Learn How These Solid State Relays Can Improve the Performance of Data Acquisition Systems and Industrial Machines
Not too long ago, all relays performed their switching duties through electromechanical means. Today, however, 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 desirable electrical characteristics and design advantages including:
• Low power consumption
• Low leakage current
• Stable on-resistance over lifetime
• High reliability with extremely long life
• Small size
• Fast switching speeds
• High vibration and shock resistance
• No contact bounce or switching noise
In this article, you will learn more about the operating principles of optically-isolated relays, how to apply them in different applications and how to maximize their already-long lifecycles.
Account for LED Power Losses to Maximize Relay Life
Optically-isolated relays inherently have a long lifespan, thanks to their lack of moving parts and the robustness of their solid state electronics. You can, however, make them last even longer by accounting for LED power losses.
With optically-isolated relays, including PhotoMOS, this loss of LED power affects the device’s operating characteristics and lifecycle.
As LED power falls, the relay’s operating currents will rise accordingly. On a typical PhotoMOS relay, for example, LED power might drop by roughly 3% after a 5mA input current has been applied for 100,000 hours. As a result, the relay’s operating (IFon) and turn-off (IFoff) currents would rise from their initial value by 3%.
This change in the electrical characteristics of the PhotoMOS has lifecycle implications. As LED sensitivity degrades with continued usage, more current is needed to generate the same amount of light. This light is used to charge the gates of internal MOSFETs and ultimately turn the relay on.
Slower Turn-On Time
The turn-on time of optically-isolated relays slows as LED power falls. Going back to our example of a 3% degradation of LED power after 100,000 hours at 5mA, the turn-on time would likewise slow down by 3%. Put differently, a PhotoMOS with a turn-on time of 0.03mS out of the box will have a turn-on time of 0.0309mS after 100,000 hours of use at 5mA.
This slowdown occurs because light intensity diminishes, which reduces the voltage and current output of the photo diode array in the IC. So it takes longer to bias the MOSFET gates.
Elevated Temperature Effects
At elevated ambient temperatures, more LED current is needed to generate the same amount of lamination. This lamination will then be converted to produce the necessary electrical voltage and current to charge the gates of MOSFETs and maintain ON-state.
Calculating Input Resistance (RF) Correctly
When calculating the correct RF value for the resistors used with optically-isolated relays, make sure you take the forward voltage (VF) into account.
Since the LED operating current increases as the temperature rises, we must use the typical recommended IF value of 5mA at the maximum operating temperature of +85ºC to ensure safe operation. The LED VF depends on the forward current (IF) and the temperature.
Let’s for example calculate the RF value for a popular Panasonic optically-isolated relay, the AQV210 PhotoMOS. Figure 1 shows the LED VF versus ambient temperature graph for the AQV210 PhotoMOS. The LED VF with IF of 5mA at +85ºC is 1.03V.
The maximum RF value can be calculated as follows:
Assuming a 5% tolerance and a temperature coefficient of 250 ppm (parts per million) per ºC, the appropriate RF value will be the next lower value from the standard resistors: RF=680Ω. This margin will ensure safe operation over the entire temperature range. If the supply voltage (VCC) contains a ripple, the lowest possible VCC value should be used for the calculations.
Although power consumption and drive current for optically-isolated relays are significantly lower than electromechanical relays, some logic circuits cannot drive the PhotoMOS directly and require some additional components. Using a transistor as a control mechanism to switch an external power supply is one method that is typically used by circuit designers.
In this scenario, the transistor is controlled by the output of the logic circuit. When the transistor is turned on, it will create a path to ground for the power supply VCC thus turning on the LED. When calculating the RF in this circuit, we must account for the voltage drop, typically 0.4V to 0.7V, between the collector and the emitter of the transistor.
Many Types of PhotoMOS Relays
More than 300 different types of PhotoMOS optically-isolated relays are available to meet a wide variety of electrical and package size requirements. The PhotoMOS products most suited to motor protection and other industrial uses include:
• AQZ207 SIL package with 1A load current.
• AQY277A DIP4 SMD with 0.65A load current.
• AQV252G with 2.5A load current.
For test and measurement applications, consider our Low CxR PhotoMOS Model AQY221N2M. It offers:
• Low capacitance of 1.1pF
• Low on-resistance of 9.5Ω
• Fast switching and physical isolation
• Linearity: Optical MOSFET-based relays like PhotoMOS have highly linear input and output characteristics that outshine those of alternatives such as Triacs or OptoCouplers.
• Minimal signal propagation delay
Besides using Low CxR PhotoMOS relays for switching signals and I/O lines to devices being tested, these relays may also be employed in data acquisition circuits. For instance, they can be used to select the gain of operational amplifiers. With the help of an optically-isolated relay, the device’s digital control unit and the analog signal system can be physically isolated, enhancing the precision of the device by minimizing noise.