Lamp tolerances

[djhm]

Hi. I am cleaning up an old microscope illuminator. The transformer in it takes 240v mains and outputs 6v. Checking with a multimeter the output is 8v. The bulbs that it uses are 6v 48w. Could anyone tell me the tolerances of bulbs, will it burn out quickly? At about £25 each they are expensive. Thanks . David.

 

[shrtrnd]

I can't say for certain, but will speculate.
First, almost nobody calibrates their voltmeters, so it may be inaccurate if it's old.
If you're reading the transformer voltage with no load (no bulb), it's going to read a little higher than the actual voltage with the bulb in place.
If the instrument calls for 6V at 48W, you should use that bulb as a replacement.
(an 8 volt bulb is going to be extemely hard to find).
If you actually are getting 8 volts out, your bulb life would probably be a little shorter than a 6v bulb (ESPECIALLY at 48W).
I'm guessing a 48 watt bulb is going to experience high thermal stress.
If you're worried about the life of the bulb, because of the expense. You can consider
looking for a bulb (if it's not some type of pre-focused specialty bulb), in a higher voltage that will fit your lamp mount. 6.3V is common, some 8.6v available. I guess a lot will depend on how much illumination you require, and if other bulbs would fit the application.
Excellent technical data on bulbs can be found on internet 'Don's Bulbs'. He's too
expensive for me, but he has excellent data on a heck of a lot of bulbs.
Good Luck.

 

[(*steve*)]

It's quite likely that the voltage will fall to 6 volts under load.

You don't mention, but I expect that you measured the open circuit voltage.