27 Apr
2011
27 Apr
'11
3:33 p.m.
On 4/26/2011 2:17 PM, Tony Duell wrote:
<snip>
I'm sure it's just a typo, and I apologize in advance for being too dumb
to figure it out, but what is a baretter? (Sounds quite interesting,
It is a typo, or rather a mis-spelling. It should be 'barretter;.
A barretter is a constant-current device, essentially a special type of
filament lime. Often the filament was iron wire in a hydrogen-filled bulb
(or so I've read). The current floowing though the device was fairly
constant for quite a wide range of voltages across it.
Some UK series-string radio sets had one in series with the heater chain
(rather than a dropping resistor) so that the heater current would be
correct for jsut about any mains input voltage.
The were also used as a ballast for the Nernst glower in some IR sources
(which is where I first came across one at school [I managed to convicne
the powers that be that getting me to chase a ball around an area of
grass was a waste of everyone's time, so I got to fiddle with electronics
stuff instead. Getting the old IR source to work again was just one of
the many things I tried]
FWIW, if you have an old device using a barretter, you can often use a
mains-voltage lamp to replace it. I am pretty sure I used a normal 60W
bulb (so 0.25A at 240V) to replace a 0.3A barrretter in said IR source.
-tony
The
reason is quite simple--tungsten-filament lamps (not that I
expect anyone to have any carbon-filament ones) have a very steep
resistance-temperature curve (higher "hot" resistance).
Tugsten filament lamps approximate a constant current load over quite a
range of voltages (am I the only person to rememebr the baretter?).