This doesn't fill me with confidence

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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


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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



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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.
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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



Re: This doesn't fill me with confidence



12V lamps but
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That makes no sense at all. If there are still 19 lamps in their the most
you will get is still 12volts once you actually draw some current.



Re: This doesn't fill me with confidence



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You will get ~12V if you draw as much current as the absent lamp would have
drawn.  Loads presenting a lower current will obvioulsy see a higher voltage,
and on OC it will be the full supply voltage.  Basic Ohms Law stuff.

Re: This doesn't fill me with confidence



wrote:
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shoves
have
voltage,

Right, and since you only need ~10mA or so to be potentially lethal, the
right amount of skin resistance should do the trick.

MrT.



Re: This doesn't fill me with confidence


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shoves

What if you touch the lamp closest to the cord and ground yourself at the
same time?

Michael



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