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This doesn't fill me with confidence
- 09-04-2007
- Michael C
September 4, 2007, 7:07 am

I have a medical device with a blown globe. I measured 240volts at the globe
but the suppliers says it takes a 120V globe. I asked them if they are sure
because I measured 240volts at the terminal. Their reply was that they run
the 120V globe at 240V but at a lower amperage. They do this to get a better
colour temp out of it. Interesting. I'm guessing I measured 240V only
because there was no load. Either that or they are breaking the laws of
physics.

Re: This doesn't fill me with confidence

Connect a similar wattage (if practical) 240v globe to the socket in
the device and see if the voltage drops.
Unless the bulb is really low wattage, (which I doubt, or they would
be running it from a low voltage) I would hate to think they were
using a resistor or linear regulator as a voltage drop method !
of course - shoving a 110v globe in a 240v socket might be a really
good reason the poor bulb blew in the first place.

Re: This doesn't fill me with confidence

I wonder how much the person you spoke to knows about the actual design of
the thing. Giving a globe higher voltage than it's design voltage will
increase the heat, and hence increase the resistance of the element, but not
by enough to reduce the current back accordingly. Running at higher voltage
will also improve the colour temperature, but the downside is it will
shorten the life of the globe quite dramatically. A blue filter would be a
more reliable way of improving colour temperature. I would be very surprised
if a globe was able to withstand double it's design voltage for any
sustained length of time.
My guess is that they have either another globe, or a resistor in series
with this globe. This would cause the voltage to read 240V when open
circuit, but of course it will drop to something more reasonable under load.


Re: This doesn't fill me with confidence

I know it's sort of unrelated but that bring something to mind...Christmas
tree lights, the old ones with 20 series connected 12V lamps. 12V lamps but
of course 240v potential if someone (read child) removes a lamp and shoves
something in there.
James
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