Form D (make-before-break) relays ...

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 The more exotic contact forms have probably been replaced by solid-state logic driving simpler relays, but you'd think that there would still be some demand for Form D. Be aware that the last time I dealt with Newark,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.

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 -

a surplus part might be my only bet.

Thanks again, Bob

Bob Armstrong wrote: (snip)

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

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"

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

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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.

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-------+-----+ | | | | | + > |

--
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-->\     \     C


and using a 3 position shorting switch wired like this:

.                  NC
.                 /
.+V---------O--->O NC+NO
.                 /  
.     +-[COIL1]--O--[-+-[COIL2]--O-------------+
.                 \
.                  NO

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

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

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

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

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