They just hold the last position. It seems to me this would be more ideal than normally opened or normally closed relays that need to waste current to hold one of their two positions. Not sure how this would be done, but one idea that comes to mind would be two permanent magnets that hold the current position normally and two electromagnets that can overcome them to switch to the other stable position, or some mechanical solution. I have to assume something like this has been done. What is it called?
Yes, latching relays.
Far-seeker:
Yes, latching relays.
Thanks a lot. I will have to look into this now that I know the terminology. I was hoping for some catchy one word name for it, but this works. Now I wonder why these seem not to be used very often in practice. Anyone know? Are they less reliable or more expensive? They are greener, if the impact of making a more expensive device or servicing a less reliable device is less than the power saved, if that is even the case.
I see Digikey carries them, there seems to be a good selection. Not terribly expensive, either, especially in quantity.
Weird, someone else is talking about latching relays right now. Maybe they are not rare and I just don't know a lot about relays.
Now I wonder why these seem not to be used very often in practice.
Because you don't know the relay's position on start up. Also when the power is pulled then the relay is still on, not a very safe situation in most circumstances.
Basically it is not what you want normally.
They are greener,
Rather insignificantly so.
Grumpy_Mike:
Because you don't know the relay's position on start up. Also when the power is pulled then the relay is still on, not a very safe situation in most circumstances.
Basically it is not what you want normally.
Good, important point. It did not occur to me because everything I have worked on so far is low voltage DC so safety basically is a side thought. Thank you.
Hey... Thats what computers are for... remembering the position of a Latching relay.. . .
Doc
Because you don't know the relay's position on start up. Also when the power is pulled then the relay is still on, not a very safe situation in most circumstances.
Basically it is not what you want normally.
As I posted on that other thread about latching relays, one way to deal with the 'problem' of not knowing the start-up state is to use latching relays that have DPDT contacts and wire one set of contacts to digital input pins on the micro so that it can determine the existing state of the latching relay at startup time.
Lefty
Or you arrange a "reset" function which drives all latching relays to a predetermined position before the process starts.
Docedison:
Hey... Thats what computers are for... remembering the position of a Latching relay.. . .Doc
When the plug is pulled!
I have used latching relays in projects but they are a pain, I was just ponting out why they are not used more often. They are also more prone to mechanical failure due to their added mechanical complexity.
Grumpy_Mike:
Also when the power is pulled then the relay is still on, not a very safe situation in most circumstances.
Basically it is not what you want normally.
By themselves, latching relays do have this problem. However, this can be remedied by using an SSR (or a normal mechanical relay for that matter) between the power supply and the latching relays. If there is a power failure to the control unit the SSR "opens" and interrupts the flow of power. It's not the most practical approach for a single relay, but for a bank of multiple latching relays it can be a reasonable solution.
As for any energy savings, it would greatly depend on the applicaton. Even though there are both normally open and normally closed mechanical relays, there are some applications where the opposite state is needed a significant minority of the time (i.e. 25% to 49%). If such an application also requires mulitiple relays, then there could be non-trival energy savings along with measurably less excess heat being generated.
That stated even if they are a good option in specific circumstances, I agree that latching relays are not really intended for general usage.
Latching relays have specific applications in which they are useful. Coming back to the OP's question, it's just as easy to wire up a double contact dual throw relay to give you what want, though one of the 'states' will consume power. See Relay - Wikipedia
One reason I considered latching relays a long time ago was thanks to a typical British electrical design on mums boat. The same folks that brought you Lucas, the prince of darkness, also did the wiring in there, to such great consternation (and violation of applicable USCG, ABYC and other regulations) that later boats imported into the US were required by the US subsidiary to be delivered un-wired to the US, where the wiring was installed. Anyhow, besides featuring such doozies as ~18AWG wiring being hooked up to a 200A bus w/o fuses or other forms of disconnects, it also featured three large relays to switch the two main buses (battery and starter) as well as a cross-connect (emergency start).
These relays were supposed to be a great feature, allowing the main buses to be connected and disconnected from the main circuit breaker panel with a simple little switch. Great, except that over 16% of the total battery bank capacity would be smoked by these relays daily - high-capacity relays require significant power to operate. Thus, I considered trying to find a latching relay equivalent and then retreated rather rapidly when I realized what kind of mega-buck territory I was approaching when 200A switches at the battery bank were not only simpler but safer as well.
I may be pretty clueless about electronics, but simple electrical connections and math I can do (voltage drop, what an apparently arcane concept in the Canvey Island area). The wiring on this boat (along with some other features I won't bother you with) was perhaps designed or installed expressly to kill the owners. Yeah, I'm still a bit bitter. We spent more time repairing issues on the boat than it took the OEM to build it.
Perhaps something like this switch would be better? http://www.batterymart.com/p-battery-cut-off-switch.html
cyclegadget:
Perhaps something like this switch would be better? http://www.batterymart.com/p-battery-cut-off-switch.html
Depends on the application. I needed bigger switches - 200 or 400A continuous, can't remember which. We have 600+Ah of AGM capacity with short-circuit current capacities that are pretty scary.
These are a little closer, take a look. Battery Disconnect Switches Electrical Wiring Ignitions & Electrical - 350 amps Continuous Amp Rating - Free Shipping on Orders Over $109 at Summit Racing
As the above have pointed out, but sometimes you do need to remember a state after a power disrupts.
Latching relays where often used in the security industry where a key-switch would set or reset the relay and the relay would stay in that position until electrically motivated to change.
K