ADVERTISEMENT FEATURE COVER STORY
THE PHOTOSMART RELAY A sound technology
“hopping” is always a concern. As the PhotoMOS relay has no moving contact parts, the chances of contact errors due to external influences are drastically reduced. Another interesting application is for
the Audio industry, entirely based around the audible “click” noise that an Electromechanical relay makes during operation. While a single Electromechanical signal relay can hardly be considered audibly loud, they can be an issue when you have many hundreds of them switching simultaneously. As a result the majority of the more frequently actuated relays were swapped to PhotoMOS technology giving near silent functionality in a sound sensitive environment. The test and instrumentation industry
Robert Dixon, Product Manager, Relays & Connectors at Panasonic Electric Works UK uncovers some surprisingly little known, but key facts behind the PhotoMOS and how to get smart with this technology via the new PhotoMOS selector app
T
he PhotoMOS relay range from Panasonic Electric Works is an
optically coupled MOSFET relay. The primary function blocks in a PhotoMOS relay when compared to a traditional electromechanical relay are: LED – to replicate the Coil Drive Circuit – This is made up of Photo Receptors and Gate Biasing components effectively replicating the Armature function. MOSFETs – These act as the contacts The ‘Coil’ is energised by driving the LED
using a current limiting resistor, this emits light, charging the Photo receptors, the voltage generated is then used to ‘actuate the armature’ circuitry. Once this takes place the MOSFET is turned on and the ‘contacts’ switch to complete the circuit. There have been many comparisons
between a PhotoMOS relay and an Opto- Isolator as physically they often look very similar and both technologies use an optically isolated LED drive side, but this is where the common points end. An Opto-Isolator typically uses a
transistor as its output and as such due to the more basic control circuit can be at varying degrees of On or Off. By contrast the PhotoMOS has only two states fully On or fully Off. This is due to the carefully managed drive circuits that
12 MARCH 2014 | ELECTRONICS
only turn the MOSFET on when full voltage saturation has been reached. One of the inherent characteristics of a
MOSFET is what is frequently considered to be a relatively high channel capacitance that is rarely compatible with High Frequency instrumentation signals, due to a block pass filter effect they cause on the signal. To be able to manage these applications better, the PhotoMOS range includes some specifically designed MOSFET channels to deal with these exact needs. The creation of these lead to a new unit
of measurement, the “picofarradOhm”, basically multiplying the channel resistance and capacitance together. Units are available with a bias either towards low resistance or a low capacitance. Typical values available vary between 3pFohms – 25pFOhms.
MOSFET CHARACTERISTICS With the inherent characteristics of a PhotoMOS the technology lends itself to a number of applications where a pure solid state relay or a more traditional relay would have led to significant compromises. For example, in the rail industry one of the parameters that can cause concern is the elevated levels of shock and vibration. With an Electromechanical relay the issue of contact miss operation or
Figure 1:
Panasonic’s new selector app offers help and support for the myriad of part numbers and specifications across the PhotoMOS range
benefits due to the wide array of channel ratings and functions, coupled with the ability to mount the PhotoMOS on the PCB in extremely high density, supported by the large variety of package sizes. In test and instrumentation applications
even with the best will in the world, even the most precise electromechanical relay contacts are subject to degradation over its switch life, either caused by pure mechanical impacts during the switch operation or over time caused by the build up of oxidisation on the contact surface. In a high duty cycle test application on a busy production line installation of a test instrument can reach a relays mechanical switch life extremely quickly. A PhotoMOS does not suffer from ‘wear
Figure 2:
The PhotoMOS relay is available in any number of channel ratings and package shape and sizes
and tear’ through mechanical switching in the same way as there are no moving parts, The contact resistance will also not degrade during its switching life. Panasonic are now able to offer an
innovative tool to aid designers in getting to the bottom of the myriad of part numbers and specifications of the PhotoMOS range. Available from both the Apple ITunes and Google Play App libraries is the new PhotoMOS selector app. From your smart phone you can now
narrow down the ideal part either by any number of specification parameters to ensure that you have the right component for the application at hand.
Panasonic Electric Works UK
www.panasonic-electric-works.co.uk 01908 231555
Enter 202 / ELECTRONICS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36