An alternative relay we have not looked at, until now, is the latching or bistable relay. This kind of relay will only use power when changing state from open to close or from close to open. Also, it keeps its last state.
Those properties could be interesting, because one of the problems we face in the preliminary design of the energy harvester, is the power usage of the external relay. One has to spend energy in order to connect the energy harvester device – via the contactor or non-latching relay – to the wind-turbine input. A latching relay could possibly solve that problem, but at a cost, as you can read further down this article.
Properties of latching and non-latching relays
Latching and non-latching describe the mechanism used to actuate/de-actuate electromechanical coils.
For non-latching relays, constant power is required to maintain the ‘ON’ position. Hence, when removing power from a non-latching or failsafe relay, the contacts will return to their normal state ‘OFF’ position. (Normally Open NO will disconnect and Normally Closed NC will connect).
To change the state of a bistable relay, a short pulse is sent to the set or reset lines on the relay. The period this pulse is active should be long enough to effectively energize the coils to change state, but not too long because then there is danger of burning the coils. The big difference between a non-latching and a bistable relay – as already mentioned – is that removing power from a bistable relay will not affect the present state of the contacts: ON remains ON and OFF remains OFF.
When to use
According to the knowledge-base on this website (and shamelessly copied to our site although the context at the origin is different), Latching or bistable relays should be used when an application requires one or more of the following:
- The last state of the relay prior to power removal must be maintained.
- Thermal stability (less heat) through power management.
- Minimal power draw while maintaining state.
- Better repeatability because they ensure more repeatable contact closures with regards to insertion loss over the life of the relay.
Non-latching relays should be used when an application requires one or more of the following:
- Cost is a factor. In general, latching relays will be less expensive than their latching equivalents.
- Relays need to be set to their normal position when power is removed: In normal operation, power removal is planned and handled elegantly. However, when power loss is not planned, it may be desirable, considering safety, to force the state to its NC or disconnect state.
Select a bistable or non-latching relay
Thus, now we would then have to investigate the effect with regard to safety when choosing a bistable relay: What happens if the energy harvester device misbehaves? The latching relay will not automatically disconnect the wind turbine from the device. Murphy’s law states that this will happen (possibly even) at the worst possible time (during a storm). At that time, power will surge into the energy harvester potentially destroying the device and or the battery bank.
Another problem is the high amperage and voltage specifications we defined for this external relay, make it very difficult to select a latching relay as an economical viable solution.
If you want to experiment with a latching relay board yourself, you can find a kit here.