Latching Relays

[zim]

Morning,

I'm looking for a latching relay that :

* When i push a button switch it'll energise a circuit

* This circuit will stay complete even if i let go my switch

* Have another pin on the relay so that when i give that 12v, it'll de-energise

I've found Durite 0-728-02, which operate the same as VWP's :

Latching relay.

This relay has a mechanical latch. When the coil is momentarily energised

the contacts operate and 'latch' in that position. To unlatch the contacts

the coil is momentarily energised again.

These relays do not fit into sockets. Mounting is by an integral spring clip.

Coil should only be energised momentarily not continuously. 12V 25A.

http://www.vehicle-wiring-products.eu/VWP-onlinestore/relays/relays.php

But these are no good for me as both the on & off are via the same pin.

This might do the trick, but i'm looking for a simpler way http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=120611987309&ssPageName=STRK:MEWAX:IT

Any suggestions ?

Cheers

Gordon

[nickwilliams]

I obviously don't know exactly what you are trying to do, but can you achieve what you want to with one of the Durite 0-728-02 relays and a pair of diodes?

I have one of the Durite 0-728-02 and could do a quick experiment with it if you tell me what you want.

Farnell list lots of different types of latching relay in their catalogue. Note that many of these are from the 'extended' range, which means they have to come from the US so they cost a significant amount extra on postage. However, using their catalogue you may at least be able to identify a part which you can look for elsewhere. You are probably looking for a 'dual coil' latching relay.

IME this sort of thing tends to be more common in the US - European and Japanese designers tend to use a clever bit of electronics to solve this sort of problem (per the e-bay item you identified) whereas Merkins use electromechanical devices.

You may struggle to find anything cheaper than the e-bay item you listed, particularly taking into account how long you might have to spend looking!

NIck.

[MrKev]

How about using two relays and wire one up as permanently latching by feeding the contacts back to the coil, and a second one as normally closed, so that when you feed 12V to that, it opens the power to the first relay, thus freeing the latch.

Kev

[LandyManLuke]

You can mostly do this with a completly bog standard auto/car relay, a NO mom switch, a NC mom switch, and a diode.

If you need a 12v signal to turn off the relay, you can add a change-over relay in place of the NC switch so that 12v on its coil opens its NC contacts.

[zim]

You can mostly do this with a completly bog standard auto/car relay, a NO mom switch, a NC mom switch, and a diode.

If you need a 12v signal to turn off the relay, you can add a change-over relay in place of the NC switch so that 12v on its coil opens its NC contacts.

Cheers. Unfortunately it's not quite as simple as that.

I am making an auto sequence thingy, that can be cancelled by a number of switches...but only switched on by one.

The ebay bloke is quite helpful and going to make something else into it Regarding delays etc.

G

[LandyManLuke]

I am making an auto sequence thingy, that can be cancelled by a number of switches...but only switched on by one.

You could build on the same basic circuit to do that. Without knowing the ins-and-outs, It's starting to sound like it'd be easy to use a mircoprocessor and handle the logic in software. That way it's easy to change or update in the future.

[Gromit]

It's starting to sound like it'd be easy to use a mircoprocessor and handle the logic in software.

Out of curiosity, where would you start with this? What hardware and what language do you code in?

I'm a software developer by trade, but haven't done any low level embedded stuff since assembly on a motorola chip in uni.

[LandyManLuke]

I've most experience with Atmel 8051 and ATmega32 and STAMP uCs, but there are loads of really quite powerful development board mounted uC out there now.

I've used a freeware compiler that cross compiles Ansi C to 8051 hex instructions, and all the 'branded' uCs/boards come with their own compile/dowload tools, mostly based on Basic or C -esque languages.

Development board mounted processors like Stamp, Picaxe, etc are obviously more costly, but mean you're pretty much up and running, in terms of running the processor, comms, I/O etc. The alternative is to start with an 8051, some breadboard and a few other bits and start from scratch.

I know Si Rafferty has used another processor that sounded good, but I can't remember what it's called.

This car used an ATmega32 development board (from here )

[Mark]

There is a lot of interest in using the Arduino system and clones thereof, which I have seen Si refer to before. I know that he has used other systems as well though...

[Dave W]

I can't think of any reason why you couldn't do what you want to do using a triac. They are dirt cheap and reliable, all you'd need to do is connect it to the coil of the relay.

A triac is often used for AC switching because you can connect, say, 12v to the trigger and it will switch on allowing current to pass both ways through it. When you remove the trigger voltage it will stay switched on until the current flowing through it drops below a certain threshold. When switching AC this happens straight away but when switching DC, as in this case, it will stay switched on forever or until you momentarily apply 12v to the other end of it or disconnect it's supply.

In the diagram below momentarily applying 12v to Input A will turn the Triac on, activating the relay. Momentarily applying 12v to Input B will reduce the current flowing through the Triac and turn it off so when you stop applying 12v to Input B the relay will deactivate.

[Dave W]

If you do want to use a Triac then something like this one would easily do the job....

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3552841

A normal relay should easily take more than the 5mA it needs to hold it on and it'll handle 5A all day and 73A surge so it's more than capable of handling a relay (normally they take around .5A).

Stud types are easier to mount than this kind of pin through but they are significantly more expensive - over 10 quid each rather than 70p each ! You can get them for free though if you have access to scrap washing machines as they're often used to control the power in those.

This is a typical stud mount one...

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=7005588

You just need to be careful with the higher powered ones that the holding current is below the current the relay draws or it'll keep turning off.

[zim]

Thanks for the info

G