By Aiman Kiwan, Panasonic Electric Works of America Corp.
Solid-state optically isolated relays can improve the performance of data acquisition systems and industrial machines.
Not too long ago, relays were exclusively electromechanical devices. However, today engineers can also opt for solid-state relays that use semiconductors to switch their output circuits. The choice between traditional electromechanical relays and the solid-state varieties often comes down to reliability and performance.
With no moving parts, solid-state relays avoid all the obvious mechanical failure modes associated with traditional relays. They also tend to offer more desirable electrical characteristics and design advantages. For instance, some of the biggest advantages are low power consumption and low leakage current as well as a stable on-resistance over the relay’s lifetime. Solid-state relays also offer high reliability with extremely long life. On the mechanical side, they are physically smaller, offer fast switching speeds, high vibration and shock resistance and no contact bounce or switching noise.
However, not all solid-state devices are created equal when it comes to these performance advantages. Optically isolated solid-state relays, in particular, can offer some benefits over other solid-state devices that use electrical or magnetic operating principles.
Principles of Operation
Optically isolated relays use a light emitting diode (LED) on their input side, MOSFETs on the output side and an array of photo sensors in between. In operation, current flows through the LED, which then emits light. The photo sensor array detects the emitted light, triggering a voltage drop that drives the MOSFETs, which finally switch the load circuit.
The design and packaging of the optical and electronic components can impact the relay’s performance. The LED and photo array, for example, are molded in a translucent resin that allows light to pass through while providing a dielectric barrier between the input and output.
The most basic method to drive an optically isolated relay is to apply a switchable voltage directly to the input pin of the relay through a resistor to limit the current through the LED. Choosing the correct RF value for the resistor will ensure that the LED reaches full intensity while preventing it from being overdriven by the input voltage.
Test and Measurement Uses
Most optically isolated relays will ultimately become part of sophisticated test and measurement systems. To keep pace with advances in the electronics industry, these systems increasingly require solid-state relays that combine low capacitance, low on-resistance, physical isolation and high linearity. All of these characteristics play an important role as data acquisition devices become faster and more precise.
For starters, low capacitance improves switching times and isolation characteristics for high frequency load signals. And low on-resistance reduces power dissipation when switching high currents and increases switching speeds to improve the precision of measurement. When considering on-resistance values, pay close attention to the temperature range the relay must withstand. Rising temperatures decrease the mobility of electrons, driving up the on-resistance. Starting with a relay that has low on-resistance will minimize the effects of temperature drift.
Another important variable, physical isolation, is sometimes referred to as galvanic separation. Physical isolation between the relay’s input and output or between different output channels enhances precision by minimizing noise. Optically isolated relays offer a true physical separation of the input and output, and the best of these products exhibit isolation voltages as high as 5,000 Vac. And lastly, high linearity ensures accurate measurements.
With a variety of signals at work in a typical test system, it’s particularly important to find relays that offer the right combination of electrical characteristics. For example, many systems have both DC and AC switching needs and will require relays that combine low on-resistance and low capacitance. The low on-resistance minimizes signal loss when switching DC signals, while low capacitance improves isolation when switching AC signals.
Not all optically isolated relays end up in test and measurement applications. Increasingly, these relays also switch and protect small motors, power supplies and control devices with load currents up to 10 A.
These industrial uses represent the next wave of applications for optically isolated relay technology, which has been widely accepted in high-precision data acquisition and measurement systems.
Like test and measurement systems, industrial equipment can benefit from high switching speeds, low on-resistance, low capacitance and small package size. Yet motors, power supplies and controls can reap additional benefits by moving from traditional electromechanical relays to optically isolated relays.
Low Power Consumption
A typical optically isolated relay requires 10 to 20 times less power than an equivalent electromechanical relay. For example, a 5 mA optically isolated relay can often do the same job as an electromechanical relay that requires anywhere from 50 to 100 mA, depending on the electromagnetic force needed to close the coil. A few milliamps here or there may not sound like a big deal, but in a plant with many small devices the savings can add up quickly.
Thanks to a built-in protective circuit in some latching-type models, some optically isolated relays can safeguard motors, power supplies and other industrial devices from possible disturbances on the output side. These disturbances (such as voltage peaks or overcurrent conditions) can arise due to short circuits or improper use. The protective circuit is located on the output side of the component and recognizes high currents. This arrangement protects both the DMOSFET on the output side and the load circuit against overcurrent conditions. As soon as a dangerous load current arises, the load circuit switches off completely. It can be switched on again only after the input signal has been reset.
Elevated Temperature Tolerance
These protective circuits can play a particularly important role when the relay must perform at elevated operating temperatures. Because the voltage drop across the shunt increases as rising temperatures drive up resistance in the component, the protective circuit responds to lower and lower current levels as temperatures rise. In essence, it exhibits a negative temperature coefficient, which allows it to offset the increased power dissipation associated with elevated temperatures.
Solid-state relays shine when it comes to reliability. Without the moving parts of an electromechanical relay, solid-state relays typically have an excellent mean time to failure (MTTF). In general, solid-state relays better tolerate shock and vibration loads that threaten electromechanical relays. Solid-state relays also eliminate the buzzing that can affect electromechanical relays driven by PWM and other methods intended to conserve input power.
Low operating cost
Solid-state relays may have a higher price tag than electromechanical relays. The total cost over the relay’s life cycle, however, tips the scales back in favor of solid-state technology. Most of the operating cost advantages come from reductions in power consumption and a longer life cycle for fewer relay replacements. Factor in the cost benefit of motor protection and the value proposition becomes even more compelling. Keep in mind, too, that the savings can be greater in applications that require the relay to remain in its closed state for long periods of time. Solid-state relays can be operated closed without the elevated temperatures and extra current draw of their electromechanical counterparts.
Saves space, speeds development
Integrating the protective mechanism in the relay, rather than relying on a separate component, saves space. And it speeds development time because there’s one less component to work into your design.
Panasonic Electric Works of America Corp.
Filed Under: Power Electronic Tips