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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Replacing mechanical Latching Relays with dual coil relays...
Date: Mon, 26 Aug 2024 15:23:45 -0700
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On 8/26/2024 3:12 PM, John Robertson wrote:
>> Depending on the number (and form) of the contacts being used, you
>> could design a little SR latch on a board, driving a single relay
>> (to give you volt-free contacts) with the appropriate number (and
>> form) of contacts.
> 
> There are two DPDT relays that change state together. 8A non-inductive at 24VDC 
> contacts.

But, one *passively* changes state (its restoring spring is allowed to move
the armature) while the other actively changes state (its armature moving
because of the magnetic force created in the energized coil)

>> One possible complication would be if these were used to maintain state
>> in a nonvolatile manner -- adding that to your circuit would require
>> a nonvolatile store.
> 
> Ah, yes these MUST be non-volatile. The relay must stay set/reset until the 
> mechanical operation resets it, otherwise fuses blow after the gears jam. Power 
> interruptions can't change that state...

<frown>

>> Of course, the "easy" way is a tiny 6 pin MCU with a FET driving the
>> relay...  persistent state could be maintained in FLASH.
> 
> Now you are just getting fancy! (ducking)

No, it was just the easiest way to get the nonvolatile support.

Without the MCU, a cross-coupled NAND (SR latch) could be driven by the
two "low side" coil controls:  pull line A to ground to engage coil A
and release coil B; pull line B to ground to engage coil B and
release coil A.  A pullup and clamp to V(logic) for each input.
The output of the latch driving a FET or BJT to energize the
*4PDT* relay (with two poles wired in the NO configuration and
the other two in the NC configuration to mimic the two relays
being in different states).

If these have to persist in the absence of power (assuming you can
tolerate the brief delay while the relay reenergizes to resume its
previous state when power reappears), then you need something that
will ALWAYS remember.

You likely don't want to have to put coin cells on each such board
to preserve the state of the SR latch.  The MCU made this a bit easier
as you could (hopefully) just set aside a piece of FLASH to store
state (taking into account wear)