Can I substitute a carbon composition resistor with another type? - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: September 13, 2020, 06:55:03 pm
content: Hello,I am trying to repair the power supply from a Tektronix 2710 spectrum analyzer and, part of that, I found a carbon composition resistor which is way out of spec.Normally, I would look to replace the resistor with the same type but, for the resistance and power specs of this specific one (i.e. 6.2MOhms @ 1/4W), I can't seem to find a suitable replacement.Given this, would it be ok if I were to substitute this type of resistor with one using a different composition (but matching the specs)?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2020, 07:08:43 pm
content: Yes.

author: giosif
author_rank: Frequent Contributor
timestamp: September 13, 2020, 07:23:33 pm
content: Haha... Quick and short answer.Maybe I should have asked a different question: any considerations / caveats coming from changing the resistor type?Thanks!

author: Benta
author_rank: Super Contributor
timestamp: September 13, 2020, 07:36:26 pm
content: Leaded carbon composistion resistors exhibit very low inductance compared to film resistors that are trimmed using a spiral cut on the surface.In a power supply that shouldn't be a big issue.

author: giosif
author_rank: Frequent Contributor
timestamp: September 13, 2020, 08:01:43 pm
content: Thank you both for your answers!

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2020, 08:03:06 pm
content: Quote from: Benta on September 13, 2020, 07:36:26 pmThat's irrelevant for such a high resistance, because the inductive impedance is going to be tiny, compared to 6M2. Inductance is more of an issue for lower resistances.Quote from: giosif on September 13, 2020, 06:55:03 pmJust use any type of resistor, as long as it has a high enough voltage rating. Carbon composition resistors have a poor tolerance and high surge capacity, although it would need to be a high voltage, for that to be necessary.

author: Benta
author_rank: Super Contributor
timestamp: September 13, 2020, 08:15:06 pm
content: Quote from: Zero999 on September 13, 2020, 08:03:06 pmDid you notice that my reply was a general one? That apart, we're on the same page.

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2020, 08:24:56 pm
content: One problem with high resistance carbon composition resistors is internal capacitance between the carbon grains through the talc between the grains. For resistors above 1 megohm, if you model the part as a resistor and capacitor in parallel, the parallel resistance falls dramatically above a few MHz. 30 years ago, I had to choose carefully between different types of 1 megohm leaded parts as the gate resistor for a high-impedance FET amplifier after a high-Q LC circuit in MRI. The best part for minimum Q reduction was a “carbon ink” part from IRC, which had much higher effective resistance than the traditional Allen-Bradley composition unit. In general, the inductance of resistors is often less important than the self-capacitance. If the high frequency reactance is important to the application, checking if it be inductive or capacitive at the relevant frequencies will be useful. (Some LCR meters, including my DE-5000, will give an ambiguous display in inductance mode if the part is capacitive at high frequency. A true admittance bridge should distinguish positive and negative susceptance.)

author: giosif
author_rank: Frequent Contributor
timestamp: September 13, 2020, 09:19:13 pm
content: Quote from: TimFox on September 13, 2020, 08:24:56 pmI think I liked your first answer better.Seriously now, thank you for these details, although I cannot claim I understood everything.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2020, 09:23:57 pm
content: Quote from: Benta on September 13, 2020, 08:15:06 pmYes I did and I darn well knew you were aware of that, but neglected to mention it and as this is the beginners' section, so it needed to be pointed out.The same is true for capacitance, which affects high value, but not low value resistors. The point at which parasitic component dominates, depends on the physical construction of the resistor and its value.

author: giosif
author_rank: Frequent Contributor
timestamp: September 13, 2020, 09:45:11 pm
content: Quote from: Zero999 on September 13, 2020, 08:03:06 pmFunny you should mention that, but the resistor in question is part of the circuitry where 2.5 kV are present (please see attached picture).That's not to say 2.5 kV flow through that resistor, though (also confirmed by the dimensions of the original resistor, which looks to match those of a 1/4W Allen Bradley resistor).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2020, 09:55:35 pm
content: More generally, no component is pure; once you understand that a resistor is an RLC network like any other component, it becomes not a question of "best resistor for X", but "which resistor has the most bandwidth / flattest impedance / least EPC/ESL for X application".And then you understand ways to game the system: you can increase the bandwidth/flatness of a desired resistance by using series/parallel combinations of parts with wider bandwidth. The bandwidth is invariant (it can't be greater than the part's EPC/ESL and R determine) under series/parallel combination, so if you start with wider band parts, you can potentially improve things. Example: maybe that 6.2M resistor could be made from six 1M resistors and one 200k, which will each have the same EPC (effective parallel capacitance) as a single 6.2M, but six in series is 1/6 the capacitance. Mind I said BW can't be greater than the part's own property; it can always be lower due to circuit strays. This isn't a slam-dunk solution, it's just an option in your toolkit. (Scope probes constructed this way, usually need to deal with the distributed stray capacitance along the resistance element / divider chain -- sometimes this is done by swamping the stray capacitance with much more external capacitance, sometimes this is done by nulling cancelling out poles and zeroes in the frequency response. Basically, this is why some scope probes have multiple trimmers on the probe or plug body.)So, if you ever wanted to know much, much more than you thought you wanted; this is one of many ways to get there.As for OP's exact situation, it's very likely that a direct replacement, respecting voltage and power ratings, will suffice. A large value like that, is suggestive of a bleeder application (often, high-value resistors are used line-to-line to discharge capacitors of nuisance voltage; the value is noncritical), or a bias resistor (the value should be accurate), or something for high voltages. Do check around for connected high voltage components -- if present, it should probably be replaced with a high voltage part, or at least a series chain of regular rated parts. (Often, carbon-comp resistors were abused in this way, and their values typically drift high when operated at high voltages!) It's very unlikely that the resistor's bandwidth matters, but if that is the case, some compensation is probably necessary (you'll have to inspect the circuit much more closely to figure how to do that).Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2020, 12:14:55 am
content: That resistor absorbs leakage between the grid and cathode. The adjacent series connected gas tubes keep the maximum voltage below 200 volts. If the resistor failed, nothing destructive would happen.The voltage requirement still requires a physically larger higher power part so the cheap replacement is a 1/2 watt carbon or metal film part. If you want something physically smaller, then a metal film glazed or fusible 1/4 watt part could be used and these are what Tektronix later replaced carbon composition resistors with in high voltage circuits.https://www.mouser.com/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/_/N-7gz41Z1yzvvqx?P=1z0vki1Z1z0vk78Z1yzbpod&Rl=7gz41ZerghZ1z0x86hZ1y8fujgSGT

author: wizard69
author_rank: Super Contributor
timestamp: September 14, 2020, 01:57:33 am
content: Interesting how you learn so much from a simple question!!!I might add that many will replace old carbon composition resistors in circuits where noise may be a problem with metal film. Now is that right or wrong; I can't say but some resistors are better than others for audio range applications and really old resistors can have issues simply due to age.In the end I would never have thought about the effective capacitance of a resistor. This even though I've seen diodes used as capacitors in circuits.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2020, 02:50:47 am
content: Metal film resistors are quieter than carbon resistors when there is a DC voltage across them (and, therefore, a direct current through them). This "excess" or "flicker" noise is over and above the inherent "Johnson" thermal noise of the resistor due to its resistance and finite temperature. Thermal noise has a flat or "white" spectrum, while the noise power density (W/Hz) of excess noise typically is proportional to 1/f. In a situation where there is no DC voltage across the resistor, the noise should only be thermal.

author: giosif
author_rank: Frequent Contributor
timestamp: September 14, 2020, 06:51:12 pm
content: Thank you all for your input!Some of the information included here is definitely above my level of understanding, but I like to believe it will all make sense later on.And I think I understood enough to help me my situation here.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2020, 12:49:18 am
content: The high voltage focus resistor chain made up of R207, R207, R209, etc. is an example where Tektronix later used HV or HVR series high voltage metal film resistors in place of the original carbon composition resistors.