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Thermal Photocells
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Author:  meppso [ Fri Feb 03, 2017 1:27 pm ]
Post subject:  Thermal Photocells

Hi All,

I hope everyone is well.

I was looking through my photocells last night and found these two Royce Thompson controllers. As well as asking for some advice I thought it would be nice to open a discussion on the subject

Does anybody have any information on these units? I know sometimes with the electronic ones like the Zodion SS55 controllers there needs to be a link left in the base between two of the terminals. Is there anything like that needed on these?



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Author:  meppso [ Fri Feb 03, 2017 1:41 pm ]
Post subject:  Re: Thermal Photocells

Hi All,

As mentioned in my last message I have been going through my photocell controllers, I have quite a few that don't appear to be working. On opening them up they all seem to have a common point failure, the connection to the heater breaks! This would have led to the lantern day burning when it was used in service.

This week I received a couple of Horstmann T20 controllers, one works and the other doesn't! I opened it up and surprise surprise the connection to the heater had failed. I decided to get hands on with the broken one and try and fix it in the hope I can fix the others. An unusual feature with the T20 is it has two heaters as opposed to one like the Royce Thompson P42 controllers. This particular example had all four connections broken.

With some help from a Mechanical Engineer friend we cleaned the contacts on the heaters, cleaned the wire and metal contacts to the controller, sometime later we have managed to attach the heaters to their respective contacts.

Please see attached photographs.

This weekend I hope to test the controller, fingers crossed.



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Author:  Phosco152 [ Sat Feb 04, 2017 8:13 am ]
Post subject:  Re: Thermal Photocells

The RT one with a dial is probably a part night type, with the dial setting the off time.

The green RT one has the cell connections to the 2 way terminal block on the front. L, Load and N will be on the base, shouldn't be the need for any link wires. 1998 is quite late for a thermal by RT, take the lid off and see what is inside.

RT units used to be black and then went to yellow covers for the 100 Lux units. Yellow was retained for the 100 and 75 Lux electronic variants. The green cover may be because its 55 Lux.

The 3 failure modes of thermal controllers are usually:

1. Spring failure of the snap contact on the bimetallic strip. Usually affects RT cells. As the strip bends, instead of the spring snapping open the contact, it fractures along the fold line meaning it can't push open the contact.

2. Heater detaches from bimetallic strip. Usually affects Zodion cells as they use heatshrink to attach the heater. This becomes brittle over time and splits, the heater falls off and the controller dayburns. RT controllers are far superior, as one heater contact is riveted on, and the other uses a clamp contact.

3. Heater leads become un-soldered - as you have found, again a common fault on Zodions.

The Horstman controller must have been particularly inefficient with 2 heaters. They are usually 4W, so that would be 8W. Doesn't sound a lot but if you have 50000 columns in a city....

The very last produced RT thermals had an "economiser" resistor fitted to them, to reduce the heater load once the contacts had opened. The theory being that, less power is needed to keep the contacts open once they had snapped open. I think this reduced power to approx 2W. I'll have to look through my collection for one and put up pics of some of the other types.

Author:  meppso [ Sat Feb 04, 2017 8:28 pm ]
Post subject:  Re: Thermal Photocells

Hi All,

Good news, my Horstmann T 20 now works, I put it in my test rig and the light now switches off :)



Author:  meppso [ Sat Feb 04, 2017 8:45 pm ]
Post subject:  Re: Thermal Photocells

Hi Phosco,

You were right about the green RT photocell controller, I opened it up and sure enough it is electronic. The switching device is a Relay so it's not completely solid state.

I liked your comments about the RT photocell controllers, I have witnessed all three failure modes which you mentioned on various ones of mine.

The failure mode where the  "snap action" of the spring strip fails is quite an interesting one. I was testing an RT P42 last night and just before it switched the light off I could hear arcing inside the unit and the light started flickering, then it switched off. I remember a Philips MA50 near my house flickering on a morning just before switch off, this too was controlled by a P42. You can tell when it's going to be a "lazy" spring action as the click isn't quite as sharp, if that makes sense!

It is a good testament to both RT and Horstmann that after 40+ years of service their controllers are still working (well some of them) :-)

I do find the old thermal controllers a miss off the streets of Cumbria. I liked the fact that as they became old they drifted out of calibration, some coming on with cloud cover and some not coming on until almost completely dark. However some of them remained bang on.

Were RT P42s used a lot down your neck of the woods?



Author:  meppso [ Sat Feb 04, 2017 11:34 pm ]
Post subject:  Re: Thermal Photocells

Hi All,

As you may know by now I have been going through my Photocell controllers and testing them, it's a long drawn out process but enjoyable. It's nice when I spot an old one and it still functions :)

I am now testing Zodion SS55s and RT P42s in parallel, it's great to see how they alter each others switching pattern (Sad I know). The Zodion controlled light flickers and flashes until the P42 controlled light switches on, the Zodion light then switches off or slows down the flash rate. It's interesting to see this lack of hysteresis in the Zodion controllers (or very small Hysteresis loop) was prominent in their 1990 model and continued right through to their SS6 one part cell.

I came across a P42 with a difference and thought it deserved a picture. It is the 78-84 Model but it has an orange switch as opposed to the white one. I don't know what this denotes but I do know that it came from either James Street or Charlotte street and controlled a 250w SON Alpha 3, as all these streets were fitted with these controllers and lanterns in 79 to 82ish. I can only assume it was to distinguish a switching lux level,



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Author:  Phosco152 [ Sun Feb 05, 2017 7:51 am ]
Post subject:  Re: Thermal Photocells

RT controllers with a failed spring will often initially switch off, then as it becomes more light and the heater gets hotter and causes the bimetallic strip to bend more, the spring would be subject to more force and it would then "fold over" along the weakened fold, causing the contact to close again, and the lamp to dayburn.

The early detector cells would eventually tend to go high resistance and this would mean not enough current to the heater, especially on overcast days, again causing dayburning.

RT controllers have a more elaborate bimetallic switch with adjustment screws for on and off points. Zodions only have an on adjustment, hence their poor hysteresis. This was not normally an issue with the old style detector as it was relatively insensitive and had its own hysteresis, but with a modern cell it made flashing at switch points more likely.

The orange test switch on the RT P42, doesn't denote anything special, ones of that vintage always had orange switches. Earlier and later ones were white. I guess the orange was chosen to stand out, but no doubt as a "special" switch due to its colour, it would have cost more and was discontinued.

2 part switching was extremely common in Hampshire from the late 1960s - when it was used to convert from timeswitches - to around 1980, when NEMA cells came in. Casual replacements caused it to die out initially but there were still some around until the South Coast PFI swept them all away. By the 2000s, the thermal controllers had been replaced with RT P42e electronic controllers.

Wiltshire also used 2 part controllers until a couple of years ago, these were replaced by Telensa RF nodes.

Author:  civiciridium [ Wed Mar 01, 2017 9:06 pm ]
Post subject:  Re: Thermal Photocells


I have a few Zodion SS54B's that have got, however I have never had or seen a thermal 2 part photocells, please could someone tell me where the 2 wires from the detector sensor eye connect too? I have attached a photo of the inside of the SS54B.

Many Thanks

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Author:  Phosco152 [ Wed Mar 01, 2017 9:23 pm ]
Post subject:  Re: Thermal Photocells

Left - Brown = Live in/supply.
Centre - Blue = detector/cell terminal one.
Right - Red = Switched live/load.

Other detector/cell wire goes to neutral and is combined with the load neutral.

Author:  civiciridium [ Thu Mar 02, 2017 6:07 pm ]
Post subject:  Re: Thermal Photocells

Thanks, I think I understand, I have attached a edited photo. Is this what you mean for the wiring setup?

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