This isn't meant to be an educational posting where you will learn a bunch of stuff about relays, but I've being doing some work recently that I thought might be of interest to some, particularly old-school engineers, to see how things change over time.
I would think we're all used to seeing modern relays that plug into breadboards and PCBs to switch signals. I've opened one up, mostly, to see what's inside - mostly - I wanted it to still work after surgery!:
A 9V DPDT relay - you can see the contacts, which are Normally Open (NO) in the image above.
Same relay from above and side, contacts on left. Here you can see the movable armature(contact) springs.
Not shown in the images above is the coil - that's hidden in the body. Just as a recap of how a relay works - a voltage is applied across the coil terminals (pins on the right) which magnetises the coil itself. That operates the armature(s) closing* the contacts and allowing a separate signal to flow through the relay terminals (3 pins on the left.) That's a simplistic explanation but suffices.
*relays will come in different types and operate in different ways:
- Normally Open (NO) - in the non-energised state, contacts are not made
- Normally Closed (NC) - in the non-energised state, contacts are made
- Changeover - when energised, contacts switch from one position to an alternate position
- Make before Break - when energised, one set of contacts switch from one position to an alternate position THEN the contact which is carrying a signal will open, transferring the signal to the made contact
- Break before Make - when energised, the set of contacts that is carrying the signal open THEN a separate contact will close and pick up the signal.
So we're in a world of miniature relays but in the words of Crocodile Dundee: "Call that a relay? That's not a relay. THIS is a relay"
The image above is a British Tabulating Machine Company Ltd Type A Multi-Relay, here with the coils and contact spring terminals visible. There was no, one, configuration for a Type A relay - they were built up from a parts bin into the configuration required and BTM used them for the punch card machines they built. Here, we see a relay that we (the Bombe rebuild team) use on the Bombe for indicating letters in the machine's Indicator Unit - BTM were responsible for the design and build of the Bombe under the codename CANTAB and chief engineer Harold 'Doc' Keen.
Firstly, I need to point out that the Type A relay as seen above was not used on WW2 Bombes, we've had to use 5 of these because we can't find enough of the Multi Relays that were actually used on the Bombe:
As you can imagine, getting hold of any parts that are 80 years or so old isn't easy but we can, and have, built up alternatives that whilst still maybe 50 - 60 years old are more findable. The images above show that similar mechanisms to small, modern relays are used, just on a different scale. The coil, under the yoke, is energised creating a magnetic field that pulls in the armature at the front of the coil, which is hinged and lifts the armature (contact) springs in the relay pile up above the yoke. This relay consists of two pile ups with a mix of changeover and break before make contacts.
Back to the Type A relay unit which is in a bit of a sorry state, but it's had a hard life before we got our hands on it, and I've removed it from the machine to clean up as far as possible. It consists of 3 coils and 3 pairs of Relay Pile Ups (6 Pile Ups altogether) used to indicate one of 12 letters on the machine's indicator unit - really it's 3 linked Type A relays, you should be able to see the shorting bars on some of the contact spring terminals. On the Bombe itself we have 5 sets of these plus 2 of the contemporary 'correct' multi relays that can indicate one or more of 26 letters across 3 Chains. I'm aware I'm using terminology from the Bombe here but basically:
- The Bombe is running through a set of checks looking for what it would call a Good Stop.
- At a Good Stop it stops running and, amongst a host of other things that happen, a contact is made on the appropriate indicator relay to flag a letter (typically, but could be more than one) in an Indicator Unit which consists of 3 lots of 26 letters.
- The Bombe can check 3 different set ups simultaneously each of which is termed a 'Chain'.
- Each Chain corresponds to one of the 3 groups in the indicator unit so that the operator knows which Chain has found the good stop.
That's it for the Bombe and contextualising the relay. As I say, I'm taking this apart to clean it - there's lots of gunk and rust in the mechanism and it is way, way out of operating spec. Normally I would use an electrolysis bath to clean up the steel parts but as it is the first time I've done this I wanted to be a lot more hands on and get a feel for the parts and the operation. This time then, I'm using very fine steel wool, cleaning paste, IPA and more patience than is right for any one person to possess - to give you an idea, it has taken about 45 - 50 hours to clean the whole lot. Here's a comparison and a good photo of one of the pile ups:
Anyone who knows anything about these things can see immediately how far out of adjustment it must be (also, how dirty it is.) There's quite a few parts in each relay pile up and below is an exploded view of just one of the 3 relay units:
Here it is cleaned and re-assembled:
A lot cleaner but still out of spec. As a comparison of size:
Of course, there are bigger relays used in industry than we use on PCBs and breadboards but the Type A relay was in common use until at least the 1960s (as far as I can tell!)
What this thing needs now is adjusting and testing but I don't have the tools to hand and need to wait until April to get going on that. If anyone's interested I'll follow up with information on that process seperately.