Voltage question.

[Dominique De Bast]

Something Strange...I have a Siemens standard/regular 8 projector that is set for the normal European voltage (220 volts) but uses a (p28) 100 volts 500 watts bulb. I have used that projector for years and projected several features mounted on large spools. The bulbs become unusual and expensive so I found a possible replacement one but it's a 120 volts one. So far, nothing difficult even for me The big puzzle is that I put my voltmeter on the soquet to see if I get really 100 volts or if it's more 105 or 110 volts (given that the voltage from the wall is 230 volts, not 220. And there, surprise, I read 230 volts. Since none of my 100 volts bulbs exploded or seemed to give more light than expected, I'm completely puzzled. How is that possibe ?

UPDATE : I found two adds for the same projector and both confirm the bulb is 100 v 500 watts, although Nothing on the projector suggests it. To see the projector, it's necessary to scroll down since the auctions ended.

https://www.ebay.fr/itm/SIEMENS-PROJ...-/133361177301

https://www.catawiki.eu/l/25551781-s...-8mm-projector

[Steve Klare]

Hi Dominique,

This is at best a guess, and maybe even a reach, but maybe this is a case where they reduced the voltage by putting a power resistor in series with the lamp. This would be a colossal waste of power and would make the thing run really hot! (really old electronics are funny sometimes...)

The thing is a voltmeter draws almost no current, so there would be almost no drop across this resistor during measurement, so you'd see the full voltage of the source. If you stick a bulb in there, the current through it would pass through the resistor too, and pull the socket voltage down.

Here's something you can try. Unplug the machine (!!!!). Change your meter to Ohms. Plug the leads into the socket and measure the resistance with the lamp switch on.

If it's just a transformer winding, you should see some very low resistance. If it really is a resistor, you should see something like 20 Ohms.

If all you see is an open circuit, try shorting the two prongs of the power cord together. (BECAUSE you unplugged it!)

EDIT!




This picture is from one of the websites you sent. That character over on the left is a majorly serious power resistor! I've seen things like this in electric heaters a number of times!

-so maybe my theory is not as whacky as I originally thought!

Could you check if the resistor is connected directly and solely to the lamp socket at one end? (-AGAIN! Unplugged it! We need our members healthy!)

(I like this projector: I'd buy it just for the color!)

[Dominique De Bast]

Thanks a lot for your quick and clear answer, Steve !

I set my meter on x10 ans x 1k in the ohm zobe

and at x10, the "arrow" went aound the number 20. Not Always, I had to try several times to get some réaction.

My projector has the same scarying thing 😀

I always wondered why Siemens choosed a 100 volts bulb in a 220 volts projector...

Do you think I can safely replug the machine, put the bulb in and try to light on ?

[Steve Klare]

I think it should be fine to put the regular bulb back. (It looks like your resistor has had some hot times!)


Here's a test after the bulb is back in:

• Plug it in.
• Run the machine about a minute without the lamp lit.
• UNPLUG the machine (!!!!!).
• Carefully feel if the resistor is hot. (It may get VERY hot!)
• Replug
• Run the machine about a minute with the lamp lit.
• UNPLUG the machine (!!!!!).
• Carefully feel if the resistor is hot again.

If the resistor is only hot the second time, then the resistor is in series with the lamp.

(Please be careful!)

[Dominique De Bast]

I'll try that at once.

UPDATE : test made. The bulb did not explode The first time, the resistor was cool and the second time it was hot. You really know what you're talking about, Steve ! Thanks a lot.

[Janice Glesser]

Outstanding!!! What a great thread! Steve...I'm so impressed! It's troubleshooting techniques like you have listed that are invaluable to us electrically challenged souls.

[Dominique De Bast]

Janice, I would have called myself a dummy but "electrically challenged soul" sounds better. Steve has impressed me more than I can tell.

[Steve Klare]

As long as we got through this without Dominique getting burned or electrocuted!

[Dominique De Bast]

I'm still safe after having projected "Our Congressman" (400 ft/120 m) as a test. Everything went well so I will be able to screen other films but not now since it's already 1 am, here

[Brian Fretwell]

Of course with a series resistor like that any change in mains voltage would be at least halved at the lamp and maybe as the UK has always been 240/230 volts they erred on the side of caution during manufacture, for people not being too careful on the projector setting.

[Maurice Leakey]

The same arrangement is used on the Specto 500 projector. A large resistance unit standing alongside the lamp, both in the same air current from the fan.


Maurice

[Steve Klare]

Is it that they were just trying to avoid the cost of a transformer?

With that they could have made the lamp voltage whatever they needed without all this wasted power. With 220V in and a 100 VAC on the lamp there was more voltage on the resistor than the lamp so more power being radiated in waste heat than even supplied to the lamp. (Never mind how little of the lamp power actually becomes light...)

I guess "conserving resources" wasn't a thing yet!

[Paul Adsett]

Maybe Maurice can correct me if i'm wrong, but at the time the original British made Specto was designed much of the UK was still on DC mains. I know when i had my first projector in 1953 (Pathescope Ace) I had to use an external dropping resistor, which was supplied by Pathe, to drop from 230vdc to about 12v for the lamp!. It was a great heater, and very welcome on cold winter nights. As Gerald McKee observed in his book Film Collecting, the smell of the hot resistor wires and ceramic mount really added to the charm of film projecting!
A school friend of mine up the road had a Hornby 0-0 electric train set which required AC power, so he used a thing called a "rotary converter" to convert his mains power to AC.
i vividly remember the council digging up all the roads in my little Welsh town to convert every house for AC power. This must have been in the early 1950's.
Having had electrical shocks from both 230v DC and 230V AC I can testify that DC shock is a whole lot worse, no idea why.
Steve, if you have'nt seen last years film The Current Wars, I recommend you watch it, it gets into all the original AC vS DC wars between Westinghouse and Edison.

[Maurice Leakey]

I confirm Paul's comment.

When the Specto came out in 1936 it had a transformer in its base, however, for those with a DC supply it could bought with a resistance, and for those with no mains electricity there was a 12volt model which could be coupled to a car battery.

In the early days most large towns generated their own electricity, some DC, some AC. However, when the 33kV National Grid on steel pylons covered the country in the early 30s most of the local power stations closed down and the AC supply became more commonplace.


Maurice

[Steve Klare]

That is really interesting.

Thanks!