Resistors Physical Size

[John10b]

Why are modern resistors physically so small compared to older ones?
Just replacing some in a Bush AC34, ordered 1/2 watt and surprised to see them very small compared to original. What is the difference in manufacturing process to allow this?
Cheers
John

[julie_m]

The materials and construction have been improved, making modern resistors safer to use at higher temperatures. So they can be physically smaller and still dissipate the same amount of heat to their surroundings, because they drift less with changing temperature and do not suffer from weak spots in the resistive layer.

[GrimJosef]

I'm afraid it may be a difference in the way that power ratings are done. Back in the day a 1/2 watt resistor would survive indefinitely with 1/2 watt being dissipated in it as long as it was in more-or-less free air. Nowadays a 1/2 watt resistor is rated to survive 1/2 watt only if you can keep its surface at or below the specified temperature (often 70C). To do that you might have to immerse it in coolant or fasten it very closely to a large heatsink or at least cool it by keeping the leads short and soldering them to a large copper pad. If you just leave it in free air then the surface will quickly get hot and you may have to start using the power derating curve which is probably published in the datasheet somewhere.

Cheers,

GJ

[Argus25]

It depends on how long you want the part to last.

I have seen what are 1/8W resistors rated as 1/4W and looking at a modern 1/2 watt resistors, they are a fraction of the size of those from yesteryear. The smaller the object is, the hotter it gets for any given power dissipation.

But it is true that modern resistors are made of superior materials compared to old fashioned ones and a higher temperature is tolerable without degrading the value or stability. But the question is; how hard do you want to push it. Probably if you calculate that you need a 1/4 W resistor or close, if you want reliability, use a half watt one.

For most of my valve set restorations I have come to use 2W rated metal film resistors. They are about the same physical size as 1/4 to 1/2 W vintage resistors and they have a good look to them and are more stable with lower noise than carbon composition types and running them well under their ratings means they will last.

Also I agree with Grimjosef's remarks about modern power ratings, there are some severe liberties going on these days with all kinds of component power ratings. It is more of a contest between the marketing departments of various companies and the Engineers who really know what is going on have been locked out the back with cold cups of instant coffee. What about a manufacturer of a part in an 8 pin SOIC IC package claiming that two of its pins can support 30A ? This sort of thing is what we have to contend with.

[MrBungle]

If it's any recommendation, if you look at the Vishay CMF and RN55 resistors, the same size resistors are rated for 1/8W on milspec and 1/2W on commercial...

YMMV but the milspec ones are like it for a reason.

On the subject of SMD packages, a lot of them chuck the heat out through the pads. I'm amazed to find that a well mounted 1/8W resistor will sit there and burn a watt quite happily where as its through hole brethren would be pouring smoke out.

[John10b]

Thank you for the information.
What physical size resistors do vintage Wireless restorers use? Should I buy, for example, 2 watt just to get comparable size or not?
I will be interested in your comments.
Finally is there a preferred supplier who specify physical size, the one I use does not give size.
Cheers
John

[GrimJosef]

Farnell generally give sizes either in the product listing or in a linked datasheet. The same components may be available from their sister company, CPC, at a lower price but CPC don't link to a datasheet so often.

Cheers,

GJ

[Radio Wrangler]

Yes, put the power rating up to get the size to match.

They look less different in the chassis or on the board, but more importantly they'll run at a similar surface temperature to the originals, not hotter. The temperature you want to run resistors to isn't just a matter of what the resistor will stand, but also what the parts around them or wires touching them can stand.

David

[G8HQP Dave]

For some circuit positions in valve equipment you may need 1W or 2W resistors just to get the needed voltage rating. The common 0.6W may only cope with 250V - enough for many radio receivers but not necessarily audio amplifiers.

[Biggles]

Watch out for maximum voltage ratings when sourcing replacement resistors for valve equipment too. I prefer to use the two watt resistors also. They are the right physical size and are running well within their ratings when replacing your standard half watt carbon jobs.
Alan.

Edit; crossed posts with Dave about voltage!

[Argus25]

Originally Posted by Biggles

Watch out for maximum voltage ratings when sourcing replacement resistors for valve equipment too. I prefer to use the two watt resistors also.

I have had some interesting experiences using resistors in high voltage applications.

I have attached some photos of resistors to help make a point, just pretend they are all 1M ohms (three of them are).

Looking at the 1 meg 2W metal film one first, since the power dissipated in the resistor is V^2/R then when the resistor is dissipating 2w the voltage across it is 1400V. But these MF resistors are not reliable over about 750V in the long term and 1400V is far far too high. So in my view, in this 2W MF resistor series, once you get to about 270k, which is dissipating 2W with 750V across it, you have reached the limits for reliability. In my experience, once the voltage gets to the 750-1kv region, it requires a special resistor (or two in series perhaps)

So what I'm saying here is despite the 2W rating for the 1 meg MF resistor, you can in fact never dissipate the 2W in that part, if it is a high resistance part (>270k) without exceeding a voltage compatible with long term reliability.


With 1/4 W types I regard 350v as the very upper limit even then one wonders about some with 500V ratings and I struggle to believe it.

I've included in the picture a 1M 2W carbon film resistor, these go high in value or O/C if exposed to > 750V, usually after a week or two.

Philips recognized the problem and made resistors called "focus" resistors, typically used in TV focus chains. These were better than ordinary resistors for voltage tolerance, they can still go open though but they could reliably handle 1kV.

Over 1200V now, for a single resistor, I have moved to Japanese Electrohm high voltage resistors for valve Tv work which are long term stable and don't fail due to high voltages, but you can see from the photo how big they are compared to the 2W MF type of the same value.

So the overall point is, a resistor's power rating cannot necessarily be used to calculate the maximum allowable voltage it can handle. And be very cautious if a resistor, even a 2W MF type, is exposed to 750V or more or have very detailed high voltage specs on it.

[suebutcher]

Can you give us the dimensions of those resistors?

[Argus25]

The brown resistor is 8mm dia and 24mm long.

Recently a switch in a 10A rated appliance of mine failed (A dryer that can draw this much current). I was amazed at how small the contacts were and the pathetic size of the mechanism, I would have been reluctant to push it to 2 Amps myself. No surprise, the body melted and the contacts overheated. But it said "10A 240V" on the switch body, made in the far east.

So I looked through my parts bin and found along the way a vintage Japanese 5A rated switch with contacts over twice as big. In the end I replaced it with a 20A rated switch.

I have found that the Japanese are very conservative with their electrical product specs. And if a Japanese part is rated, say a switch for some current, generally you can trust the rating.

I'm very reluctant now to trust switches and connectors up to their supposed max ratings without inspecting them first to see if it looks like they are up to the task.

[trobbins]

Originally Posted by GrimJosef

I'm afraid it may be a difference in the way that power ratings are done.

I couldn't easily locate any good vintage datasheets except for the 1961 ERIE sheets posted by Lucien:
https://www.vintage-radio.net/forum/...ad.php?t=69039

They definitely show a 'poorer' spec level in general, such that 0.5W at 40C rated parts were 0.25W at 70C. Many had a constant derating from 40C, but a few have the 70C break-point. Of note was the 107C max surface temp rating back then, as compared to a typical 155C rating nowadays.

[Craig Sawyers]

Originally Posted by suebutcher

Can you give us the dimensions of those resistors?

Thanks for the information on the large brown Electrohm resistor - that is valuable information. Where do you get them from? A web search fails to find a stockist.

[John10b]

How very interesting, i didn’t realise there was so much to resistors!
Cheers
John

[Argus25]

Originally Posted by Craig Sawyers

Thanks for the information on the large brown Electrohm resistor - that is valuable information. Where do you get them from? A web search fails to find a stockist.

That doesn't surprise me. The only place I have been able to get these high voltage resistors (and a lot of other useful parts) is at the electronics markets in Akehabara in Tokyo. I only get to go there every 3 or four years to stock up, but it might have been 4 years since my last visit.

Once an electronics engineer has been to the Akehabara markets, it leaves them longing to go back. All the Japanese made parts are super high quality.

I have attached a photo of a typical shop. If you look in the top right you will see all sorts of can electrolytic capacitors. Off to the left (not visible on the same photo) they have all sorts of specialist resistors. This would be one of about 50 to 100 or more small shops on one block, jam packed with goodies, some now specialize in valves , others semiconductors and IC's etc. Other shops with amazing nickel plated brass hardware and engineering materials with stuff not stocked by RS or Farnell/E14. This is the place where I get many of the quality parts for my home grown radio/video projects etc.

The trouble is, very few of these sellers are on the net. The only way to get the parts is to physically go there, but it is a great excuse & place for a holiday if you can talk your partner into it, allow at least 2 to 3 days to comb the markets as you will see things and want to go back. A lot of the small shops are upstairs too.

[John10b]

What a great place to visit, I think I could happily spend my holidays at such a place, my wife no way! Unless there is a an M&S close bye .
Cheers
John

[ionburn]

I would love to go there. So much to do with the different culture that a month visit would be good. Getting stocked up would be too, but maybe spoilt for choice.

[Argus25]

I have attached a few more photos. One shows very large 10k metal film resistors they sell there, I think these ones are 5 or 6W rated, and precision non inductive 4W carbon resistors with gold plated wires, very good for the anode loads in valve video output stages etc. Of course they have every type of WW resistor under the sun.

Most of the market stalls are under that green railway bridge but some have moved, but across the road and down the street there are more.