On Fri, 24 May 2013 11:18:05 -0700, janders wrote:
On 5/24/2013 8:47 AM, Lloyd E. Sponenburgh wrote:
I couldn't be sure without seeing it, but that sounds like a gas-
discharge lamp (like a neon bulb, but perhaps a gas blend that gives a
different color).
If so, you need only a limiting resistor.
Picture's worth a thousand words... G
https://picasaweb.google.com/lh/photo/
HR9gnIj6F9INyHaO0T1HqtMTjNZETYmyPJy0liipFm0?feat=d irectlink
Doubt highly there's any electronics involved. So, for an electronics
idiot, would you mind explaining a bit more on current limiting
resistors for something like this? I assume it'll have to dissipate
heat. Might it get hot enough that using part of the base as a heat sink
could result in a burn? But I like the idea of something as simple as a
resistor, make it a lot easier to hide in the base.
This won't be run all the time, just for effect now and then, maybe few
hours at a time.
Or is the small power transformer a better way to go?
A gas discharge lamp like what's inside of that runs at pretty much a
constant voltage, lower than 120V, and can accept a moderately wide range
of input currents. So it "wants" to see a source that can supply a wide
range of voltages and a fairly constant input current.
Wall power is a fairly constant voltage that will supply just about any
current that you ask of it, until the breaker pops. So it's exactly the
reverse of what the lamp needs.
The easy way to turn wall power into something that a gas discharge lamp
can handle is to use a resistor in series with the lamp. That lamp
almost certainly has just that sort of resistor in its base.
If you knew the current draw of the lamp when it's running from 120V this
would be easy -- you'd just need a resistor that would drop 100V at that
current, and had a power rating sufficient to burn up the lost energy
without overheating.
Were it me, I'd measure the AC current draw, then calculate a resistance
from R = (100V) / (current in amps), then go buy one that's close. I'm
expecting that the lamp will pull 10mA to 50mA, which means it'll be a
large resistance, probably around 4k-ohm to 20k-ohm. I'd also calculate
the power dissipation needed, P = (100V) * (current in amps). If I'm
right about the current, then it'll be in the one to five watt range.
Then I'd order parts on line or buy enough resistors from Radio Shack to
make up a string of the right dissipation and resistance.
But -- that's only easy if you already know your way around electronics.
If you don't it's way freaking complicated. An autotransformer will be
far heavier and proportionally far more expensive, but it's "more
expensive" as in dollars instead of dimes, it should be way simpler and
it should only be a couple of inches on a side at most.
Get one that has a 220V/120V primary. You don't care about the secondary
voltage -- you won't be using it. Hook it up like Lloyd says, ignore the
secondary (well, tape it off), and have fun.
Or, get one of the travel transformers and just use it. Hook it up, plug
it in, run it for an hour, then unplug and check the transformer -- if
its cool enough that you can hold it comfortably, then you're probably
OK. If you're feeling paranoid, do the same test again but leave it
plugged in for twelve hours or a day -- that'll get it as warm as its
going to get, and if it's still just warm and not hot then you're
probably OK.
If it does get hot, then find one with a higher current rating and try
again.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com