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Resistor and Capacitor Tolerances

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Resistor and Capacitor Tolerances

Post by Terrykc » Mon Jul 16, 2012 6:46 pm

This post was originally based on resistors but is equally applicable to run of the mill capacitors, although the latter usually have wide tolerances and fewer values per decade. The exception is, usually, in tuned circuits and similar areas where capacitors tend to have very tight tolerances.

I've copied it here because it was buried in the midst of a radio repair post and I had some difficulty finding it when I wanted to refer somebody else to it!

--------------------------------------------------------------------------------

The subject of tolerance crops up here frequently and is obviously a much misunderstood topic.

A typical post will say something like "I changed R47 because it had dropped in value from 220k to 201k."

This is the equivalent of saying "I changed all the tyres on my car because the tread depth had dropped from 8mm to 7mm ..."

But you wouldn't do that, would you?

Manufacturing accurate resistors has been possible for many years but it wasn't always so. Most electronic designs, particularly the vintage kit we are usually talking about here, is pretty tolerant - if you'll pardon the pun - of component value variation and a tolerance of ±20% was considered acceptable very early on.

Attempting to make resistors of, say, 10, 11, 12, 13, 14, 15 ohms and so on, was obviously a waste of time, given such wide tolerances and an exponential series of values was evolved to make keep the available range within sensible limits.

Take a 10Ω resistor, for example. 10Ω plus 20% = 12Ω. Similarly, a 15Ω resistor minus 20% is also 12Ω - so there is no point making anything between 10V and 15Ω ...

With a ±20% tolerance, each decade can be covered using only six values - the E6 series - because the values progress exponentially thus: 10, 15, 22, 33, 47 and 68. Hands up if you thought all those strange resistor values were chosen at random ...! If you check them, you'll find that the tolerance of each pair of adjacent values either just meet or slightly overlap each other.

No doubt as manufacturers found improved methods, accuracy improved, leading to the familiar E12 series - 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. Note that although, in the main, these were classed as ±20% components, the values actually overlap at the 10% point.

My experience of resistors in the early 60s was of ±20% Erie carbon composition resistors, then RS ±10% resistors - still carbon composition but half the size - and then to ±5% carbon film resistors which, with very rare exceptions, I used throughout my working life.

The 5% tolerance leads on to an E24 series of values - 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82 and 91 but, for virtually all practical purposes, the additional values can be ignored. The only E24 value I can ever recall in a (vintage) radio or TV was a 43k resistor used by Bush in the line stage of the TV125 series (and probably the earlier TV105 and TV115 series before.

Anybody involved with RF work will, of course, find the 75Ω value useful!

The picture below shows the three series with the upper and lower limits for each value at the appropriate tolerance. Hopefully it will help to clarify the issue and stop much non-essential resistor changing in the future! Spend your time (and money) finding the real fault!

Resistor values can vary between hot and cold - but it is rare (and few resistors actually get warm, anyway!)

Res_Tolerance.png
Component tolerances

Click image to enlarge. Click again to view full size.

 
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Re: Resistor and Capacitor Tolerances

Post by CTV » Mon Jul 16, 2012 9:07 pm

Terrykc wrote:I've copied it here because it was buried in the midst of a radio repair post and I had some difficulty finding it when I wanted to refer somebody else to it!


I've made this thread a sticky :thumbl:

 

Re: Resistor and Capacitor Tolerances

Post by XTC » Mon Jul 16, 2012 11:04 pm

Critical RF capacitors were usually silver mica, often odd values such as 60pF and 109pF and +/- 1%. Sometimes the temperature coefficient was also considered for drift cancelling. Resistors and most capacitors weren't critical and +/- 10% was fine and +/-20% common. Valves usually had quite a spread of characteristics when new and were subject to ageing.

You wouldn't expect to see close tolerance resistors or caps outside of test equipment and there in a few chosen locations. +/- 1% tolerance resistors and large capacitors were available, but they were damned expensive.

There were-hi stab carbon resistors, which may have been used in domestic equipment, but only in the odd selected location. They were widely advertised. From what I've seen of them, they haven't coped with ageing very well.

Generally, manufacturing techniques have improved and the specifications of readily available parts is now very good. There wouldn't be a market for +/- 20% resistors now because +/- 5% parts are so easy to make and +/- 1% isn't much dearer.

In another thread there was a question about a cap used in a TV timebase. I suppose it's possible it was Select On Test, although were that the case it was usually pointed out in the service documentation. Generally, sifting through a pile of 10K +/- 5% resistors and selecting one to +/- 1% with a test rig, is not the same as taking a 10K +/- 1% resistor made as that. The +/-1% resistor will have been designed as +/- 1% tolerance from all sorts of angles; soldering tolerance, ageing, shock and vibe, moisture and probably have a defined temperature coefficient. But, if you are operating under commercial pressures SOT maybe good enough, if a preset isn't appropriate.

As for replacing resistors in vintage equipment, something often overlooked is the voltage rating of a resistor. Exceeding the voltage rating (while not necessarily exceeding the power rating) will very likely permanently change its value. Most 0.5 W and less resistors intended for transistor circuits only have a voltage rating of 250V or even 125V. I tend to use 2 or 3 W metal film types because not only is the size similar to the original, the voltage rating is usually 500V. I'm not a fan of NOS resistors or caps.

As for Jeffrey's point about considering the voltage rating of caps, one of the factors is the possible effect of fault conditions.

Pete.

 
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Re: Resistor and Capacitor Tolerances

Post by Terrykc » Tue Jul 17, 2012 12:18 am

My intention in posting this topic in the first place was to help demystify the topic of tolerance variations for the benefit of the beginner who, armed with a much more accurate measuring device than those used at the time their set was built, can produce resistance readings with 3 or more digit accuracy which appear to show a significant difference to the rather accurate value indicated on the device.

They reason that, as the manufacturer went to all the bother of marking a resistor as 680Ω say, a reading of 709Ω must be wrong when, in fact it is well within 5% of the marked value on what may well be a 20% tolerance component.

Capacitors are less likely to be measured and are changed for other reasons that we all know well but the beginner faces a problem when no replacements of the correctly marked value appear to be available. Of course, if you had a supply of 0.5uF waxed paper capacitors available, the last thing you would do with them today would be to use them as replacements!

Much of what you write is totally irrelevant to the repair of vintage equipment and only likely to cause more confusion. In fact, it is only at the end of your post that you raise the issue of vintage repair at all and, whilst I accept what you say about resistor voltage ratings, it would be rare to find resistors with voltage drops across them anywhere near the 250V rating of a third or half watt component.

It is worth pointing out though, in case someone is tempted to use a supply of 0.25W or even 0.125W sub-miniature resistors in an old receiver, though I think it more likely that chunkier 1W types might be seen as more desirable from the point of view of matching the size of vintage components.

 
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Re: Resistor and Capacitor Tolerances

Post by Terrykc » Tue Jul 17, 2012 10:31 am

Sorry, Trevor, but you are missing something that I was planning to expand on - I was on was on my way to bed when I made my previous post!

The voltage rating of a resistor is also value dependent.

Take 0.5W resistors. Suppose the manufacturer's maximum rating is 250V. That does not apply to most resistors in use, either modern or vintage!

The maximum permissible voltage across a 1Ω resistor is half a volt!

At half a volt the current is half an amp and, as a half times a half is a quarter (Watt), the resistor is dissipating its full power.

The maximum voltage across a resistor rises with value until 250kΩ is reached. At this point the voltage is 250 and the current is 1mA. Again, 250V x 0.001A = 0.25W. Only above this point does the manufacturer's maximum voltage rating cut in.

So, old or new, up to 220kΩ, there should be no problem. Only for values from 270kΩ up might voltage be an issue but, as I said previously, 'it would be rare to find resistors with voltage drops across them anywhere near the 250V rating'. Note that 'rare' does not exclude exceptions!

Paraphrasing my sig. line, if in doubt, read the manufacturer's data, but it would be interesting to see how many half watt resistors folk can find in their vintage kit that have >250V across them ...

 
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Re: Resistor and Capacitor Tolerances

Post by Refugee » Tue Jul 17, 2012 11:23 am

This applies to modern devices 8))
In the devil there is an SMPSU.
In it there is a "power-up" resistor that provides a trickle currant to the chip from rectified mains and failure to open circuit of this component is in the top 3 stock faults on modern TV and computer power supplies. There is often two lower value resistors connected in series indicating that the manufactures sometimes know about it.

 
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Re: Resistor and Capacitor Tolerances

Post by GlowingAnode » Tue Jul 17, 2012 3:29 pm

Refugee wrote:... and failure to open circuit of this component is in the top 3 stock faults on modern TV and computer power supplies.

Ref, out of interest, what are the other two?
I don't get involved in modern stuff, so would be useful to know for future reference.
Rob.

 
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Re: Resistor and Capacitor Tolerances

Post by Refugee » Tue Jul 17, 2012 3:48 pm

Blown semiconductors and blown capacitors on both sides of the transformer.

 
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Re: Resistor and Capacitor Tolerances

Post by Terrykc » Tue Jul 17, 2012 6:10 pm

In view of the previous comments about the maximum voltage rating of modern resistors, I tried an experiment.

My previous figure of 250V was derived from the data for the Philips CR25 series which nobody, I'm sure, would describe as 'modern', so I headed for Farnell's website (other suppliers are available) because I know they have data sheets readily available.

I took 5% carbon film resistors as a typical choice for most day to day use and looked at what they had.

Their main choices centre on the Multicomp range, to which the following applies.

Starting with the smallest and working up, we have the following:

Type/Power Rating/Max Voltage/Available Resistance Range

MCRE 0.125W Series 200V max, 1R0 - 1M0
MCF 0.25W Series 250V max, 1R0 - 10M
MCCFR 0.5W Series 350V max, 0R5 - 10M
MCF 1W Series 500V max, 1R0 - 10M

Pretty much as I expected and not the serious down rating warned of earlier in this thread. So, a modern 0.5W carbon film resistor can be expected to do everything its composite carbon predecessor was capable of.

I did find one inconsistency, though. There is another 1W range from the same manufacturer and a 2W range in the same series. If you look at Farnell's 'More Details' link, both series are stated to be 350V max yet, if you read the manufacturer's data sheet, it clearly says 500V!

MCCFR 1W Series 350V max, (Farnell) 500V max, (Multicomp), 4R7 - 1M0
MCCFR 2W Series 350V max, (Farnell) 500V max, (Multicomp), 4R7 - 1M0

Personally, I'd believe the manufacturer.

The data sheets also give the formula for RCWV (Rated Continuous Working Voltage) which saved me having to work it out!

RCWV = Square Root (Rated Power x Resistance Value)

From this a compiled the following chart showing maximum voltage for each power range against resistance (to keep it within a reasonable length I've pruned the list of values down to 5 per decade)

Image

Note how the maximum resistance limit for maximum voltage for all types is the same (or very close) to the value I previously calculated!

On the basis of this I think it is fair to say that the humble, cheap, 0.5W carbon film resistor is perfectly adequate for all but the most demanding of applications you may come across - assuming, of course, that the manufacturer hasn't specified a component with a higher rating!

If the value is above 220k though, you might want to double check whether an uprated component might be justified - which is why I've highlighted that section of the chart - but that should only be necessary occasionally.

 

Re: Resistor and Capacitor Tolerances

Post by XTC » Tue Jul 17, 2012 9:51 pm

Murphyv310 wrote:Back to vintage, on the Bush TV22/24 the line osc anode load is a 820k, my advice to anyone is to fit a good carbon as the modern resistor is guaranteed to fail!


There are these which aren't too dear.

http://uk.farnell.com/welwyn/vrw37-820k ... dp/1292568

or maybe these in parallel.

http://uk.farnell.com/vishay-bc-compone ... dp/9477306

http://uk.farnell.com/multicomp/mgrf1wj ... dp/1357951

HV resistor failure is quite a common thing in scopes. Occasionally you come across scopes which someone has tried to fix with strings of carbon composition resistors, 1/4W probably 500V resistors, but they've obviously failed PDQ.

HV resistors are pretty much like high voltage zenners and high voltage diodes, you're pretty much stuck dealing with someone like Farnell or RS, although Maplins used to do HV resistors, along with lots of other things...................

Pete.

 

Re: Resistor and Capacitor Tolerances

Post by Dave » Mon Jun 10, 2013 1:01 am

Just my two pennyworth on capacitor tolerances dragged from the recesses of 50 plus year memories at Erie Resistor.
1) Always remember the voltage rating is very important. I recall at Erie all ceramic capacitors were voltage tested to 2 times working for 1 second
2) All ceramic capacitors are temperature sensitive. I recall Erie made some which were P100 (Positive 100 parts /million per degree C I think) and N 750 are 2 I recall. Due to the make up of the ceramic dielectric, some of the more exotic dielectric mixes used to make the higher value capacitors only allowed tolerances of -20+80%
3) The very low value ceramics , say up to 50 pico F, were measured for value in a shielded jig with only a few mms of lead wire exposed to eliminate stray capacitance.
4) The pressing and firing of ceramic discs sometimes resulted in small.invisible, voids within the dielectric. This voids were filled with gas which with the application of high voltage, could result in them becoming small ionisation chambers and generating a signal. Erie would if asked select ionisation free capacitors.

Somewhere or other, I have a photo of a very high voltage , 230KV, capacitor I made up as a special project. A series parallel arrangement about 30 ins long!

I can also recall a device I made up to pulse high meg ohm resistors with high voltage. Basically a Runbaken oil coil for automotive use driven by a pulsed mercury vapour thyratron. It would arc over a couple of inches or more. Far exceeded the Runbaken claims!

In the 50s and early 60s , Erie also made the resistors used in the undersea telegraph cable repeaters. Basically very carefully assembled and tested standard solid carbon resistors .


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