Latching Relays.... revisited.

I got here via google search for 'ratchet relay'.

The thread linked to was about a year old

http://cr4.globalspec.com/thread/15310/Regarding-Latching-Relay

...and I know some forums frown on resurrecting old threads, so here's a revisit.

Somebody began that thread asking direction on learning about these things, and the initial responses (reasonably) were hand slaps about beginning the search with a forum post rather than via the low fruit of google. Fair enough.

Then the thread kind of went on to validate his confusion. I'm not an EE, just a tinkerer, so I welcome any and all criticism without any damage to my pride. One of the reasons, perhaps, there is so much confusion about these relays is that the electromechanical acrobatics performed by them have largely migrated to microcontrollers on a separate pcb. In the 'old days' specialized relays got pretty exotic, especially inside of a major telecommunications switching facility. But so much of the logic is handled by ICs today that many of these special relays are out of production.

It seems like many of the descriptive tags get muddled; is it because they're falling out of general use? Again, correct me if I'm wrong anywhere here, but a Latching relay is pretty much the broadest category of 'bistable' relays. Bistable in that whether held in place mechanically, or with magnetism, the arm remains latched where it is regardless of whether current remains on the coil or is removed.

I've seen quite a few threads on the net where someone is specifically describing an 'impulse' relay, and the respondants misapprehend the term and shout back that EVERY latching relay is an 'impulse' relay. But strictly speaking, an 'impulse' relay is one that will latch back and forth with a signal applied across the same two pins, without the need to reverse polarity (or energize a second coil). It's accomplished with internal IC microcontrol, (like the Magnecraft 711) or it's done with a heart shaped cam (like tycos' version) or it's done with ratchets and pawls (like omrons' version). There used to be many, many more of this type available, but signal (vs power, which include the above three) versions of them have completely disappeared with the advent of pcb microcontrol. (i challenge anyone to still find one. seriously! that's what got me started on discovery)

And the meaning/terminology just gets more fragmented from there. Sequencing relays are a subcategory of impulse relays since they may or may not actually sequence beyond two positions. Again, the telephone company used to, (and probably still does in some rare cases) employ monstrous versions of these. I've still seen these multi poled boat anchors in surplus catalogs. But this is mostly done in the firmware/software these days.

Ratcheting, alternating, flip-flop, sequence, impulse, bi-stable, latching, magnetic, cammed, etc, etc. I really can't blame that first poster at all. It's possible that he even did go to google for a couple of days before posting. I've seen these terms used completely interchangeably by quite a few engineers and salespeople. I can't find the signal sized impuse relay anymore that I needed, so I'm going to have to fudge one with a push solenoid and a latching dpdt pushbutton. What a drag.