Re: RC filter resistor

["Jim Lux" <jimlux-at-xxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

A 100W light bulb, when hot, is roughly a 100 ohm resistor.  Sure, it would
work, but here are the practical problems:
1) The resistance varies a LOT from cold to hot. Very low when cold
relative to the hot temperature. Standard 300W halogen bulb (nominally
around 48 ohms when hot is about 3.5 ohms when cold. 

2) Breakdown voltage. A standard 100W bulb and socket isn't going to hold
off the high voltage real well because the clearances are kind of close
(look how close the wires are inside the bulb.  In theory, the resistor
potentially needs to take the full supply voltage at some point in the
cycle.

That said, something like a 150W halogen bulb or 300W halogen bulb might
not be a bad idea.  They are long enough that breakdown voltage isn't an
issue, and if you size it for the cold resistance, then you might have a
winner. If you have a 30 mA NST, the bulb's not even going to get warm
(considering normal operating current is around 2.5A for a 300W bulb).
OTOH, there might be easier ways to get a 3 ohm resistor (like a fairly
short length of nichrome wire).

Scavenging an old toaster or electric heater might be a way to get nichrome
cheap, if you don't just buy it.



----------
> From: Tesla list <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: RC filter resistor
> Date: Sunday, November 05, 2000 8:26 AM
> 
> Original poster: "acmnovak" <acmnovak-at-email.msn-dot-com> 
> 
> Could a light bulb (>100 watts) be used as a power resistor in a RC
filter?
> They are made for heat and are redily avaiable most everywhere...
> The fact that power resistors heat up and are not easy to find in the
right
> values really kinda makes me wanna avoid the whole RC filter situation. 
A
> filter composed of two 100 watt lightbulbs, a safety gap, and a bypass
cap
> would be a cheap and easy way for one to protect a hard to find/expensive
> xformer..
> Thanks
>     -Michael
> 
> 
> 
> 
>