I'm trying to build some simple sequential logic circuits with
relays (for fun and demonstration, not for any practical purpose
and there are a couple of situations where I really need some make
before break (aka Form D or MBB) contacts to avoid race conditions.
Apparently these used to be pretty common at one time, but today
they're hard to find.

I found an interesting article on the Potter & Brumfield web page
about relay construction that explained how mercury wetted relays were
used to make form D contacts - when the armature switches from one
contact to the other, the mercury temporarily bridges the gap between
all three contacts until surface tension forces the bridge to
collapse.

I've found several surplus sources of mercury wetted relays, but
most of them don't mention anything about make before break - does
anybody know if _all_ mercury wetted relays have this property, or
only some?

I've also found a few new production PCB mount relays that are make
before break, but I really wanted something bigger so people could see
(and hear) the relays operate. Something about the size of a 10A "ice
cube" or octal plug in relay would be about right.

Does anybody know any other tricks for finding relays w/form D
contacts these days?

Thanks,
Bob Armstrong

On Fri, 21 Dec 2007 11:34:39 -0800, Bob Armstrong wrote:

> I'm trying to build some simple sequential logic circuits with relays
> (for fun and demonstration, not for any practical purpose and there
> are a couple of situations where I really need some make before break
> (aka Form D or MBB) contacts to avoid race conditions. Apparently these
> used to be pretty common at one time, but today they're hard to find.
>...
> Does anybody know any other tricks for finding relays w/form D
> contacts these days?

Have you looked at the big supply houses, such as Newark Electronics?
The more exotic contact forms have probably been replaced by solid-state
logic driving simpler relays, but you'd think that there would still be
some demand for Form D. Be aware that the last time I dealt with Newark,
they had a $25 minimum order, and that was at least 20 years ago.

On Dec 21, 12:15=A0pm, "Stephen J. Rush" <sjr...@comcast.net> wrote:

> Have you looked at the big supply houses, such as Newark Electronics? =A0
> The more exotic contact forms have probably been replaced by solid-state
> logic driving simpler relays, but you'd think that there would still be
> some demand for Form D. =A0

Yes, and Mouser and D-K, but thanks for the suggestion. Lots of
relays, but no make before break (at least in any larger size). As
far as I can determine, P&B and Panasonic don't even make any form D
power relays any more -

http://pewa.panasonic.com/pcsd/index.html
http://www.potterbrumfield.com/power_Relays.asp

a surplus part might be my only bet.

Thanks again,
Bob

Bob Armstrong wrote:
(snip)
> Does anybody know any other tricks for finding relays w/form D
> contacts these days?

Not finding, but, maybe, making. I think you can cross
connect two relays that have an extra set of contacts so
that the pair performs as a MBB set. I haven't gone through
the exercise of doing it, but I am pretty sure I have seen
it done. I'll think about it.

--
Regards,

John Popelish

Bob Armstrong wrote:

> I'm trying to build some simple sequential logic circuits with
> relays (for fun and demonstration, not for any practical purpose
> and there are a couple of situations where I really need some make
> before break (aka Form D or MBB) contacts to avoid race conditions.
> Apparently these used to be pretty common at one time, but today
> they're hard to find.
>
> I found an interesting article on the Potter & Brumfield web page
> about relay construction that explained how mercury wetted relays were
> used to make form D contacts - when the armature switches from one
> contact to the other, the mercury temporarily bridges the gap between
> all three contacts until surface tension forces the bridge to
> collapse.
>
> I've found several surplus sources of mercury wetted relays, but
> most of them don't mention anything about make before break - does
> anybody know if _all_ mercury wetted relays have this property, or
> only some?
>
> I've also found a few new production PCB mount relays that are make
> before break, but I really wanted something bigger so people could see
> (and hear) the relays operate. Something about the size of a 10A "ice
> cube" or octal plug in relay would be about right.
>
> Does anybody know any other tricks for finding relays w/form D
> contacts these days?
>
> Thanks,
> Bob Armstrong
If you use DC coiled relays, you can feed each one via a diode to
isolate back drain and place a small electrolytic cap across the coil
poles to maintain a slight delay off.. This will allow another relay
time enough to close before the last one opens.

Actually, if you check the spec's on some DC relays, you'll find some
have a longer opening delay.


--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

Bob Armstrong wrote:
> I'm trying to build some simple sequential logic circuits with
> relays (for fun and demonstration, not for any practical purpose
> and there are a couple of situations where I really need some make
> before break (aka Form D or MBB) contacts to avoid race conditions.
> Apparently these used to be pretty common at one time, but today
> they're hard to find.
>
> I found an interesting article on the Potter & Brumfield web page
> about relay construction that explained how mercury wetted relays were
> used to make form D contacts - when the armature switches from one
> contact to the other, the mercury temporarily bridges the gap between
> all three contacts until surface tension forces the bridge to
> collapse.
>
> I've found several surplus sources of mercury wetted relays, but
> most of them don't mention anything about make before break - does
> anybody know if _all_ mercury wetted relays have this property, or
> only some?
>
> I've also found a few new production PCB mount relays that are make
> before break, but I really wanted something bigger so people could see
> (and hear) the relays operate. Something about the size of a 10A "ice
> cube" or octal plug in relay would be about right.
>
> Does anybody know any other tricks for finding relays w/form D
> contacts these days?
>
> Thanks,
> Bob Armstrong


Well, you can use two relays to get a make before
break function, if that is acceptable, by using the
normally open points of one to turn the other one off:

+-------- out N/C
|
v
+---o--- Ry1-1 (N/C)
|
| Ry2-1 (N/O)
+12 -------+------+---o---
| ^---+---- out N/O
\e |
PNP |---+---[330]---+
/c |
| +---[10K]--- On signal (gnd)
[Ry1] for Ry1
|
|
Gnd -------+

Out N/O will go to +12 before out N/C drops +12.
Ry2-1 will make first and put + on the base
which will cause Ry1 to drop. You can delay
the drop of Ry1 a bit (to make up for contact
bounce) if required with a cap from base to
ground or with a large cap in parallel with
Ry1.

Ed

On Fri, 21 Dec 2007 11:34:39 -0800 (PST), Bob Armstrong <bob@jfcl.com>
wrote:

> I'm trying to build some simple sequential logic circuits with
>relays (for fun and demonstration, not for any practical purpose
>and there are a couple of situations where I really need some make
>before break (aka Form D or MBB) contacts to avoid race conditions.
>Apparently these used to be pretty common at one time, but today
>they're hard to find.
>
> I found an interesting article on the Potter & Brumfield web page
>about relay construction that explained how mercury wetted relays
were
>used to make form D contacts - when the armature switches from one
>contact to the other, the mercury temporarily bridges the gap between
>all three contacts until surface tension forces the bridge to
>collapse.
>
> I've found several surplus sources of mercury wetted relays, but
>most of them don't mention anything about make before break - does
>anybody know if _all_ mercury wetted relays have this property, or
>only some?
>
> I've also found a few new production PCB mount relays that are make
>before break, but I really wanted something bigger so people could
see
>(and hear) the relays operate. Something about the size of a 10A
"ice
>cube" or octal plug in relay would be about right.
>
> Does anybody know any other tricks for finding relays w/form D
>contacts these days?
>
>Thanks,
>Bob Armstrong


There is an old "rule of thumb" saying for relay contact operation on
any relay;

"Make contacts MAKE before break contacts BREAK". This action may or
may not be possible on off the shelf relays because they rarely (ie.
never) allow for mechanical adjustments to be made.

If you do have relays where the above action occurs then combining the
function of a MAKE contact and a BREAK contact together, you can
reproduce a MBB function. I know it is wasteful of contacts but unless
you can find a source of old telephone exchange relays I doubt you
will find anything off the shelf which contains a springpile with
genuine MBB contacts.

On Dec 21, 2:04 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
> If you use DC coiled relays, you can feed each one via a diode to
> isolate back drain and place a small electrolytic cap across the coil
> poles to maintain a slight delay off.. This will allow another relay
> time enough to close before the last one opens.

On Dec 21, 4:05=A0pm, ehsjr <eh...@bellatlantic.net> wrote:
> Well, you can use two relays to get a make before
> break function, if that is acceptable, by using the
> normally open points of one to turn the other one off:

Thanks, but using solid state components would require a lot more
explanations for the demo and obscures the simplicity of the relay
circuits (you can just trace out the electrical paths on the schematic
with your finger)

I could use the two relay version and have the NO contacts on RLY1
simply shunt the coil of RLY2 (with a current limiting resistor, of
course). I may have to resort to that if I can't find any proper form
D relays.

Thanks again,
Bob

Bob Armstrong wrote:
> On Dec 21, 2:04 pm, Jamie
> <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
>
>> If you use DC coiled relays, you can feed each one via a diode to
>>isolate back drain and place a small electrolytic cap across the coil
>>poles to maintain a slight delay off.. This will allow another relay
>>time enough to close before the last one opens.
>
>
> On Dec 21, 4:05 pm, ehsjr <eh...@bellatlantic.net> wrote:
>
>>Well, you can use two relays to get a make before
>>break function, if that is acceptable, by using the
>>normally open points of one to turn the other one off:
>
>
> Thanks, but using solid state components would require a lot more
> explanations for the demo and obscures the simplicity of the relay
> circuits (you can just trace out the electrical paths on the schematic
> with your finger)
>
> I could use the two relay version and have the NO contacts on RLY1
> simply shunt the coil of RLY2 (with a current limiting resistor, of
> course). I may have to resort to that if I can't find any proper form
> D relays.
>
> Thanks again,
> Bob

Jamie mentioned another approach you might consider,
in addition to the shunt idea. I think this portrays
his idea:

+ ----+
|
| Ry2-1
----o o o-------+-----+
| | | | | +
> |<-' [Ry1] [C1]
| |
Gnd -------------+-----+

You can size C1 for long enough delay before
Ry1 drops, to let Ry2 transfer and stop bouncing.

Ed

On Sat, 22 Dec 2007 07:54:16 -0800 (PST), Bob Armstrong
<bob@jfcl.com> wrote:

>On Dec 21, 2:04 pm, Jamie
><jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
>> If you use DC coiled relays, you can feed each one via a diode to
>> isolate back drain and place a small electrolytic cap across the coil
>> poles to maintain a slight delay off.. This will allow another relay
>> time enough to close before the last one opens.
>
>On Dec 21, 4:05 pm, ehsjr <eh...@bellatlantic.net> wrote:
>> Well, you can use two relays to get a make before
>> break function, if that is acceptable, by using the
>> normally open points of one to turn the other one off:
>
> Thanks, but using solid state components would require a lot more
>explanations for the demo and obscures the simplicity of the relay
>circuits (you can just trace out the electrical paths on the schematic
>with your finger)
>
> I could use the two relay version and have the NO contacts on RLY1
>simply shunt the coil of RLY2 (with a current limiting resistor, of
>course). I may have to resort to that if I can't find any proper form
>D relays.


---
If you want to use two relays you can get the make-before-break
function in both directions by wiring the contacts like this:

View in Courier:

+------------------+-->NO
| |
+------------|-----+------------|-->NC
| | | |
O-->\ <--O O-->\ <--O
NC1 \ NO1 NC2 \ NO2
OC1 OC2
| |
+------------------+--------->C


and using a 3 position shorting switch wired like this:

.. NC
.. /
..+V---------O--->O NC+NO
.. /
.. +-[COIL1]--O--[<1N400X]--+
.. | |
..GND>-+-[COIL2]--O-------------+
.. \
.. NO


--
JF

On Dec 22, 9:54=A0am, Bob Armstrong <b...@jfcl.com> wrote:
> On Dec 21, 2:04 pm, Jamie
>
> <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
> > =A0 If you use DC coiled relays, you can feed each one via a diode to
> > isolate back drain and place a small electrolytic cap across the coil
> > poles to maintain a slight delay off.. This will allow another relay
> > time enough to close before the last one opens.
>
> On Dec 21, 4:05=A0pm, ehsjr <eh...@bellatlantic.net> wrote:
>
> > Well, you can use two relays to get a make before
> > break function, if that is acceptable, by using the
> > normally open points of one to turn the other one off:
>
> =A0 Thanks, but using solid state components would require a lot more
> explanations for the demo and obscures the simplicity of the relay
> circuits (you can just trace out the electrical paths on the schematic
> with your finger)
>
> =A0 I could use the two relay version and have the NO contacts on RLY1
> simply shunt the coil of RLY2 (with a current limiting resistor, of
> course). =A0I may have to resort to that if I can't find any proper form
> D relays.
>
> Thanks again,
> Bob

Hi, Bob. Most old-line relay manufacturers will always do specials,
but you have to deal with minimum order and enormous lead times. I'll
bet Magnecraft or P&B will do your relay, but it'll be very expensive
and a long way down the road. I'd give up on trying to find what
you're looking for as a single purchase part.

For demo purposes, you'll have to cobble something together. In order
to quell all chase conditions, you'll have to have a one coil delay
(usually in the 10ms. ballpark for standard 10 amp industrial-type
relays). A good way to do this is with two relays, where the contact
of one drives the coil of the second, like this (view in fixed font or
cut&paste to M$ Notepad):

| |
| T _ |
| --- / \ |
o----o o---------------------(RY1)---o
| \_/ |
| |
| RY1 _ |
| ||/ / \ |
o----||----------------------(RY2)---o
| /|| \_/ |
| |
(created by AACircuit v1.28.6 beta 04/19/05 http://www.tech-chat.de)

Here's a timing diagram of the two relays:


|
1| .--------------------.
| | |
SW1 | | |
| | |
0| ---' '------
|
|
1| .--------------------.
| | |
RY1 | | |
| | |
0| -----' '----
|
|
1| -------. .--
| | |
RY2 | | |
| | |
0| '--------------------'
|
(created by AACircuit v1.28.6 beta 04/19/05 http://www.tech-chat.de)

MAKE =3D RY1 + 'RY2
BREAK =3D '(RY1 * 'RY2), or using DeMorgan:
BREAK =3D 'RY1 + RY2

This leads to an independent Form D make-before-break contact like
this:
| |
| RY2 |
| ||/ |
| .--||----. |
| | /|| | |
| | | |
| | ||NO | |
| | ||----o--------NO |
| | || |
| C---o--o RY1 |
| C||/ || |
| ||----||-------NC |
| /||NC || |
| RY2 |
(created by AACircuit v1.28.6 beta 04/19/05 http://www.tech-chat.de)

Note that you'll need two 3PDT relays to get a single-pole Form D
contact like this, but sometimes it's necessary. Note also that if
you can arrange to wire up the common of your Form D contacts to the
left supply rail, you can save a relay contact on RY1:

| |
| T _ |
| --- / \ |
o----o o---------------------(RY1)---o
| \_/ |
| |
| RY2 |
| ||/ |
| .--||----. |
| | /|| | |
| | | |
| | ||NO | |
| | ||----o--------NO |
| | || |
o------o--o RY1 |
| C||/ || |
| ||--o-||-------NC |
| /||NC| || |
| | RY2 |
| | _ |
| | / \ |
| '--------------(RY2)---o
| \_/ |
| |
(created by AACircuit v1.28.6 beta 04/19/05 http://www.tech-chat.de)

A lot of the mystery in industrial relay logic is made clear when you
are aware of the little tricks. You can see things like this in the
wiring diagram, and then the spaghetti makes sense.

Cheers
Chris