Electrical help needed Magnetic relay 5C/1937 Type J

You will need to use the 5C/2472 as a battery relay as the one you have is only used for the starter. I have had no problems with these units on the Dragon, Fox Moth and the Auster.
Sorry OneEighthBit but these items are BRITISH and are therefore designed to last many lifetimes.

Wonderful information everyone. I knew this relay was rated for 600 amps so thought it would be bullet proof. After reading Pete and Terrys’ documents though it seems like energising them continuously is not a good idea (2 min recommended maximum). It just goes to show you should use the appropriate parts rather than just whatever you have lying around, will try to source some modern replacements now.
Regards Paul

Relays Type J;

Of probably no use to you but when I was researching the Horsa modifications it gives the use of a 5C/649 Magnetic Relay to isolate the low-current 12v circuit from the 12v 40amp/h circuit for the landing lamp. I think the equivalent for 24v would be 5C/862 maybe?

That said, if you can’t see the relays once installed I’d say it’s be safer putting a modern new one in that some NOS 60+ year old kit 🙂

Hi

The power dissipated once the relay has closed is just V x A, this is just the ‘loss’ through the resistance of the coil, you will have added some power to make the magnetic circuit but once this is established it does not need further power to be added – you have ‘stored’ it in the magnetic field set up. That stored power is given back to you when you try to ‘break’ the circuit and the field collapses.

If you measure V and A of the coil supply then this will tell you how much power will be dissipated as heat, the temperature (not the same as heat of course) at which the relay stabilises will depend on the cooling efficiency of the whole relay assembly. In the words of Yoda “there is no expect”. If you successfully and completely prevent the heat from a small candle escaping from an enclosure the temperature inside will rise to infinity.

Of course it is unlikely that the relay would have been designed to run too hot.

The reason why two coils are used to pick up and hold a relay is because when the relay is open the magnetic circuit has a physical ‘gap’ – that of the open armature. To a magnetic circuit this is the equivalent of a ‘resistance’ as air is many times less permeable than iron. To overcome the resistance you need to pour on the current which generates a high magnetic field. This pulls the armature ‘in’ and as a result the air gap is lost and the magnetic ‘resistance’ is gone and now you don’t need anything like the current to maintain the circuit, thus the use of a second smaller coil and lower current. (The big coil that you used to pull the armature in will still have the same resistance so you can’t expect it to suddenly run cooler when the armature closes). What you could do though is to introduce a resistance in series with the coil supply, another way of reducing dissipation.

Don’t forget that there is another resistance involved in the relay – that of the contacts themselves, this may seem tiny but if it is an old relay the contacts may be dirty and thus a higher resistance, with a 20A current passing though a resistance of say 0.05 ohms the power dissipated at the contacts would be I squared R or 20W. While this may not heat the whole relay it is enough to cause smoke if the contacts are dirty.

Hope that is informative even if not helpful!

Cheer
James

I think a commercially available 24v relay would be much better for the job. They don’t cost much either.

[ATTACH=CONFIG]235166[/ATTACH] I would suspect they are only intermittently rated, looking at the coil resistances, it appears that the type K is the same relay but with a continuous rated coil.

Pete

Thanks for the answers, I was starting to think it may be designed for only short use cycles which Beaufighter has confirmed, still don’t know why they’re getting so hot though, there is not much power going through them.

The Type J relay is a start relay, for passing high current during the engine start cycle but has also been utilised for continuous use where current drain is low.

Something like a type “D”?Rated for 200 amps,used a lot in 1950s aircraft.

Paul
I am not familiar with the relays, bit before my time, but it sounds like you need relays with 2 coils. One to close and one to hold closed.

The closing coil has less resistance and therefore more current to energise the relay. As it closes it makes a set of contacts to complete the circuit of the holding coil which is much higher resistance and less current and less heat.At the same time another set of contacts opens the circuit to the closing coil. When you switch off all returns to default
Cheers
VW