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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Replacing mechanical Latching Relays with dual coil relays...
Date: Tue, 27 Aug 2024 11:10:10 -0400
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On Tue, 27 Aug 2024 07:06:03 -0700, John Robertson <jrr@flippers.com>
wrote:

>On 2024-08-27 2:44 a.m., piglet wrote:
>> John Robertson <jrr@flippers.com> wrote:
>>> Anyone have experience with dual coil latching relays? I'm trying to
>>> replace old (1950s) Guardian latch relays as used in 1950s style
>>> jukeboxes with more modern two coil latching relays but have had trouble
>>> with some relays going to an indeterminate state if the power to the
>>> coil is  below optimum.
>>>
>>> What I actually need to do is a circuit that only allows the Latch/Reset
>>> coils to trip when there is sufficient voltage and current available...
>>>
>>> This somewhat plays into the discussion on DC relay latching voltage
>>> thread and I'm wondering if the Zener diode across the windings or in
>>> series with the windings might help my design.
>>>
>>> As for the old original relays, the contacts are getting so pitted they
>>> can't be saved and Guardian hasn't made this coil assy for decades. They
>>> don't turn up on eBay either.
>>>
>>> Thanks,
>>>
>>> John :-#)#
>> 
>> Millions of latching relays are still made. If the problem is under
>> voltages causing erratic activity I guess a driver with threshold detection
>> using LM339s or TL431s or whatever might be possible. Either to trigger
>> dual coil latch relays or single coil bipolar latch relays.
>
>The problem is switch noise.
>
>> 
>> Can you give numbers, like correct coil voltage and the troublesome
>> incorrect voltages ?
>
>Correct coil voltage is 24VDC.
>
>Modern latching relays don't have a mechanical interlock - at least I 
>haven't seen any - like the original Guardians. So if the microswitch to 
>trip the relay is noisy then the modern latch relays go into a middle 
>indeterminate state and then we get jammed gears - blown fuses.
>
>> 
>> Something as simple as series SCR with zener anode to gate might be enough?
>>
>
>Yes, something like that may work, I'll have to post the schematic of a 
>typical interlock relay when I have some time. Whatever is figured out 
>has to not allow an indeterminate state.

There is a large literature on electronic designs for interlocking
safety-critical functions, like firing a rocket or detonating a bomb.
This literature can be a source of circuit ideas and hazards to be
addressed.

Joe Gwinn