[EE] Latching relay question

[David C Brown]

I am planning to use two coil latching relays - Panasonic  *TX2-L2-12V*  -
in a control circuit.  I have discovered a rare condition in which both set
and reset coils couls be energised.

Would this damage the relay and, if not, how would it operate?  IO presume
that it depends on which coil is energised first
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak                           Phone: [phone]
Derbyshire                eMail: [hidden email]
SK23 7ND          web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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[Dave Tweed]

David C Brown wrote:
> I am planning to use two coil latching relays - Panasonic *TX2-L2-12V* -
> in a control circuit. I have discovered a rare condition in which both set
> and reset coils couls be energised.
>
> Would this damage the relay ...

Probably not.

> ... and, if not, how would it operate? I'd presume that it depends on which
> coil is energised first

Actually, just like any electronic S-R latch, it's more likely that it depends
on which coil is energized *last*.

-- Dave Tweed
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[Harold Hallikainen-3]

The datasheet gives very little information on how this relay actually
works, but I think there is a permanent magnet in there that holds the
contacts in the latched position.

I think we have three magnetic fields: the permanent magnet, the latch
coil, and the unlatch coil. When neither coil is energized, the permanent
magnet is strong enough to hold the contacts in the latch state, but not
strong enough to pull the contacts from the unlatched state to the latched
state. When the latch coil is energized, its magnetic field is added to
the permanent magnet field creating a field that is strong enough to
overcome the relay hysteresis and pull the contacts to the latched
position. When the latch coil current is removed, the permanent magnet
holds the contacts in the latched position.

When the unlatch coil is energized, it generates a magnetic field that
opposes that of the permanent magnet. The total field is no longer strong
enough to hold the contacts in the latched position, and the contacts move
to the unlatched position. When the unlatch coil current is removed, the
permanent magnet field is the only field present, but it is not strong
enough to pull the contacts to the latched position.

There are several possibilities for when both coils are energized. In
general, I think the contacts will stay in the position of the first coil
to be energized while both are energized, but then move to the position
indicated by the last to be energized. For example:

1. Latch coil energized - move to latched positon.

2. Unlatch coil energized - Total magnetic field now just the permanent
magnet (latch and unlatch fields cancel), so we stay in the current
latched position.

3. Latch coil de-energized - Now the unlatch coil cancels the permanent
magnet field, so the contacts move to the unlatched position.

Latching relays are interesting! Many years ago I needed to convert
momentary contact closures to ground to a set of latched contacts. I did
this by putting two resistors in series with the relay coil, one on each
side. So the circuit was +12V to resistor to coil to resistor to ground.

To latch the relay, the resistor to ground was shorted by grounding that
side of the relay coil. This increased the coil current above the pull-in
current so the relay pulled in. When the momentary closure was removed,
the relay coil current was above the hold current, so the relay stayed
latched.

To unlatch the relay, the side of the coil with the resistor to plus
supply was grounded. This removed the relay current, so the relay
released. When the short to ground was removed, relay current returned,
but it was not enough to pull in the relay, so it stayed released.

Finally, on powering both coils, the only issue I can think of is that if
this is done for a long period of time, the dissipation in the coils will
be higher than normal. This would result in higher coil temperature,
higher coil resistance, and change in relay characteristics. As I recall,
the datasheet has information on temperature characteristics.

Harold

> David C Brown wrote:
>> I am planning to use two coil latching relays - Panasonic *TX2-L2-12V* -
>> in a control circuit. I have discovered a rare condition in which both
>> set
>> and reset coils couls be energised.
>>
>> Would this damage the relay ...
>
> Probably not.
>
>> ... and, if not, how would it operate? I'd presume that it depends on
>> which
>> coil is energised first
>
> Actually, just like any electronic S-R latch, it's more likely that it
> depends
> on which coil is energized *last*.
>
> -- Dave Tweed
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

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[David VanHorn-2]

It is possible that there would be contact damage due to low pressure.
When a normal relay is "off" the spring holds the NC contacts closed with a
certain amount of force.
When "on", the coil holds the armature, and there is usually some spring
action in the armature, actually reducing the contact force.
Without seeing the mechanicals, my best guess would be that the armature
would be weakly touching one side or the other.
At high currents this could cause heating of the contacts, or if you get it
just perfectly wrong it could arc.
Just my supposition, not saying this will happen.



On Sat, Jun 27, 2020 at 11:06 AM Harold Hallikainen <
[hidden email]> wrote:

> The datasheet gives very little information on how this relay actually
> works, but I think there is a permanent magnet in there that holds the
> contacts in the latched position.
>
> I think we have three magnetic fields: the permanent magnet, the latch
> coil, and the unlatch coil. When neither coil is energized, the permanent
> magnet is strong enough to hold the contacts in the latch state, but not
> strong enough to pull the contacts from the unlatched state to the latched
> state. When the latch coil is energized, its magnetic field is added to
> the permanent magnet field creating a field that is strong enough to
> overcome the relay hysteresis and pull the contacts to the latched
> position. When the latch coil current is removed, the permanent magnet
> holds the contacts in the latched position.
>
> When the unlatch coil is energized, it generates a magnetic field that
> opposes that of the permanent magnet. The total field is no longer strong
> enough to hold the contacts in the latched position, and the contacts move
> to the unlatched position. When the unlatch coil current is removed, the
> permanent magnet field is the only field present, but it is not strong
> enough to pull the contacts to the latched position.
>
> There are several possibilities for when both coils are energized. In
> general, I think the contacts will stay in the position of the first coil
> to be energized while both are energized, but then move to the position
> indicated by the last to be energized. For example:
>
> 1. Latch coil energized - move to latched positon.
>
> 2. Unlatch coil energized - Total magnetic field now just the permanent
> magnet (latch and unlatch fields cancel), so we stay in the current
> latched position.
>
> 3. Latch coil de-energized - Now the unlatch coil cancels the permanent
> magnet field, so the contacts move to the unlatched position.
>
> Latching relays are interesting! Many years ago I needed to convert
> momentary contact closures to ground to a set of latched contacts. I did
> this by putting two resistors in series with the relay coil, one on each
> side. So the circuit was +12V to resistor to coil to resistor to ground.
>
> To latch the relay, the resistor to ground was shorted by grounding that
> side of the relay coil. This increased the coil current above the pull-in
> current so the relay pulled in. When the momentary closure was removed,
> the relay coil current was above the hold current, so the relay stayed
> latched.
>
> To unlatch the relay, the side of the coil with the resistor to plus
> supply was grounded. This removed the relay current, so the relay
> released. When the short to ground was removed, relay current returned,
> but it was not enough to pull in the relay, so it stayed released.
>
> Finally, on powering both coils, the only issue I can think of is that if
> this is done for a long period of time, the dissipation in the coils will
> be higher than normal. This would result in higher coil temperature,
> higher coil resistance, and change in relay characteristics. As I recall,
> the datasheet has information on temperature characteristics.
>
> Harold
>
>
>
> > David C Brown wrote:
> >> I am planning to use two coil latching relays - Panasonic *TX2-L2-12V* -
> >> in a control circuit. I have discovered a rare condition in which both
> >> set
> >> and reset coils couls be energised.
> >>
> >> Would this damage the relay ...
> >
> > Probably not.
> >
> >> ... and, if not, how would it operate? I'd presume that it depends on
> >> which
> >> coil is energised first
> >
> > Actually, just like any electronic S-R latch, it's more likely that it
> > depends
> > on which coil is energized *last*.
> >
> > -- Dave Tweed
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
>
>
> --
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> http://mailman.mit.edu/mailman/listinfo/piclist
>

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