Carbon Resistor Thoughts

[45Overload]

There was a time - some years ago now - that I believed resistors were the only really reliable components in electronics.  Boy was I wrong.  It is true that with some exceptions resistors are more reliable than most other circuit components.  But consider the lowly carbon resistor.  At some point in my career, I started looking closely at resistors over 500K or so.  Definitely resistors over 1 Meg.  More times than not, 1/2 watt or better carbons would drift far out of their spec, yet show no obvious signs of burning or give any visual cues at all.  Other times (less often) the resistor would be the correct value, but would generate noise in a sensitive circuit.  Guess how that could happen?  I can't.

Anyway, it came to my attention that new carbon resistors routinely change value while sitting on the shelf.  What a blow!  Lucky that we now have metal films, which seem much hardier in most cases.

Wanted to say thanks for forum tips on fixing HV problems with the Tek 2215 scope.  I noticed that the trace would only begin to focus at the extreme CCW position of the focus adjustment.  That's a good clue something is amiss and I'm glad I did not have to re-invent the wheel and go testing every component in the HV area.  6 510K carbon resistors did in fact exhibit all kinds of resistance from 400K to almost 2 megohms.  One other 6.2 Meg had dropped to 4Meg.  I did not order new carbon replacements, but thanks to suggestions on the forum the focus was back to normal after resistor replacements along with some electrolytics. 

Funny how my 3 handheld ESR meters all read very low in-circuit ESR so that was a useless troubleshooting aid.  Checking actual out-of-circuit capacitance was more strange because I could never get a stable reading from the cap meters.  Well, I know that does not check them at rated voltage, but the meters were still telling me that something was not right.

That being said, I do not routinely replace just any electrolytic in older equipment.  I have examined many HP capacitors, Sprague and Mallory for example, that were 50 years old with in-spec capacitance and low ESR - lower than you could easily get from a parts house.  Another surprise for me, anyway. 

Luckily the Tek 2215 only has one Tunnel diode in the trigger circuit.  I suppose these days they could use a Schotky, but we know that the value in a scope is really in the design of the trigger...that's my opinion anyway.

OK, enough bs.  Have a good day.
45O

[Ian.M]

Carbon resistors have been notorious for being noisy and drifty since the days when they were colour coded body tip dot.

Little changes, except the miniaturisation!

[james_s]

I believe the carbon composition absorbs moisture causing the value to change, there may be other mechanisms at play. As was already mentioned they have always been notorious for drifting value. They are non-inductive and relatively tolerant of high voltage though.

I rarely saw bad resistors until I was fixing a lot of CRT monitors, I remember seeing a lot of open resistors in those.

[floobydust]

Carbon composition has the best pulse-overload/surge power handling, so they are still used and for sale.
Their parasitics- inductance and capacitance are quite low as a slug. I took apart a Philips CR37 220k 1/2W carbon film and it was only 8 turns spiral and rated past 10MHz before reactance causes error beyond its tolerance. I think metal film resistors have higher turns (inductance).

Copy pasta from Ohmite:
"Carbon composition resistors are manufactured by extruding a blend of carbon and organic binders inside a phenolic outer body. The extrusion is cut to length, leads inserted, cured, and marked to form a finished resistor. The carbon and binder mixture is adjusted to produce different resistance values. The resistors are sorted for 5%, 10%, and 20% tolerance values.
Carbon composition resistors are able to withstand larger short-term pulses and higher voltages than film resistors and are virtually impervious to ESD events (Electro-static discharge). Carbon composition resistors are also sensitive to moisture and, therefore, storage recommendations should be adhered to. Generally, any moisture absorbed during storage will be "baked out" during the soldering operation. If the product is stored properly the resistance shift during the soldering operation will be minimal, less than 2% or 3%.
Carbon composition resistors are highly hygroscopic and changes in resistance value can occur if too much moisture is absorbed. For this reason, it is recommended not to use water or water-soluble solvents to clean these components. Alcohol or hydrocarbon solvents are recommended for rinsing."

[bob91343]

The original dogbone or body-end-dot resistors were not even insulated.  The color code paint was the only insulation.  They were a solid piece of carbon that was adulterated to obtain the desired resistance.  Tolerance was typically 20% unless you wanted to pay extra.

Then the nice Allen-Bradley units became popular.  They were much better, and truly insulated.  That era lasted many years until carbon film and metal film appeared.

Stability of resistance is partly a measure of manufacturing quality.  But the operative word here is 'partly' because some performance variations over time were hard to predict.

I have had excellent luck with longevity of components, including resistors.  Not to say they are all good, but better than one might guess.  I routinely throw away components that are seriously out of tolerance.  And don't get me started on parts that are coded incorrectly.

[tautech]

Wanted to say thanks for forum tips on fixing HV problems with the Tek 2215 scope.  I noticed that the trace would only begin to focus at the extreme CCW position of the focus adjustment.  That's a good clue something is amiss and I'm glad I did not have to re-invent the wheel and go testing every component in the HV area.  6 510K carbon resistors did in fact exhibit all kinds of resistance from 400K to almost 2 megohms.  One other 6.2 Meg had dropped to 4Meg.  I did not order new carbon replacements, but thanks to suggestions on the forum the focus was back to normal after resistor replacements along with some electrolytics. 

Quite normal for the high value R's dividers in CRO EHT circuits to drift widely over time.
So much so they're one of the first things I check now after the compulsory PSU checks.
It's little wonder LCD displays have replaced CRT's in test equipment such are the problems with CRT circuitry with a bit of age on it.
Typically these high value R's are of a few watts rating which gives some clues to the thermal stress they are under more so in later high performance CRO's. So consider them expendable items in much the same way as electrolytics.

[T3sl4co1l]

Yeah, carbon comp is notorious.  Film is fine, I've got tons of them and have never measured one in error -- even a few I've toasted in the course of breadboarding....

Carbon composition has the best pulse-overload/surge power handling, so they are still used and for sale.
Their parasitics- inductance and capacitance are quite low as a slug. I took apart a Philips CR37 220k 1/2W carbon film and it was only 8 turns spiral and rated past 10MHz before reactance causes error beyond its tolerance. I think metal film resistors have higher turns (inductance).

And note that the capacitance between those turns is what's dominating.  Resistors over 200Ω or so tend towards capacitive reactance at high frequencies, while resistors below 50Ω or so tend inductive.  Simply the ratio of DC resistance to characteristic impedance of the component.  It's hard to make an axial (flat to the board, not standing) or chip resistor with an impedance much different from this.

The spiral cutting will tend to increase characteristic impedance, at least over the modest range where the spiral's length is dominant (low GHz?).

There are wide-style chip resistors, which can have lower characteristic impedance.  Good choice for current shunts.  Or just use a bunch in parallel, connecting with wide pours.

Wirewound resistors, somewhat obviously, have a lower bandwidth, but which way that swings isn't actually all that obvious.

The basic material property in play here is the ratio of resistivity to inductivity of the material: the bulk time constant, as it were.  A longer time constant (lower resistivity) means inductance takes over at a lower frequency.  This is a good reason to avoid using PCB traces as resistors -- besides the horrendous tempco and poor fab tolerance, they go inductive above perhaps 10s to 100s of kHz.

A high resistance material, or bulk equivalent (carbon granules are pretty conductive, but carbon composition has poor contacts and random paths between grains, so approximates a high resistivity material in bulk), is attractive for resistors, for this reason.

Films can also be made ever thinner; a PCB with 35um copper isn't very good, but if you could get one with say 1um, it would be passable (assuming the thin layer doesn't corrode away, of course).

That's why metal film (thick and thin) are pretty well standard.

Anyway, wirewound -- because the metal has a longer time constant, and on top of that, the winding design tends to emphasize inductance (i.e. a helix).  There are more turns than a film resistor needs, so the characteristic impedance is high and the cutoff frequency is low (typically low MHz).  Large values (>1k, 10k?) will again tend to be capacitive, while small values tend to be inductive.  Noninductive (Ayerton-Perry) winding helps cancel most of this.  (Note that the inductance due to sheer component length is unavoidable.)

Tim

[PKTKS]

The original dogbone or body-end-dot resistors were not even insulated.  The color code paint was the only insulation.  They were a solid piece of carbon that was adulterated to obtain the desired resistance.  Tolerance was typically 20% unless you wanted to pay extra.
(..)

Yes and  still today they may be badly fitted.

ATX PSUs use them **ALOT**
Some cheap ATX can have a common issue like this:
- the resistor(s) are "GLUED" or fixed in the DISSIPATOR.
- that would be to avoid vibration.. not heat sinking
- with ever increasing power ratings.. this glue MELTS over time
- well the melting action always take the painted coat with it..
- leading the body to touch the heat sink...

Guess what?  a primary side over 300V dead short...
luckily the fuse blown in 90% cases..
or the PCB track goes first..

Paul

[james_s]

Quite normal for the high value R's dividers in CRO EHT circuits to drift widely over time.
So much so they're one of the first things I check now after the compulsory PSU checks.
It's little wonder LCD displays have replaced CRT's in test equipment such are the problems with CRT circuitry with a bit of age on it.
Typically these high value R's are of a few watts rating which gives some clues to the thermal stress they are under more so in later high performance CRO's. So consider them expendable items in much the same way as electrolytics.

I have not found LCDs to have significantly better longevity. I just recently replaced the LCD screen in a ~20 year old oscilloscope because the rear polarizing film had started to shrink and delaminate. I've seen other LCD panels where the light spreader for the backlight turned yellow from UV and heat from the CCFL tubes, it was so bad that the sheets crumbled into bits when I touched them and in newer panels I've experienced LED backlight failures. The problems will be different, but we'll be seeing similar age related problems with LCD panels when they reach the age that the CRT displays with drifting resistors are now.

[David Hess]

The higher valued carbon composition resistors drift the most and even suffer from what is effectively dielectric absorption.  They all have excess noise.

They are especially suited to applications where high pulse power handling is required and where it is desirable that the resistor fail open so they were sometimes used in place of fuses.

Ceramic composition resistors are a potential modern replacement but they have a limited selection.  Where it matters, carbon composition resistors have been replaced with high voltage fusible film resistors like the ones 45Overload mentioned.

[tautech]

Quite normal for the high value R's dividers in CRO EHT circuits to drift widely over time.
So much so they're one of the first things I check now after the compulsory PSU checks.
It's little wonder LCD displays have replaced CRT's in test equipment such are the problems with CRT circuitry with a bit of age on it.
Typically these high value R's are of a few watts rating which gives some clues to the thermal stress they are under more so in later high performance CRO's. So consider them expendable items in much the same way as electrolytics.

I have not found LCDs to have significantly better longevity. I just recently replaced the LCD screen in a ~20 year old oscilloscope because the rear polarizing film had started to shrink and delaminate. I've seen other LCD panels where the light spreader for the backlight turned yellow from UV and heat from the CCFL tubes, it was so bad that the sheets crumbled into bits when I touched them and in newer panels I've experienced LED backlight failures. The problems will be different, but we'll be seeing similar age related problems with LCD panels when they reach the age that the CRT displays with drifting resistors are now.

Sure backlight circuits have their own sets of issues and it's technology that's still evolving whereas CRT's used in analog CRO's had to provide a high level of performance to display the full analog BW whereas today all that's managed in the digital domain and the display needs only be refreshed fast enough to hide visible flicker.
Yet I've experienced even TDS Teks have backlight issues, one I was given by a EE tech had a nF cap in the RC timing for push-pull backlight primary drift into the low pF's where the poor little TO-92's stopped switching and of course backlight stopped. Easy fix with a 30c replacement cap.