Guest
Hi
Can anyone help?
I am looking for carbon resistors - in Johannesburg.
spec: 2.2 mega ohm, 2 watt - 450 needed.
Can anyone help?
I am looking for carbon resistors - in Johannesburg.
spec: 2.2 mega ohm, 2 watt - 450 needed.
S
Salmon Egg
Guest
On 3/22/07 11:46 AM, in article
1174589185.104727.136990@o5g2000hsb.googlegroups.com, "fischerl@polka.co.za"
<fischerl@polka.co.za> wrote:
use the term "composition" rather than "carbon" to describe them.
Bill
-- Fermez le Bush--about two years to go.
1174589185.104727.136990@o5g2000hsb.googlegroups.com, "fischerl@polka.co.za"
<fischerl@polka.co.za> wrote:
That used to be a staple product. I suggest that when searching for them,
use the term "composition" rather than "carbon" to describe them.
Bill
-- Fermez le Bush--about two years to go.
R
Ross Herbert
Guest
On Thu, 22 Mar 2007 13:30:19 -0700, Salmon Egg
<salmonegg@sbcglobal.net> wrote:
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.
<salmonegg@sbcglobal.net> wrote:
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.
L
Lostgallifreyan
Guest
Ross Herbert <rherber1@bigpond.net.au> wrote in
news:7uv603t40eh8bcu8eoe7miur3f4tn7nih4@4ax.com:
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.
news:7uv603t40eh8bcu8eoe7miur3f4tn7nih4@4ax.com:
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.
L
Lostgallifreyan
Guest
Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns98FC55011E48Ezoodlewurdle@140.99.99.130:
I dropped the tube, but the
resistor assembly and PSU survived all.
news:Xns98FC55011E48Ezoodlewurdle@140.99.99.130:
I ought to add that it never failed.
I dropped the tube, but the resistor assembly and PSU survived all.
A
Al
Guest
In article <Xns98FC55011E48Ezoodlewurdle@140.99.99.130>,
Lostgallifreyan <no-one@nowhere.net> wrote:
fills a hollow tube; the composition varies according to resistance. The
film resistors are spiral cut to get high values and can be inductive.
You can also get arcing across the cuts if the voltage is high enough
(despite the coating).
Al
Lostgallifreyan <no-one@nowhere.net> wrote:
Plus the construction is different. The composition resistor material
fills a hollow tube; the composition varies according to resistance. The
film resistors are spiral cut to get high values and can be inductive.
You can also get arcing across the cuts if the voltage is high enough
(despite the coating).
Al
R
Ross Herbert
Guest
On Fri, 23 Mar 2007 08:21:23 GMT, Lostgallifreyan <no-one@nowhere.net>
wrote:
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
wrote:
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
L
Lostgallifreyan
Guest
Ross Herbert <rherber1@bigpond.net.au> wrote in
news:02e903l3rj11h6ndruhdk9jm1vq9tuvv7a@4ax.com:
The emphasis on high voltage stuff being commonplace is vanishing steadily
along with CRT's, as LCD's and such take over.
I think there will always be specialised high-volt parts manufacturing
though. You won't find (m)any oil filled multi-KV rated caps or solid
carbon resistors at Digikey or RS, but people do still make them. I guess
the problem is that most aren't made to general preferred values anymore,
but to specific bulk customer order only.
Nice find on the ceramic composites. I like the look of those. I bet they
cost a bit, though..
news:02e903l3rj11h6ndruhdk9jm1vq9tuvv7a@4ax.com:
I pass. >
The emphasis on high voltage stuff being commonplace is vanishing steadily
along with CRT's, as LCD's and such take over.
I think there will always be specialised high-volt parts manufacturing
though. You won't find (m)any oil filled multi-KV rated caps or solid
carbon resistors at Digikey or RS, but people do still make them. I guess
the problem is that most aren't made to general preferred values anymore,
but to specific bulk customer order only.
Nice find on the ceramic composites. I like the look of those. I bet they
cost a bit, though..
P
Prepair Ltd
Guest
On Sat, 24 Mar 2007 10:03:11 GMT, Lostgallifreyan <no-one@nowhere.net> wrote:
conventional carbon and metal film product is vanishing fast, as the demand from
high-volume producers is almost over.
I can remember some years ago picking up a bulk load of resistors, probably a
couple of million for less than $10 the lot. It took 2 van loads to clear and
was all good quality product.
That was then. Now I find that demand is rising as manufacturers wake up to the
fact that unless they start buying this stuff from the main manufacturers, their
product lines are going to stop through lack of parts.
The ranges that RS Components (for example) carry these days in conventional
leaded resistors is very small compared with 10 years ago.
Even specialist parts like flame-proof resistors are getting expensive and hard
to find from regular distribution, and the makers don't want to know for
relatively small quantities.
Peter
--
Peter A Forbes
Prepair Ltd, Luton, UK
prepair@easynet.co.uk
http://www.prepair.co.uk
Since the resistor market has been cornered by a couple of companies,
conventional carbon and metal film product is vanishing fast, as the demand from
high-volume producers is almost over.
I can remember some years ago picking up a bulk load of resistors, probably a
couple of million for less than $10 the lot. It took 2 van loads to clear and
was all good quality product.
That was then. Now I find that demand is rising as manufacturers wake up to the
fact that unless they start buying this stuff from the main manufacturers, their
product lines are going to stop through lack of parts.
The ranges that RS Components (for example) carry these days in conventional
leaded resistors is very small compared with 10 years ago.
Even specialist parts like flame-proof resistors are getting expensive and hard
to find from regular distribution, and the makers don't want to know for
relatively small quantities.
Peter
--
Peter A Forbes
Prepair Ltd, Luton, UK
prepair@easynet.co.uk
http://www.prepair.co.uk
F
Franc Zabkar
Guest
On Fri, 23 Mar 2007 08:21:23 GMT, Lostgallifreyan <no-one@nowhere.net>
put finger to keyboard and composed:
"focus" resistors. My catalogue lists a 1W, 10KVDC, 7KVAC RMS type
with values in the range 100K to 10M. I can understand a pulse rating
of 10KVDC, but I can't understand the RMS rating. Even at 10M, the
maximum sustainable voltage for a 1W resistor is only 3kV.
Furthermore, my experience with switchmode PSUs in all manner of mains
powered equipment suggests that there is an unusually high failure
rate in the 100K-500K "startup" resistors that are used to kickstart
the chopper transistor, even though these resistors are operating well
within their specified wattages.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
put finger to keyboard and composed:
This has always bothered me. In TV sets there are special high voltage
"focus" resistors. My catalogue lists a 1W, 10KVDC, 7KVAC RMS type
with values in the range 100K to 10M. I can understand a pulse rating
of 10KVDC, but I can't understand the RMS rating. Even at 10M, the
maximum sustainable voltage for a 1W resistor is only 3kV.
Furthermore, my experience with switchmode PSUs in all manner of mains
powered equipment suggests that there is an unusually high failure
rate in the 100K-500K "startup" resistors that are used to kickstart
the chopper transistor, even though these resistors are operating well
within their specified wattages.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
L
Lostgallifreyan
Guest
Franc Zabkar <fzabkar@iinternode.on.net> wrote in
news:255b03ppoqc0uqp87s3ghktq6vler5n94g@4ax.com:
force the resistor to dissipate more than 2W, it would at least not arc
over until that voltage was reached. It might be designed as a short-term
max rating.
I don't know enough about switchmode PSU's to comment other than suggesting
that pulses that reach those resistors them might have a high enough peak
current to erode their ability to withstand what should be a safe average
current. The term 'kick start' suggests to me that some heavy peak currents
might hit them.
news:255b03ppoqc0uqp87s3ghktq6vler5n94g@4ax.com:
My guess (not a very good one) is that although their RMS rating would
force the resistor to dissipate more than 2W, it would at least not arc
over until that voltage was reached. It might be designed as a short-term
max rating.
I don't know enough about switchmode PSU's to comment other than suggesting
that pulses that reach those resistors them might have a high enough peak
current to erode their ability to withstand what should be a safe average
current. The term 'kick start' suggests to me that some heavy peak currents
might hit them.
F
Franc Zabkar
Guest
On Sun, 25 Mar 2007 09:21:17 GMT, Lostgallifreyan <no-one@nowhere.net>
put finger to keyboard and composed:
which in 240VAC locales is 340VDC. A 330K resistor would therefore see
a max current of around 1mA, and a continuous wattage of around 350mW.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
put finger to keyboard and composed:
These resistors only ever see the peak rectified AC mains voltage
which in 240VAC locales is 340VDC. A 330K resistor would therefore see
a max current of around 1mA, and a continuous wattage of around 350mW.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
L
Lostgallifreyan
Guest
Franc Zabkar <fzabkar@iinternode.on.net> wrote in
news:kmse03hkaktvk2d1domhifm88cmf3r116u@4ax.com:
either. The only thing I can think of is that they run hot, and might not
be as good at running hot permanently as they're supposed to be. Power
cycling would aggravate that. Also, the fams in many PSU's might draw gunk
onto them that reacts with them. Heat would accelerate that, and if the
resistors rely on a film, it won't take much to damage it if there's a
heat-assisted reaction. I doubt the makers test for that much, but it would
be interesting to know if those nicely glazed and shiny looking ceramic
types have a lower failure rate in the same situations, compared to the
more porous coatings.
news:kmse03hkaktvk2d1domhifm88cmf3r116u@4ax.com:
I had a look at Repairfaq.org and a couple of diagrams, and I can't see why
either. The only thing I can think of is that they run hot, and might not
be as good at running hot permanently as they're supposed to be. Power
cycling would aggravate that. Also, the fams in many PSU's might draw gunk
onto them that reacts with them. Heat would accelerate that, and if the
resistors rely on a film, it won't take much to damage it if there's a
heat-assisted reaction. I doubt the makers test for that much, but it would
be interesting to know if those nicely glazed and shiny looking ceramic
types have a lower failure rate in the same situations, compared to the
more porous coatings.