Relay Technology: Arc Extinguishing
How Fujitsu’s Arc Extinguishing Technologies Contribute to Miniaturization
Ever since Tesla and Edison crossed their ‘AC / DC sabres’, flipping a switch of a power circuit introduced a shocking risk: arcing. Arcing, both inventors learned later, is caused when the potential difference (measured in volts) between poles causes the air between them to heat up so extremely that it turns into a plasma. In other ominous words: the insulator becomes a conductor.
When switching the power on, this added conductivity is not the biggest deal. You want electrons to move from one pole to the other, so the airgap is bridged and the arc will cease to exist the moment the circuit is closed. When switching the power off, you don’t want electrons to keep moving, let alone give you unexpected potentially dangerous fireworks.
Here, AC system yields an advantage. The inherent phase shift keeps potential differences between poles in check, allowing for relatively small air gaps and modest insulation precautions. You can safely switch (on/off) 220 Volts with a very affordable design.
It is the reason why, despite the surge in available DC power generated by many solar panels on ample rooftops, your light switches at home still use AC power.
While moving electrons in an alternating fashion might be efficient for high voltages, when descending into the lower voltage ranges direct is the only way to go. To switch it, you need a DC solution. Why?
Unfortunately, the DC switch can’t ‘magically’ make a big chunk of the potential difference disappear by 'phase shifting' its way out of it. In other words: it needs to be able to handle the full electromechanical force that threatens to excite the air molecules in the airgap into a plasma state, causing the electrons to jump across when the switch is in – or moves to! – the ‘off’ position.
The first line of defense of any DC switch, and hence relay, is making the gap bigger. A 24 Volts ‘urge’ from electrons to make the jump can only generate so much heat. Increasing the gap means more air has to heated with the same amount of energy, preventing a plasma to form.
When you have unlimited space, enlarging the gap might be the only answer you need. In cars and many other appliances however, space comes at a premium, which means you want a small a gap as possible. To enable this, Fujitsu has developed several technologies for arc extinguishing that ensure a compact, as well as a safe, robust, and affordable solution.
One of our ‘secret’ ingredients is a permanent magnet. Since electrons are sensitive to magnetic fields, we can adjust the arcs path using one or two cleverly placed permanent magnets, making the arc’s path longer within the same airgap. Another tool we successfully use to enlarge the arc’s trajectory is the use of arc extinguishing plates, strategically placed within the airgap, preventing electrons to take the shortest route across and providing a cooling effect. In our most compact relays we use both.
Another option is to replace air with another gas, having better insulating properties than air, allowing for a smaller gap. Adding a gaseous medium that has a cooling effect is also effective, as it prevents the plasma from forming.
We choose the best option depending on the technical relay requirements and the space required in the application.