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whats this socket used for?

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Ian Parkin29/06/2020 13:14:45
1021 forum posts
239 photos

I recently bought a oldish projector which came with a 5 amp round pin plug fitted also a extension with a round pin socket to a 13 amp plug

the socket has a switch which is marked "start" in the off position and "run" in the on position

the plug has "gnome" transfer on it

it wasnt a gnome product that it was connected to

In the socket across the switch is a 213 ohm resister

any ideas what this is for ?

to keep a bulb warmed up in the off position "start" then full beans when in "run" ?img_0056.jpg




Nick Clarke 329/06/2020 13:18:38
1427 forum posts
63 photos

It was to allow the bulb to warm up and so not blow when the power is switched on. Other versions had a NTC thermistor instead of a switch and resistor.

Ian Parkin29/06/2020 13:36:01
1021 forum posts
239 photos

I did wonder if that was the case but the projector it was fitted to has a motor running the minute you plug it in and the bulb on a switch

i cant imagine the motor would have liked running on reduced voltage but maybe it wasnt meant for this projector

Swarf, Mostly!29/06/2020 13:41:09
668 forum posts
73 photos

That 'resistor' looks like a 'Brimistor' to me. I can't remeber whether they were NTC or PTC but my now-vague memories associate them with the heater chains of themionic valve radio receivers of the 'AC/DC' variety. The high tension supply was derived by half-wave rectifying the mains and the heaters of all the valves were connected in series, including a suitable Brimistor, across the mains but with a 'dropper' resistor included to make the chain match 230 volts. Valves were available with high voltage heaters for use in such receivers - they were designed and manufactured to have high cathode to heater voltage ratings. The temperature coefficients of the various components in the heater chain were chosen both to limit 'inrush current' while the valve heaters were yet cold and to stabilise the heater current once the set had warmed up.

I do have a copy of the 'Brimar Valve Book' in the bookcase downstairs - the Brimistor is probably included. I'll have a look later this afternoon.

Best regards,

Swarf, Mostly!

P.S.: I apologise for any typos - I'm due to have my post-cataract surgery eye test tomorrow - it should have been in mid-March!!! Dang lock-down!!!!!!!!

Michael Gilligan29/06/2020 13:43:20
20183 forum posts
1053 photos

The motor load is probably trivial in comparison with the inrush current of the cold lamp.

More sophisticated electronic versions are called ‘soft start’ [or some-such]

... excellent idea when you consider the price of some bulbs.


Bazyle29/06/2020 14:13:03
6324 forum posts
222 photos

I have a Gnome photographic enlarger that runs on a 12v car bulb. We had it in Africa to run off a car battery then a transformer when mains electricity became available.

John Haine29/06/2020 14:19:11
4675 forum posts
273 photos

I got an interesting variant of the AC/DC radio when I were a lad from a jumble sale, probably for sixpence. Got it proudly home but it was d.o.a. My dad (professional electrical engineer with a non-nonsense approach to electrical safety) did some faultfinding and established that it had what he called a "line cord" - basically the dropper resistor was made of resistance wire forming one conductor of the cable! There was a break near one end, the plug end I guess in retrospect, so he shortened the cord slightly, refitted the plug and it worked! I listened to it for quite a long time until something else went wrong. As well as the nice light from the dial for reading in bed you could warm your hands on the cable.


Rod Renshaw29/06/2020 15:58:45
376 forum posts
2 photos

Just in case anyone is puzzled, a "Brimister" is a resistor with a high negative thermal coefficient of resistance. The resistance is high when it is cold and reduces as its temperature rises. So, by placing a Brimister in series with a load, its high cold resistance limits the current surge at switch-on, and as its temperature rises, (due to the current passing through it) its resistance reduces allowing the current to rise to the normal operating condition. This low starting current limits any thermal shock to the load. This operation is normally regarded as automatic which makes me wonder if the OPs equipment was modified in the past and that there was originally an ordinary resister which had to be switched in and out of use.


Georgineer29/06/2020 18:04:55
577 forum posts
32 photos

I can't see from the photo, but my guess is that the resistor is connected as a bypass between the two sides of the switch, and also that it isn't a thermistor.

Photographic bulbs are notoriously short-lived because they are run at such high temperatures, which firstly causes the tungsten filament to evaporate quickly, and secondly causes a huge thermal shock at switch-on, which administers the coup-de-grace to an already tired filament.

I'm sure that the owner of the Gnome didn't want to modify his latest acquisition, so made a special-to-purpose adaptor lead to allow a soft start and make his expensive bulbs last longer. My father made something similar for his slide projector.

For similar reasons I made an adaptor panel I could plug my photoflood lights into, so I could run them in series for setting up and in parallel for the actual shoot.

George B.

Michael Gilligan29/06/2020 18:25:44
20183 forum posts
1053 photos
Posted by Swarf, Mostly! on 29/06/2020 13:41:09:

That 'resistor' looks like a 'Brimistor' to me. […]

- I'm due to have my post-cataract surgery eye test tomorrow -


All the best for the eye-test

Meanwhile, I’ve just found this: **LINK**


Nicholas Farr29/06/2020 18:40:59
3360 forum posts
1542 photos

Hi, I agree, probably a soft start. I fitted a similar thing in my Disco valve amp that I built, only this soft start was on the HT to the valves, so I could get the amp warmed up while stilt setting up, as it wasn't good plugging in speakers etc. in with it fully juiced up, a DIY standby if you like.

Regards Nick.

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