When I come across a burnt resistor, I usually replace it with one rated for twice the wattage as the original, but I was told it's sometimes bad to do that. Why? I'm not referring to fuse resistors but ordinary carbon composition resistors.
Sometimes, ordinary resistors ARE used as fusable. Anything to save a cent, or a fraction of a cent anyway.
-- Abandon all hope ye who have entered cyberspace.
Hmm. Not obvious why there would be a worry. The differences amount to the following:
1) Longer body, and 2) Larger body diameter, and 3) More weight, and 3) Lower temperature at given power dissipation.Other than the thought that it does have a fuse function, which you dispute above, there can be:
A) The longer body or larger body diameter causes the wiring to be too close to nearby parts, mechanically stresses something, or blocks some hole that needs to be clear, or B) The greater weight causes some problem (such as if it were at the end of a propeller, for example), or C) The lower temperature affects something else that depends on the earlier higher temperature.
In short, I can't think of a problem that would not be pretty obvious when you were replacing it. But then my imagination ain't what it used to be, either.
Jon
No. The first step of repairing this thing is diagnosing what caused the first one to fail. If it was just a failure of the part itself, then the replacement only needs to be as big as the original, albeit one of higher wattage rating can do no harm - the thing is, if you use that (higher wattage R) without knowing what caused the first one to burn up, you might be masking a more fundamental problem.
Hope This HElps! Rich
Resistors mostly cook due to two reasons -
1) Under-specked. Runs too hot for the application. Good idea in this case. 2) (Most common reason) Something else failed and pulled excessive current through the resistor. If you replace just the resistor, and use a higher wattage part, you might cook other (more expensive) stuff.Lord Valve American - so far
No, it shouldn't be a problem. Different wattage is usually associated with size. If the resistor was 'cooked' then it was running at or beyond it's rating. Which means there was something else in the circuit that is defective.
Most resistors are metal film nowadays, carbon comps are hard to come by. (But Carbon comps do handle peak currents much better than metal films.)
Cheers
A larger resistor may have different stray inductances and capacitances, which might affect some RF circuits.
"larry moe 'n curly"
** Carbon comp resistors are rare and almost obsolete these days - did you mean carbon film ?Most resistor burn ups are the result of misuse or OTHER failures in the equipment, so no upgrade is needed
If a new resistor of the same size gets unusually hot, then an upgrade is justified.
BTW
2 and 3 watt carbon comp resistors that run hot ( 100C or more ) gradually fall in value and get even hotter, ie a vicious cycle exists until complete failure occurs.Replace them with a wire wound type.
.... Phil
snipped-for-privacy@cf8g2000vbb.googlegroups.com...
sCarbon composition resistors have a negative temperature coefficient of resistance - put enough current through them and you form a "hot channel" with almost all the current being carried by thin filament that is almost incandescent, and presents a very low resistance.
Back in 1974, my then boss amazed me - and several other engineers - by persuading a nominally 10k carbon film resistor to pass something like an amp with a voltage drop of about 100mV for about half an hour. There was a very narrow stripe of discoloured paint above the hot channel, but that was the only evidence of what was going on.
He been having trouble with an "intrinsic safety" standards committee, and this demonstration (and a couple of repeat performances) managed to persuade them that carbon film and carbon composition resistors had to be banned in intrinsicly safe devices.
-- Bill Sloman, Nijmegen
-- JF
The safest thing is to use an exact replacement unless you understand the circuit more than well enough to have designed it in the first place. Or get an ECO that allows the use of the substitute part.
Replacing a current-sense resistor with the wrong type (too inductive) could cause a whole lot of pain, for example.
If you actually are talking about carbon composition resistors (fairly uncommon and relatively expensive these days, as others have said), then there isn't much problem with a higher wattage other than getting it to fit, but most resistors these days are metal film (with some carbon film still around in through-hole applications, and wirewound used in power applications). There's also metal foil, fusible, high voltage rating, metal-oxide-film (MOF), tight tolerance, low tempco, positive TCR, low inductance, and lots of other types of resistors.
Wattage rating is only one parameter (and it really isn't a simple parameter- it might be lower in a given application than the big print value if you limit how hot the body of the resistor can get or limit the ambient temperature or have pulses of current that yield high peak power).
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
...
Nah, all the major parts houses in the USA carry 'em...Mouser, Digi-Key, etc. Hell, you can even get 'em as SMDs.
LV
"Most" (by far) by quantity used, but even by types offered, fewer than 3% of the resistor part numbers that Digikey has in stock are carbon comp.
If the original resistor burnt, something is wrong and needs fixed.
Replacing the resistor with a bigger one is not in any manner a solution.
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Depending on the circuit, there's probably no reason not to super-size it.
For smaller resistors, I really wonder what the actual wattage rating is in the first place, and how repeatable that limit is part-to-part. I mean, if you took a hundred 1/10th watt resistors, ran them up past their rated wattage, what would the graphs look like for all 100 parts?
If the standard deviation is high, then there you go. Upsize to your heart's content.
Except high-frequency stuff that may burp if too much stray inductance/ capacitance, etc... I assume we're not talking such circuits here.
For me, personally, if I need a 4.7k resistor, I'll most likely use the one I can actually FIND in my parts bin :)
-mpm
That _may_ be the case. You don't know and neither do I. But the OP is asking about whether or not it could be bad, so I think it's good you point this out -- others already have, though, so there's no need to do it twice. And I already know, so it doesn't help telling me.
Designers aren't always perfect. Some of them are even hobbyists or worse. These are carbon and the devices may be old and just gradually failed -- I've seen that happen. They do develop fissures and fail, sometimes just from vibration sometimes from just aging or part variability that wasn't accounted for. It _may_ be a problem elsewhere in the circuit. It _may_ just be a failed part, itself.
I'm just glad the OP is getting all the options laid out.
Jon
S/he should keep in mind that all absolute statements made on Usenet are false.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
-- How firmly do you believe that? ;)
Absolutely.
-- www.wescottdesign.com
That's totally wrong.
George H.
"The Journey is the reward"
eff.com
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