RE: using Lamps RC filter resistor

["Dale Hall" <Dale.Hall-at-xxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Michael,
Short answer:
 Not likely - need too many series lamps to get to the desired Rcold to be
effective.
 or fewer at run hot at full illumination, but a fault would quickly burn
them out, 
  (which may be preferred to stressing the xfmr)

The use of incandescent lamps for current limiting is common and well known
in low voltage applications.
One needs to carefully select the correct lamp characteristics for each job.

Decide if current limiting is the aim 
  and if a lamp is able to provide sufficient voltage drop to provide
protection
  and/or if an average value of resistance is desired to affect filter
properties.
  (then what is average current developed to determine if a lamp may be
selected)

Main deterrent to the use of filament lamps for AC TC/xfmr protection 
is the relatively small voltage they drop in full illumination compared to
xfmr Vsec.
240VAC lamps (or higher if available would be required) and use of many in
series.

Lamp usage in the Primary is likely most effective, 
series/paralleling lamps to dimly glow in normal operation offering minimal
insertion loss
but glow brightly in fault offering maximal insertion loss.

I, Terry and others use lamps regularly to current limit when first firing
up electrical loads.

Consider first, the average circuit current. 
The lamps low cold resistance won't increase much 
until significant AVERAGE current begins flowing.
The effect varies with lamps and ratings so it's 
recommended to characterize the lamp after some educated guessing.
ball park about half rated. (for 100W lamp = .5 * 100W/120V = .417A)

For current limiting it is desired the lamp run 
under normal operation in its cold to warm low resistance region 
so a fault causes abrupt filament heating increasing its resistance causing
bright illumination.

In a fault condition, 
as with a shorted Cpri due in dielectric breakdown, 
much more current may be available 
so it is desired to drop some voltage 
as with a hot higher resistance filament that illuminates brightly 
indicating a problem and at the same time reducing the current 
that otherwise would have flowed.

I use two 7v lamps in parallel with my 
12 v dc to 7kV dc SSTC converter for just that type of protection.
When a Mylar or PP cap takes a little extra time to "clear", 
brightly lit lamps make me aware instantly.

The slow BPS of my SS DC TC make this form of current 
limit and visual indicator protection, nearly ideal.

The application with AC & 120bps needs some thought as to desires &
effectiveness.

The chief deterrent to this approach on the Sec side for most AC TCers
is that so much voltage is available from the Xfmr 
that the incandescent lamp will drop too little to be meaningful. 

The well spaced terminals of a cartridge type lamp may be effective as a fuse
that may give a "flash" indication of fault.

Regards, Dale

----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, November 05, 2000 8:26 AM
To: tesla-at-pupman-dot-com
Subject: RC filter resistor

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