to both circuits is isolation voltage.
Isolation voltage is sometimes referred to as input to output isolation voltage and is one of the more important optocoupler specifications. Isolation voltage represents the maximum voltage that can be applied to both the input and output circuits while still maintaining electrical isolation.

Current-transfer Ratio
Current-transfer ratio, or CTR, describes the relationship between the output current and the input current that caused it. This is a minimum value expressed as a percentage of the input current. Typical CTR specs are around 10-50%; these devices operate similarly to step-down isolation transformers. Transistor optocouplers designed to step-up current into the output circuit, often those with photodarlington outputs, can reach 600% or more. Current-transfer ratio reaches its maximum value when the input light source is brightest. Knowing a device's CTR is essential to configuring it in order to effectively control the output current.

Standards
Transistor optocouplers may be designed and manufactured to meet one or many standards. In particular, the SMD 5962 family of standards includes different optocoupler product designs that conform to MIL PRF 38534 (General specifications for microcircuits).

Digital interfacing
Transistor optocouplers are ideally suited for use in digital interfacing applications in which the input and output circuits are driven by different power supplies. They can be used to interface digital ICs of the same family (TTL, CMOS, etc.) or digital ICs of different families, or to interface the digital outputs of home computers, etc., to motors, relays, and lamps, etc. This interfacing can be achieved using various special-purpose ‘digital interfacing' optocoupler devices, or by using standard optocouplers; Figures 14 to 16 show circuits of the latter type.
Figure 14 shows how to interface two TTL circuits, using an optocoupler circuit that provides a non-inverting action. Here, the optocoupler LED and current-limiting resistor R1 are connected between the 5V positive supply rail and the output-driving terminal of the TTL device (rather than between the TTL output and ground), because TTL outputs can usually sink a fairly high current (typically 16mA) but can source only a very low current (typically 400µA).

The open-circuit output voltage of a TTL IC falls to less than 0.4V when in the logic-0 state, but may rise to only 2.4V in the logic-1 state if the IC is not fitted with an internal pull-up resistor. In such a case, the transistor optocoupler LED current will not fall to zero when the TTL output is at logic-1. This snag is overcome in the Figure 14 circuit by fitting an external pull-up resistor (R3) as shown. The Figure 14 circuit’s optocoupler phototransistor is wired between the input and ground of the driven (right-hand) TTL IC because a TTL input needs to be pulled down to below 800mV at 1.6mA to ensure correct logic-0 operation.
CMOS IC outputs can source or sink currents (up to several mA) with equal ease. Consequently, these devices can be interfaced by using a sink configuration similar to that of Figure 14, or they can use the source configuration shown in Figure 15. In either case, the R2 value must be large enough to provide an output voltage swing that switches fully between the CMOS logic-0 and logic-1 states.

Figure 16 shows how the transistor optocoupler can be used to interface a computer's output signal (5V, 5mA) to a 12V DC motor that draws an operating current of less than 1A. With the computer output high, the optocoupler LED and phototransistor are both off, so the motor is driven on via Q1 and Q2. When the computer output goes low, the LED and phototransistor are driven on, so Q1-Q2 and the motor are cut off. The reverse of this action can be obtained by wiring the optocoupler's output in series between R2 and Q1-base, so that Q1-Q2 and the motor turn on only when the computer output goes low.

Analog interfacing
An transistor optocoupler can be used to interface analog signals from one circuit to another by setting up a standing current through the LED and then modulating this current with the analog signal. Figure 17 shows this technique used to make an audio-coupling circuit.

Here, the op-amp is connected in the unity-gain voltage follower mode, with the transistor optocoupler LED wired into its negative feedback loop so that the voltage across R3 (and thus the current through the LED) precisely follows the voltage applied to the op-amp’s pin 3 non-inverting input terminal. This terminal is DC-biased at half-supply volts via the R1-R2 potential divider, and can be AC-modulated by an audio signal applied via C1. The quiescent LED current is set at 1 to 2 mA via R3.
On the output side of the optocoupler, a quiescent current is set up (by the optocoupler action) in the phototransistor, and causes a quiescent voltage to be set up across RV1, which should have its value adjusted to give a quiescent output value of half-supply voltage. The audio output signal appears across RV1 and is DC-decoupled via C2.

Triac interfacing
An ideal application for the transistor optocoupler is that of interfacing the output of a low-voltage control circuit (possible with one side of its power supply grounded) to the input of a triac power-control circuit that is driven from the AC power lines and which can be used to control the power feed to lamps, heaters, and motors. Figure 18 shows an example of such a circuit; the figures in parenthesis show the component values that should be used if 115V AC (rather than 230V) supplies are used; the actual triac type must be chosen to suit individual load/supply requirements.

The Figure 18 circuit gives a non-synchronous switching action in which the triac’s initial switch-on point is not synchronized to the AC power line waveform. Here, R2-D1-ZD1 and C1 are used to develop an AC-derived 10V DC supply, which can be fed to the triac gate via Q1 and hence be used to turn the triac on and off. Thus, when SW1 is open, the optocoupler is off, so zero base drive is applied to Q1, and the triac and load are off. When SW1 is closed, the optocoupler drives Q1 on and connects the 10V DC supply to the triac gate via R3, thus applying full AC mains power to the load.

Optocoupled scrs and triacs
SCRs (silicon controlled rectifiers) and triacs are semiconductor power-switching devices that (like transistors) are inherently photosensitive. An transistor optocoupler SCR is simply an SCR and an LED mounted in a single package, and an optocoupled triac is simply a triac and an LED mounted in a single package. Such devices are readily available, in both simple and complex forms; some sophisticated triac types incorporate interference-suppressing, zero-crossing switching circuitry in the package.

Figure 19(a) and 19(b) show the typical outlines of simple transistor optocoupler SCRs and triacs (which are usually mounted in six-pin DIL packages); Figure 20 lists the typical parameters of these two particular devices, which have rather limited rms output-current ratings, the values being (in the examples shown) 300mA for the SCR and 100mA for the triac. The SCR device’s surge-current rating is 5A at a pulse width of 100µS and a duty cycle of less than 1%; the triac device’s surge rating is 1.2A at a pulse width of 10µS and a duty cycle of 10% maximum.
Transistor optocoupler SCRs and triacs are very easy to use; the input LED is driven in the manner of a normal LED, and the SCR/triac is used like a normal low-power SCR/triac. Figures 21 to 23 show various ways of using an optocoupled triac; R1 should be chosen to pass an LED current of at least 20mA; all other component values are those used with a 230V AC supply.
In Figure 21, the triac is used to directly activate an AC line-powered filament lamp, which should have an rms rating of less than 100mA and a peak inrush current rating of less than 1.2A.

Figure 22 shows how the optocoupled triac can be used to activate a slave triac and, thereby, activate a load of any desired power rating. This circuit is suitable for use only with non-inductive loads such as lamps and heating elements, using a triac of suitable rating.

Finally, Figure 23 shows how the above circuit can be modified for use with inductive loads such as electric motors. The R2-C1-R3 network provides a degree of phase-shift to the triac gate-drive network, to ensure correct triac triggering action, and R4-C2 form a snubber network, to suppress rate-of-rise (rate) effects.

Designing Optocoupler Interfaces
The main purpose of an transistor optocoupler interface is to completely isolate the input circuit from the output circuit, which normally means there will be two completely separate power supplies, one for the input circuit and one for the output. In this simple example the input and output supplies will most likely be the same in voltage and current capabilities, so the interface is just providing isolation without any major shift in voltage or current levels.
In choosing appropriate values for R1, the value for the current limiting resistor is set to produce the correct forward current (IF) through the infrared LED in the optocoupler. R2 is the load resistor for the phototransistor and the values of both resistors will depend on a number of factors.

Current Transfer Ratio
The current in each half of the circuit is linked by the Current Transfer Ratio or CTR, which is simply the ratio of output current to the input current (IC/IF) usually expressed as a percentage. Each optocoupler type will have a range of CTR values set out in the manufacturer's datasheet. The value of CTR also depends on a number of factors, first of all is the type of optocoupler, simple types may have a CTR value of between 20% and 100%, whilst special types, such as those that use a Darlington transistor configuration for their output phototransistor, may have CTR values of several hundred percent. Also the CTR of any particular device may vary considerably from that device's typical value by anything up to +/-30%. Manufacturers will normally quote a range of CTR values for different output phototransistor collector voltages (VC) and different ambient temperatures (TA) The CTR will also vary with the age of the optocoupler, as the efficiency of LEDs decreases with age (over 1000s of operating hours). Because the CTR of an optocoupler can be expected to reduce over time, it is common practice to choose a value for IF somewhat lower than the maximum, so that the intended performance can still be achieved over the intended lifetime of the circuit.
Although this example describes the design of a simple interface linking two HCT logic circuits, the difference between the results achieved here and those needed for any other optocoupler are that similar calculations can be made just using data appropriate to other voltages and currents and other optocouplers.

Calculating the Optocoupler Resistor Values
The start of the design process is to specify the input and output conditions the transistor optocoupler is to link. Typical optocouplers can handle input and output currents from a few microamps to tens of milliamps. There are many optocouplers on the market and to find the most appropriate for a particular purpose, vendor's catalogues and manufacturers datasheets should be studied.
In this case however, a popular PC817 optocoupler from Sharp will use voltages and currents available from HCT logic. Assuming that a single HCT output is only feeding this optocoupler, a logic 1 voltage of about 4.9V can be presumed.
The output current available from a HCT gate to drive the optocoupler input is limited to 4mA, which is quite low for driving an optocoupler. The PC817 must then be capable of producing the necessary output from this low input current.
Ideally the transistor optocoupler should in this case act as though it is invisible, that is the HCT gate connected to the optocoupler output should see an available current of up to 4mA, just as though it was connected to the output of another HCT gate. Therefore the output current of the PC817 needs also to be ideally about 4mA, with the forward current (IF) driving the input LED at 4mA (assuming a 100% CTR).Having found an approximate figure for the CTR, which suggests that input and output conditions should be similar, at 4mA, the next task is to calculate the values of R1 and R2.