Optocouplers are devices that transfer electrical signals via light waves. They're appropriate for discrete-motion applications in industrial environments.
Through light waves, optocouplers transfer signals of all types between components. Due to their mode of operation, they also provide electrical isolation and noise immunity not found in other signal-transmission devices. Consider their operation in a common stepmotor design: Pulse width modulated (PWM) signals and feedback encoder signals can be isolated with low speed (1 MBd) and medium speed (10 MBd) digital optocouplers, respectively. What's more, analog versions, such as digital-to-analog converted (DAC) signals, can also be isolated in this way.
The air gap over which an optocoupler operates provides inherently high insulation, so it protects both machine operators and equipment. By incorporating optical isolation, user interfaces are insulated from fatally high voltages — and expensive controllers are isolated from the high-voltage, high-current motors they control.
Where optocouplers are used as safety isolators, they must undergo rigorous testing and meet the requirements of stringent regulations and standards.
IEC 60747-5-5 electrical safety standard
Design engineers looking to boost a design's safety by incorporating optocouplers should familiarize themselves with IEC 60747-5-5 — the standard that ensures reliable isolation from optocouplers. In fact, this standard is widely recognized in other equipment safety standards, including IEC 61800 - Standard for Industrial Motor Drives, IEC 60950 - Standard for IT and Communication Equipment, IEC 60601 - Standard for Medical Equipment, and IEC 61010 - Test and Measurement Equipment.
To meet the IEC 60747-5-5 standard, optocouplers must first withstand a series of environmental and mechanical stresses, and then pass a battery of electrical conformance tests. Stress conditions include:
Temperature cycles — Five temperature cycles with a dwell time of three hours at specified minimum and maximum storage temperatures
Vibration — 10 to 500 Hz, to 10 g; 0.75-mm amplitude; stress duration of 10 cycles per axis
Mechanical shock — Three shocks in each direction at 100 g; shock duration of 6 msec; half-sine-wave shock formation
Dry heat — The maximum working insulation voltage (VIORM), or a voltage of no less than 700 V, is applied between the input and output for 16 hours at a temperature of 100° C; VIORM is the maximum continuous operating voltage that a device or insulation can be exposed to without danger of the insulation being catastrophically destroyed
Damp heat — One cycle at 55° C, followed by a stressing period of 21 days at 85° C with 85% relative humidity
Low temperature — Two hours at the minimum ambient temperature
After surviving these environmental stresses, samples must then pass two electrical tests to confirm their intact ability to isolate. In the partial discharge test, units must allow no electrical discharge greater than 5 pC (picocoulombs) when a working voltage of 1.6 times VIORM is applied across the insulation for 60 sec. The surge test, on the other hand, implements 50 discharges (1.2 µsec/50 µsec impulse) at 10 kV; afterwards, insulation resistance must be at least 109 Ω at 500 V.
Beyond these two hurdles, optocouplers are also tested during production — exposed to a working voltage of 1.875 times VIORM for 1 sec. Discharge must be less than 5 pC to pass in accordance with IEC 60747-5-5.
This month's handy tips courtesy of Vincent Ching, technical engineer, Avago Technologies, Singapore. For more information, visit avagotech.com.