Relays are essentials in designs where high current is needed to power a certain part of a design, though a dilemma can arise on using relays; What type of relays do I use for my design? You’ll most likely end up choosing between a type of contact relays or contactless relays, but in most cases a Photorelay (a sub-group of contactless relays) can easily replace a contact relay with added benefits. Toshiba has one of the most efficient lines of Photorelays that they continue to improve on. Toshiba will show you why their Photorelays are the choice for mechanical relay replacement.
What are Photorelays?
There are two main groups of relays, contact relays (mechanical relays) and contactless relays (semiconductor). Contact relays have mechanical switches in them, where in most cases you can even hear the switches change. Contactless relays use other means (no mechanical switches) of changing the state of the switch.
Photorelays are a sub-group of contactless relays, which are LED driven and use a MOSFET chip as the output device. They’re able to handle both AC and DC loads and are great for signal relay (sub-group of contact relays) replacements. Figure 1 shows how a Photorelay functions. Current first passes through the LED to cause it to emit light, then the PDA gets powered up by sensing the LED’s light and drives the gate of the MOSFET. This causes the MOSFET to turn on and change the status of the switch.
Figure 1: Photorelay’s function. (Source: Toshiba’s Photorelay Specification e-book)
Why Replace Mechanical Relays with Toshiba’s Photorelays?
Now knowing how Photorelays work, the next question might be; Why should I replace my mechanical relay with Toshiba’s Photorelays? There are 5 main points where you will see the advantage of switching to Photorelays.
Total Cost Reduction
By switching from mechanical relays to Photorelay, you’ll be saving cost for your design in the long term. Since Photorelays operates by receiving LED light, there is no mechanical contact meaning less wear and tear induced degradation like mechanical relays. With little maintenance required, Toshiba’s Photorelays have higher reliability, meaning a longer life and reduction in cost.
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Space Saving
Technology is constantly pushing the envelope and with that comes smaller and compact designs. Engineers are constantly trying to figure out how the reduce the size of their design. Replacing with Toshiba’s Photorelays can greatly contribute in miniaturization of the set. Relays usually follow the package trend of around 60mm2, while Toshiba has a line-up of small package products, such as VSON (1.45 x 2.45 mm) and S-VSON (1.45 x 2.0 mm).
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Low Power Consumption
Mechanical relays have large number of parts that require a larger current to drive it. With Photorelays the drive circuit is simpler and since an LED is used on the input side, the necessary input current for turning on the output is as low as 3 to 5 mA (max), where it can even be driven by a small battery or directly from the microcomputer. Usual input power consumption from mechanical relays are around 100mW, while Photorelays consume around 0.5mW.
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Fast Switching Speed & Reduction of Noise
Toshiba’s Photorelays have excellent switching characteristics compared to mechanical relays. Since Photorelays rely on the LED light to drive the MOSFET, the switching speed is almost instant, typically being up to 10x faster than mechanical relays.
There is also a reduction in noise. Mechanical relays tend to have noise caused by either electromotive force or bounce from the coil. With no coils on Photorelays, noise caused by these are reduced significantly. Figure 2 shows a great example of how mechanical relays cause noise.
Figure 2: Mechanical relay’s causing noise. (Source: Toshiba’s Photorelay Specification e-book)
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Hot Switch Support
“Hot Switch” is a means, by which the switch operates with voltage applied; current flows to the load at the moment the switch is closed. The opposite of a hot switch is a cold switch. Cold switch means that the switch operates without voltage applied. Therefore, no current flows when the switch is closed.
Mechanical relay’s contacts are likely to be worn out since the voltage is applied by the hot switch. Additionally, an arc is generated when the current is interrupted by the switch opening, causing the life of the relay to shorten.
Photorelays have hot switch support, as it can be used with either the hot or cold switch as long as the maximum rating is maintained.
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Toshiba’s Line of Photorelays
Now having an idea of why Toshiba’s Photorelays are great replacements for mechanical relays, you might be asking; Which Photorelays might fit my design to replace mechanical relays? Toshiba has a long line-up of high current Photorelays, featured in Figure 3, each with their own electrical characteristics and packaging. It’ll be up to you to find the one that fits your design!
Figure 3: Toshiba’s Photorelay line-up table.