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I’ve just got a multimeter to measure the voltage to the lamps.I tried it with one of my 12v 100w projectors and with the lamp on it measures 10.7v but if you unplug the lamp and switch it on it measures 12.6v.So what is the correct way to measure it ?.
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Hi Jason. Measuring the voltage without the lamp is the correct way, in your case it seems fine.
John -
Hi john, thanks for the reply.So would the lamp be getting 10.7v or 12.6v when its lit and running ?.Comment
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Both ways are telling you something. The 12.6 with the lamp removed tells you that the transformer secondary that's powering the lamp is doing its job (or at least trying). The 10.7V with it in is telling you that you are dropping voltage on things like wiring resistance and switch contacts. Of the two, this one is the bottom line: how bright your lamp is.
The 10.7 volt number is pretty important. Power is proportional to voltage squared, so you are getting 10.7^2/12^2=114.5/144=79.5% of the power you'd get if you really had 12V on that socket. Brightness isn't exactly proportional to power, so you are probably getting less than 3/4 of the full brightness.
If you are happy with the brightness, you shouldn't worry too much about it, but if you had a really dim lamp and this number was something like 8Volts, it would explain a lot.
Let's say instead you saw the 8V without the lamp: now you have a power supply problem: something like the input voltage being set for some other country or something badly wrong with the transformer.
That's the value in being able to use a meter: it gives you real information to work with instead of just guessing. Most of the time what comes next is still a guess, but at least it's a guess with a clue!Comment
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Wow . Thank you Steve ! Very insightful .Comment
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Thanks for the reply steve.So at the moment it is using the 240v setting in the projector (i’m in the UK) and getting 10.7v to a 12v lamp, if i was to use the 230v setting it would slightly raise the voltage to the lamp and be brighter, so long as it is still under 12v would it be ok?.I want to try and get as close to 12v as possible.Comment
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You need to measure the lamp voltage with the lamp plugged in and fully illuminated. When you do this you should measure very close to the rated voltage of the lamp, probably within 0.5 volts. For example the Elmo GS1200 service manual specifies lamp voltage as 23.75V when using a 24.0v 200w lamp load. They specify this value measured at the transformer terminal, but assuming you have a good lamp connector it should be very close to the same value at the lamp itself.
Just because you have your transformer set to the correct mains voltage does not necessarily mean the lamp is getting the voltage it needs for maximum brightness, transformers have specification tolerances. I have a Eumig S938 Stereo which uses the 15v 150 w EFR lamp, and with the transformer set to the correct mains voltage of 117v the lamp voltage is only getting a miserly 14.2v ! Great for long lamp life but a really significant dimming of the screen illumination - from white light to yellow light. So I reset my transformer voltage to 110v which brought the lamp voltage up to 15.1 v, very close to where it should be. Great for screen illumination but not so good for lamp life. You take your pick!Comment
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That sounds fine: the lamp manufacturer specs the lamp for full performance at 12V, so this is your goal. It probably wont get you all the way there since your source voltage is rising about 4% and you are 11% low at the socket right now, but it's something.
Besides: from what I'm seeing UK Mains voltage was reduced from 240VAC to 230 in 2003. If this is what you have you are just keeping up with the times.
You could switch ranges on the multimeter and ever so carefully poke a socket to see what you have at the wall. (Fingers on the other sides of the guards from the probe tips! Don't make 50 Hertz into 50 Hurts!)Comment
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Ok thanks steve,I’ve now set it at 230v instead of 240v and it reads 11.2v with the lamp fully on and it looks brighter now.i don't mind messing around with 12v but i don’t think i’m going to start poking things into the mains socket.Comment
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Staying within your comfort zone is just good sense!
When I was 18 years old they sat me down at a test bench in front of a 16,000V CRT power supply and showed me how to test it. Every fiber in my being told me to stand up and run for it (-so many ZEROS!), but given that summer job or mopping out the restrooms at McDonald's, I flipped the switch and gradually got comfortable with it!
(Wasn't nearly as dangerous as it sounds: that came later!)Comment
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