Use of old GPO relays for point or signal control

martin goodall
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Use of old GPO relays for point or signal control

Postby martin goodall » Thu Sep 13, 2012 11:02 am

Some years ago I acquired some old relays that appear to be of the PO type (but the precise spec. is unclear). However, I have a profound ignorance of the electrical properties of these devices. A short article on the practical use of these relays appeared in the February 1969 Railway Modeller, and Keith Norgrove also wrote a note on the use of these relays in Prototype 17, but both articles leave some questions unanswered.

First, no indication is given as to the operating voltage. What is the correct voltage to apply to these units? (I vaguely recall they were designed to work on 24 volts, but can they be operated on 12 volts DC? Or can 14/16v AC be used?) Secondly, I gather that the power they draw is significant. Can someone say how many amps they take? Thirdly, is the relay on continuously while the point is thrown or the signal is ‘off’? In which case, will it heat up (and if so, does it need a heat sink or heat shield?). Lastly, depending on the answers to the questions above, might these relays in fact need their own separate power supply?

My profound ignorance of such matters must be obvious from the foregoing, but if anyone can answer these questions, I would be grateful.

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Jol Wilkinson
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Re: Use of old GPO relays for point or signal control

Postby Jol Wilkinson » Thu Sep 13, 2012 11:19 am

Martin,

the original part of London Road uses something similar. They are used to activate the signals which are normally held "on" by a weight, so only draw current when the signal is off.

As I wasn't involved in building the original layout, I don't know exactly what the relays are, but they look like the PO type. They are fed from a 12v transformer through a simple rectifier circuit which looks capable of handling no more than a couple of amps.

Jol

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Russ Elliott
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Re: Use of old GPO relays for point or signal control

Postby Russ Elliott » Thu Sep 13, 2012 12:05 pm

There's no simple answer to what you've got, Martin. 'PO 3000' relays covered a range of coil resistances: 6500, 2000, 1000 and 500 ohms. I think the most common ones were the 1000 and 2000 ohm ones, with nominal operating voltages of 24V and 50V respectively. (DC, don't use AC.) If there is no coil resistance marking somewhere on the body, the only way to find out what you've got is to measure the resistance with a meter. Depending on operating voltage, the current draw is likely to be in the 25 to 50mA range. The nominal operating voltage is very nominal, and I think most types are likely to be happy enough on 12V DC.

They're certainly designed for continuous use, and yes they will get warm accordingly, but the power being dissipated is low ( i2r ), and they do not require extra heatsinking.
Last edited by Russ Elliott on Thu Sep 13, 2012 9:26 pm, edited 1 time in total.

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grovenor-2685
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Re: Use of old GPO relays for point or signal control

Postby grovenor-2685 » Thu Sep 13, 2012 4:06 pm

The telephone exchanges which were the primary use normally operated on a 50V nominal battery, but these relays were also used for a lot of other functions where lower voltages were in use. So its essentially a case of trying out the relay you have to see what voltage you need to get satisfactory operation. It depends also on the mechanical load you are putting on the relay, a return spring adequate for a set of points will be fairly strong and thus need a higher voltage compared to what you might use on a signal. Generally I found very few of the relays would give enough force on 12V, 24 was much better.
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Keith
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Keith
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Tim V
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Re: Use of old GPO relays for point or signal control

Postby Tim V » Thu Sep 13, 2012 4:18 pm

Suggest you try them as Keith says. Mine have a number stencilled on the arm, and some have multiple windings. Some of them work on 12V, mine are designated 2L. Current draw is insignificant in my experience. When they are "on" they draw current all the time, but don't get significantly warm.

Their best feature is they're free!
Tim V
(Not all railways in Somerset went to Dorset)

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Paul Townsend
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Re: Use of old GPO relays for point or signal control

Postby Paul Townsend » Thu Sep 13, 2012 9:09 pm

Highbridge has used these for all turnouts but not signals.
Mine were all 2000ohm, the commonest version, which runs on nominal 50v DC but is quite happy on anything between 30 and 80v continuous according to the mechanical load required and number and style of contacts retained. They usually had two stacks of contacts, each stack being 1 to 4 changeovers. I always discard one side of contacts stack totally and drive the tou from there.

NB, Russ got his respectively inverted!! The 1000ohm ones are for nominal 24v.

If you need to borrow an ohmeter to check yours you know where to come.

If you want more of the 2000ohm version I can offer you a box full.

If you arrange that all relays are off for turnout in the Normal position then the number "pulled " at any moment is small so total current draw will be only a few hundred Milliamps,

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Russ Elliott
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Re: Use of old GPO relays for point or signal control

Postby Russ Elliott » Thu Sep 13, 2012 9:25 pm

paultownsend wrote:NB, Russ got his respectively inverted!!

Whoops. Original now amended.

martin goodall
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Re: Use of old GPO relays for point or signal control

Postby martin goodall » Fri Sep 14, 2012 9:11 am

I am grateful for these various hints and tips. I will dig out the box and see what the labelling on the relays says. I even possess a multi-meter, so could measure the resistance if necessary [scary science for a non-engineer!]

I am not intending to use these relays for point control, as I have installed an entirely mechanical system to control the points on 'Burford'. The relays (if used) would be employed to operate the signals.

I do not intend that the relays should move the signal arms themselves; what I have in mind is using relays [or solenoids as an alternative] to 'tilt' the operating arm of the signal mechanism, using the ‘swinging arc’ design developed by Vincent de Bode (as modified by Ray Hammond). [I believe this is alternatively referred to as a de Bode/Doe [‘Davin’?] operating mechanism.] The 'tilting' mechanism could equally well be operated mechanically, but there are practical reasons that make it more convenient to operate this electrically on this particular layout. This may well suit the use of relays, as very little mechnical effort will be required to move the 'tilting' arm. The actual work of moving the signal arm (in both directions) is done purely by the force of gravity.

martin goodall
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Re: Use of old GPO relays for point or signal control

Postby martin goodall » Mon Oct 01, 2012 4:01 pm

I have managed to dig out my box of old relays. I have five of each of two types:

4/SCO/435
5.5 Ω a b
2300 Ω c d
1.288104 M4AE

2/SCO/434
400 Ω
2011 D

From what Paul wrote, I rather assume that the first type would need 50v DC, whereas 12v might suffice for the second type. Any comments, anyone?

Next (even sillier) question - Which terminals do you connect to the power supply? [You can tell that I could summarise my knowledge of this topic on the back of a postage stamp!]

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Tim V
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Re: Use of old GPO relays for point or signal control

Postby Tim V » Mon Oct 01, 2012 4:16 pm

Try them all, you can't break it!
Tim V
(Not all railways in Somerset went to Dorset)

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grovenor-2685
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Re: Use of old GPO relays for point or signal control

Postby grovenor-2685 » Mon Oct 01, 2012 6:55 pm

Which terminals do you connect to the power supply?

As the contact fingers are all just taken directly out as terminals the remaining terminals that are attached to the coil bobbin are the ones to connect to power, the first relay you quote has two coils one being of very low resistance and probably intended to be in series with something, the 2300 ohm coil on terminals C and D is the one to use, it might just pick up on 12V but will likely prefer 24V. Looking at the terminals with the contacts at the top I think the coil terminals will be abcd from left to right. If you put 12V on the ab coil you will need a 2 amp power supply and things will get warm pretty quick so it will be obvious.
The other relay will only have two terminals on the coil so no mistake possible there.
Keith
Refer http://www.norgrove.me.uk/pointops.htm
Regards
Keith
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Paul Townsend
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Re: Use of old GPO relays for point or signal control

Postby Paul Townsend » Tue Oct 02, 2012 11:42 am

grovenor-2685 wrote:
Which terminals do you connect to the power supply?

, the 2300 ohm coil on terminals C and D is the one to use, it might just pick up on 12V but will likely prefer 24V.

Keith
Refer http://www.norgrove.me.uk/pointops.htm


Just to flesh out the info Keith gave, the 2300ohm coil is designed to work originally on nominal 50volts, but that is to move all the fitted contacts reliably; there could be any number of light or heavy contacts between 2 and about 12. If you are not using the relay contacts, remove them and you will have enough oomph for your job on much lower voltage. You could use any voltage between say 6 and 60V CD to give the oomph you need!

BUT I recommend nowt over 50v or the Elf &Safety lawyers will want to talk to you!

The 400 ohm one will give more oomph per volt at expense of increased current. Again try anything between 5 and say 24v DC and use the lowest voltage that meets your need.

Remember current used Amps= volts/Resistance in Ohms in the steady state and so when you do your trial and choose suitable voltage the power supply must cope with this x No. of relays.
It is quite likely that you will be able to use 12v for your app on both relays; As that should be easily available try that first. They will be totally happy to sit indefinitely with 12v on their coils (NOT the 5.5 ohm ab coil tho')

Finally it is ESSENTIAL that each relay coil has a diode across it to avoid interference with your wife's radio and neighbours TV and your other electrics.
Visit me to borrow a sample diode and demonstration of fitting...it must be the right way round! Once trial done I will tell you where to buy n suitable diodes...any old one is NBG.


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