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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

Keep in mind that solid-state devices are not created equal when

it comes to these performance advantages. Optically-isolated

solid state relays, in particular, can outshine other solid-state

devices that use electrical or magnetic operating principles. In

this paper, you will learn more about the operating principles

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. The MOSFETs finally

switch the load circuit.

The design and packaging of the optical and electronic

components are crucial aspects of 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

PhotoMOS relay

Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial

PhotoMOS 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 (see Design Tip, “Calculating

resistance, physical isolation and high linearity.

All these characteristics play an important role as data

acquisition devices become faster and more precise:

Low capacitance improves switching times and isolation

characteristics for high frequency load signals.

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

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 volts AC.

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

supplies and control devices with load currents up to 10 amps.

These industrial uses represent the next wave of applications

for optically-isolated relay technology, which has been widely

accepted as a way to switch 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:

here or there may not sound like a big deal, but in a plant with

many small devices the savings add up quickly.

Protection. Thanks to a built-in protective circuit in our latching-

type models, PhotoMOS 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

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.

Reliability. Solid-state relays such as PhotoMOS shine

when it comes to reliability. Without the moving parts of an

electromechanical relay, solid-state relays typically have

Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial

state relays 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.

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

Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial

Since the LED operating current increases as the temperature

at the maximum operating temperature of 85ºC to ensure

Panasonic optically-isolated relay, the AQV210 PhotoMOS.

Figure 1 shows the LED forward voltage versus ambient

ppm (parts per

million) per ºC,

value will be the

next lower value

from the standard

safe operation over the entire

temperature range. If the

relays, some logic circuits can not 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 0.4 to 0.7 V, between

resistor to ensure that RF is lower than the maximum allowed

value of 714 Ω: RF=560 Ω.

Mean Time To Failure (MTTF) equals 1/the failure rate λ.

λ is expressed in terms of failures per unit of time (FIT), where 1 FIT=1 failure per billion device hours.

The failure rate of Panasonic’s PhotoMOS optically-isolated relays is 20 FIT which means that MTTF is

1/(20*10*-9) based on the THB test per MIL HDBK-217F.

Based on the MTTF, expected time to first failure exceeds 50 million hours of operation.

Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial

package size requirements. The PhotoMOS products most

suited to motor protection and other industrial uses include:

AQZ207 SIL package with 1 A load current.

AQY277A DIP4 SMD with 0.65 A load current.

AQV252G with 2.5 A load current.