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 Post subject: Resistor Physical size differences
PostPosted: Nov Sun 03, 2019 4:47 am 
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I get asked from time to time about physical 1/2 watt resistor size compared to new resistors. Well as I told my customer, a picture is worth 1000 words. From left to right. Old 1/2 watt, new 2 watt, new 1 watt and new 1/2 watt. The 1 watt is closer to the physical to a older 1/2watt .

Sorry for the crappy picture...

Sal


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resistor size differences.jpg [ 348.24 KiB | Viewed 652 times ]

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Last edited by Sal Brisindi on Nov Sun 03, 2019 5:20 am, edited 1 time in total.
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 Post subject: Re: Resistor Physical size differences
PostPosted: Nov Sun 03, 2019 5:14 am 
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Hello Sal,
Quote:
1 watt is closer in physical size then the old 1/2 watt.

I think what you meant to say is the 1 watt is closer to the physical to a older 1/2watt then a new 1/2 watt right ?.

Sincerely richard


Last edited by radiorich on Dec Thu 05, 2019 4:47 am, edited 1 time in total.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Nov Sun 03, 2019 5:23 am 
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I hope you don't mind, I plagiarized your sentence, you said it better...

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 Post subject: Re: Resistor Physical size differences
PostPosted: Nov Sun 03, 2019 12:03 pm 
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I suppose there might be a benefit to this.

When recreating dogbone resistors, the resistor inside can be a higher wattage than the original as it won't be able to dissipate heat as well in the enclosed environment.

-Steve

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 Post subject: Re: Resistor Physical size differences
PostPosted: Nov Wed 06, 2019 12:35 am 
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Just got my order from Mouser to restock. All 1 watt resistors.
Gives you an idea of how much the size can vary.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Nov Wed 06, 2019 12:50 am 
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I guess metal film on a ceramic rod can take more heat than carbon/clay mush in a bakelite tube...

Keep in mind the smaller one will be hotter at the same dissipation, less surface area

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 8:48 pm 
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I just got a little surprise, due to my lack of attention. I bought a bunch of 10K 1/4W carbon film resistors. A bunch being 1000 of the same value. I go thru a lot of resistors. I initially thought that they (Mouser) sent me the wrong parts, but the mistake, was all mine. Here's why:
Attachment:
YageoCFR_Dimss.jpg
YageoCFR_Dimss.jpg [ 105.61 KiB | Viewed 334 times ]

I ordered the CFR25S instead of the CFR-25. Both are 1/4W, though the CFR25S is the size of normal 1/8W, have lower voltage ratings, and higher temperature drift. I'll keep and use them, as parts of this size are used in circuits, that stay well away from their max ratings. As for tempco, if that's important, I'll use a metal film, or better.
Attachment:
Yageo_CFR_25ds.jpg
Yageo_CFR_25ds.jpg [ 278.01 KiB | Viewed 334 times ]


When it comes to higher power parts, get the datasheet, and note the maximum rate operating temp. Many have max temp specs ~350C. Note that solder melts around 190C, so if the solder melts, and the resistor falls off, yet itself is not damaged, it still counts as a fail.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 9:00 pm 
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Maybe they have ICAS ratiings, for 10 minutes, with a starting temperature of 0C. :P Or they dissipate their heat into another dimension with quantum cores.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 9:01 pm 
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Panasonic, SEI, Yageo, Rohm, Dale, and Ohmite all offer "reduced size" resistors like 3-Neurons mentioned. They are becoming more mainstream, as are 1/8W and 1/10W resistors.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 9:03 pm 
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wazz wrote:
Maybe they have ICAS ratiings, for 10 minutes, with a starting temperature of 0C. :P Or they dissipate their heat into another dimension with quantum cores.


With Super-Asymmetry, it would be infinite dimensions or none. :mrgreen:

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 9:09 pm 
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Sal, you are making a basic mistake. The American made 1/2 watt resistor can easily pass a half watt all day long without flinching. I seriously doubt those Chinese made resistors can pass full power unscathed. I always double up on power when ordering from our distant neighbors to the east.


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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Wed 04, 2019 10:40 pm 
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When I was a child (I actually was ... a very long time ago), the old timers had a rule of thumb, between actual power dissipation, and the rating that should be used:

1) If the components where in a well ventilated area, then 2x should be the limit. So if the actual dissipation was, say, 400mW, then 2x is 800mW, and then a 1W resistor should be used.

2) If it was semi-enclosed, and there was still some air flow, the rule was 3x, minimum.

3) If fully enclosed, with NO air movement, then 4x was the rule. A dissipation of 400mW, multiplied by 4 is 1.6W, meaning a 2W resistor is specified.

I often use 5W resistors, when they dissipate 1 watt, or even less, just because I don't like temperature. Using a physically larger resistor spreads that SAME heat energy over a larger volume, resulting in a lower spot temperature. Also locate your electrolytics some distance from those power resistors. You don't need to cook the electrolyte out of them.

On the flip side, for quick experiments, running a resistor well over its rating can be done, if only for a couple of minutes, when collecting data, in an experiment, or other transient use.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Thu 05, 2019 3:40 am 
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If I put a 60-watt halogen bulb next to a 60-watt ordinary incandescent, next to a 60-watt chandelier bulb next to a 60-watt CFL next to a 60-watt projection bulb, they’d all be different shapes and sizes, different color temperatures, and different life ratings. You would probably not mistake one for another, even though they all have the same wattage.

Likewise the fact that two resistors have the same wattage tells you little else about them. You’ve already observed they’re different physically. They also have different temperature and voltage coefficients, different tolerances, different breakdown voltages, and different pulse and overload handling capabilities. Some are flameproof, some are not. Each has certain advantages that make it better for some applications than others, which is why so many varieties exist.

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Thu 05, 2019 4:52 pm 
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Even worse are hidden factors, like internal inductance. Usually not a deal killer for vintage
radios, but the parasitic suppressors in high pwr RF amplifiers often used a carbon comp resistor
that was carefully matched with a parallel inductor with a reactance that was careful selected
to balance stability vs. power loss. Some newer resistors look like (pardon the
pun) carbon copies, but have an internal structure that has considerable more
inductive reactance since they are in reality carbon film parts.

from W8JI website: https://www.w8ji.com/images/Carbon_Metal_resistors.jpg

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Thu 05, 2019 8:03 pm 
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Johnnysan wrote:
Sal, you are making a basic mistake. The American made 1/2 watt resistor can easily pass a half watt all day long without flinching. I seriously doubt those Chinese made resistors can pass full power unscathed. I always double up on power when ordering from our distant neighbors to the east.


Sorry I just saw all the replies. I will subscribe to this page.

Johnny,
You said you seriously doubt the China made 1/2 watt resistor would pass a half a watt all day compared to a US made resistor. Do you know for a fact or is it just what you think? Half hour ago I put a 100 ohm 1/2 watt resistor across my power supply set at 7.07vdc. So far 30 minutes had passed and the and the resistor is cold to the touch. I plan to leave the resistor on the power supply all day and check to see if the resistor gets warm. I will update this sub forum later this evening.

Sal


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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Thu 05, 2019 10:56 pm 
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This will be interesting. What's fact and what's fiction.

Freeman.


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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Thu 05, 2019 11:43 pm 
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The power rating on most resistors is good up to about 160 degrees F and with free air flow conditions. After that, it is a fairly steep slope down derating the resistor.


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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Fri 06, 2019 12:13 am 
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Well after 5 hours so it is not even warm to the touch...

To be fair and impartial, anyone here wants to take the challenge?

Sal

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Fri 06, 2019 12:48 am 
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Sal, crank up that voltage! Let us know when it smokes muahahahahahah

I've never had any problem with Chineseium resistors. I suppose it's possible, but the REAL problem with the imports, especially the film type, is that the color bands are sometimes very hard to make out.

I just got a cabinet full of 1/2 watt Ohmite carbons.... because I wanted the drawers. But I think I'll take the time to inventory the resistors and use them. :)

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 Post subject: Re: Resistor Physical size differences
PostPosted: Dec Fri 06, 2019 7:00 am 
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The verdict is in, The resistor has no noticeable rise in temperature and I had it on my power supply for 11 hours and 50 minutes.

Sal

EDIT: I raised the voltage to 10 volts for 2 hours and the resistor is very slightly warm with 3/4 watts.

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