Very sensitive dual coil relay

[Refugee]

Inspired by this thread-:
https://www.vintage-radio.net/forum/...d.php?t=162696
I spotted the device when looking for a mechanical part and dragged it out along with a couple of others of the same kind.
In operation it is a bit like the high sensitivity telegraph relays in the linked thread but with two coils on a single former.
Unfortunately the manufacturers label has gone teflon so any help with an ID would be nice.
Starting with the one that had been opened I took the top off again and put a DMM across it. I could just about see the coil move on the K ohm range before the auto ranging went to ohms and then it moved about half way to the stop and then it read a bit over 500 ohms. The diode range drove it to the stop but not violently. That was about 1ma.
I then set about some more testing.
When I reached out for another DMM an AVO 9 came out of my stash because the meter I was reaching for was in another room.
I added a 22K resister in series with the DMM on ua range from a 30 volt power supply and connected it to one that has never been opened.
With one coil energized the SPCO center off switch contact operated reliably at just under 200ua in both directions. The operation was consistent and repeatable once I had cleaned the contacts by setting the AVO to low ohms and operating it a few times. These items had been laying in a tin with some GPO relays and a few switches so they needed cleaning.
So it looks like an electro-mechanical comparator.
I tested it with an opto resistor of the ORP kind about 40 years ago on one coil again and got myself a fancy daylight sensor.
I would guess that they would originally have been biased on both coils and then switch on the difference.
They came out of an unknown chassis from some kind of automotive test set that had OC series germanium transistors in it so they would date from the early 1960s.

[G6Tanuki]

I remember a very similar moving-coil relay was used in the R4187 military receiver.

This [as used in Vulcan bombers] had a load of crystal-controlled channels, and a front-end variable capacitor tuned by a massively-geared-down DC motor. There was a potentiometer attached to the tuning-capacitor spindle, and a series of multi-turn preset potentiometers - one per channel - also switched by the crystal-select switch.

The potentiometers, the moving-coil relay and another bigger relay formed a 'wheatstone bridge' arrangement which switched power to the tuning-motor - which would run in the appropriate direction until the bridge was balanced, the moving-coil relay sat in its centre-off position, and motor-power was then cut.

Change channel - a different preset potentiometer was selected, the bridge unbalanced, the relay went left or right and the motor turned again. From memory the R4187 relay was marked as 115-0-115 Microamps - which is kinda sensitive.

[Refugee]

That sounds like the same sort of thing.
The chassis it mine came out of was all built on .15 inch varo board.
I adapted one or two of the circuits for experiments all those years ago.
I still have some elapsed time indicators and mechanical counters from another chassis from the same place.

[Radio Wrangler]

I had a single coil ultra sensitive relay. It came out of an unknown avionics chassis from Padgett's in Liversedge. I can't remember the maker's name but the label on it said 37uA sensitivity. I'd never heard of a relay like that before. There were Teflon B7A valveholders as well.

David

[woodchips]

You don't give any idea of size, but I have several that look the same, overall length about 1.5/16".

Mine are marked:

Made in England
Model R350B
4mA nom | one
8mA max | coil
Elliott
Date 1 67
Connection diagram with 2 coils and SPCO

Presumably they are all the same but not looked.

Came out of some sort of ex-military equipment, can't now remember, but seemed worth keeping.

[Refugee]

Those dimensions sound about right.

[woodchips]

Correction, nominal current is 0.4mA, not 4mA.

[Refugee]

That sounds a lot closer to the spec of mine.
I take it yours just looks like mine.

[russell_w_b]

We used those in the Marconi B6122 (or BD272) HF transmitters, in - I think - the Unbalanced Power protection circuits. The relay is basically a meter movement in its own enclosure and driving contacts instead of a pointer.

[woodchips]

Yes, my ones look like yours, but seems to be more than one manufacturer.

[kalee20]

Originally Posted by russell_w_b

We used those in the Marconi B6122 (or BD272) HF transmitters, in - I think - the Unbalanced Power protection circuits. The relay is basically a meter movement in its own enclosure and driving contacts instead of a pointer.

I recall an F G Rayer project in Practical Wireless, around 1954, for a radio control receiver in which a 27MHz tuned circuit fed a germanium diode and a 'microamp relay' which he described how to make out of a surplus microammeter with a contact on the pointer. This then switched current from a battery to the coil to a conventional relay.

It had the advantage that power was only drawn from the battery when the transmitter was keyed. No LT and HT batteries needed whatsoever.

[Refugee]

Originally Posted by russell_w_b

We used those in the Marconi B6122 (or BD272) HF transmitters, in - I think - the Unbalanced Power protection circuits. The relay is basically a meter movement in its own enclosure and driving contacts instead of a pointer.

The spec printed on yours is consistent with my findings on test.
Well done.