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. Assuming that a single HCT output is only feeding this optocoupler, a logic 1 voltage of about 4.9V can be presumed. In this case however, a popular PC817 optocoupler from Sharp will use voltages and currents available from HCT logic. 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. Typical optocouplers can handle input and output currents from a few microamps to tens of milliamps. The start of the design process is to specify the input and output conditions the optocoupler is to link.
Calculating the Optocoupler Resistor Valuesįig. Because the CTR of an optocoupler can be expected to reduce over time, it is common practice to choose a value for I F somewhat lower than the maximum, so that the intended performance can still be achieved over the intended lifetime of the circuit.Īlthough 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. Manufacturers will normally quote a range of CTR values for different output phototransistor collector voltages (V C) and different ambient temperatures (T A) The CTR will also vary with the age of the optocoupler, as the efficiency of LEDs decreases with age (over 1000s of operating hours). Also the CTR of any particular device may vary considerably from that device's typical value by anything up to +/-30%. 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. Each optocoupler type will have a range of CTR values set out in the manufacturer's datasheet. 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 (I C/I F) usually expressed as a percentage. R2 is the load resistor for the phototransistor and the values of both resistors will depend on a number of factors. In choosing appropriate values for R1, the value for the current limiting resistor is set to produce the correct forward current (I F) through the infrared LED in the optocoupler. 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.
The main purpose of an 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. 5.2.1 A Simple Optocoupler Interface for HCT Designing Optocoupler Interfaces What is necessary is to ensure that R1 creates an appropriate current level from the input circuit to correctly drive the LED side of the optocoupler, and that R2 creates appropriate voltage and current levels to supply the output circuit via the inverter.įig. LSTTL or CMOS types) may have different logic voltage levels and different input and output current requirements, and optocouplers can provide a convenient way of interfacing two circuits with different logic levels. The Schmitt inverter at the output performs several functions it ensures that the output conforms to HCT voltage and current specifications, it also provides very fast rise and fall times for the output, and corrects the signal inversion caused by the phototransistor being operated in common emitter mode. In this example a PC817 optocoupler is shown isolating a circuit using HCT logic via a 7414 Schmitt inverter gate. There are many different applications for optocoupler circuits, so there are many different design requirements, but a basic design for an optocoupler providing isolation for example between two circuits, simply involves the choice of appropriate resistor values for the two resistors R1 and R2 shown in Fig. Understand the requirements for a typical optocoupler application.Calculating component values for optocouplers.Using the Current Transfer Ratio (CTR).Understand the design of optocoupler circuits.Describe basic applications of optocouplers:.After studying this section, you should be able to:.
0 kommentar(er)
