Re: resistors wanted

On Fri, 23 Mar 2007 08:21:23 GMT, Lostgallifreyan <no-one@xxxxxxxxxxx>
wrote:

Ross Herbert <rherber1@xxxxxxxxxxxxxx> wrote in
news:7uv603t40eh8bcu8eoe7miur3f4tn7nih4@xxxxxxx:

Carbon film on ceramic substrate would be the way to go. A number
of
asian manfs make them but the OP may have difficulty in locating a
distributor locally who carries 2W types.


Might not be... Metal or carbon films break down catastrophically
with high
voltage, but solid carbon resistors don't, until much more is forced
on
them. I found that out when making a limit resistor for a HeNe laser
tube,
I ended up with 2W solid carbon resistors, 4 in series, wrapped in
adhesive
lined heatshrink. It lasted for years, but previous experiments with
metal
film AND carbon film types ended up like little fireworks in seconds,
even
when selected for an apparent voltage rating that could stand the
supply
voltage. My guess is the supply went high to ignite the tube, and
having
started a burn, continued it even though tube ignition brought the
voltage
within safe limits. Solid carbon doesn't care about that, it just
works.

The OP didn't say why he wanted them, but my guess is similar high
voltage
systems where the current path must be safely distrubuted through the
whole
volume of the resistor.


I agree that the generally available metal film resistors don't tend
to stand up well to high pulse voltages but I have found carbon film
to be much better in this regard. Carbon composition may be preferable
in this regard depending upon the application.

The problem is that fewer (none?) manufacturesrs these days make
carbon composition resistors, and even then they will usually be max
1W.

For example, check out the product lines of two well known
manufacturers and see if you can locate a 2W, 2.2Mohm carbon
composition - or carbon film even.
http://www.irctt.com/index.aspx
http://www.ohmite.com/cgi-bin/products.cgi

Ohmite do make a ceramic composition resistor as a replacement for the
carbon composition and the OY series is 2W. I would go for this option
myself.
http://www.ohmite.com/cgi-bin/showpage.cgi?product=ox_oy_series
.

Relevant Pages

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