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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Apr 2020, 5:11 pm | #1 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Resistor power rating
Hi folks, does the team think this is a 1 or 2W?
many thanks |
1st Apr 2020, 5:50 pm | #2 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
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Re: Resistor power rating
1W for me.
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1st Apr 2020, 6:16 pm | #3 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Resistor power rating
Thanks Kalee, that's what I suspected
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1st Apr 2020, 6:39 pm | #4 |
Hexode
Join Date: Apr 2008
Location: Ramsgate, Kent, UK.
Posts: 252
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Re: Resistor power rating
Modern resistors are amazingly small for their power dissipation capability these days . . . the laws of physics don't change so, for a given rating and element type, the smaller the resistor (hence it's surface area) the higher the permissible surface temperature is. That rules out doing any power dissipation/temperature tests!
To compound the problem, I have the following types of resistor in my stocks, the bodies of these are all about 12.5 (1/2") long: 2 Watt - RS 'Pro' CARBON Film 0.5% Body colour pink (YES - 2W!) Neohm ROX2S Metal Oxide Film, 2W 5% Body colour Pinky-Brown Blue or Green Wirewound type, manufacturer unknown. 1 Watt - Several older CARBON film types. 0.5 Watt - Few older CARBON types, up to 1970's manufacture In my experience your resistors have the appearance of metal oxide types, in which case they could well be 2 Watt rated, but to be totally on the safe side I would say don't stress them beyond 1 Watt. Adrian. |
1st Apr 2020, 7:31 pm | #5 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: Resistor power rating
Yes- I have to keep changing my expectations of how big a given power rating needs to be over the years. It can be very difficult now just to judge on appearances. I'd agree that it could well be 2W rated, I've recently been using PR02 resistors, just slightly smaller and more angular-bodied. However, I always feel that being cautious is cheap insurance on the small scale- if you're an OEM, you can save thousands by running close to the limit and by time the complaints come in, they're out of warranty (the art and science of thrifting...) but as a repairer, do you really want folk grumbling about you because you saved 10p on a resistor and something stopped working after a while?
I'd call it 1W and sleep easy! |
1st Apr 2020, 10:22 pm | #6 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
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Re: Resistor power rating
The wattage rating also depends on the temperature of the environment it is used in, the heat from adjacent components etc. and may be derated further for improved reliability.
Also if you've got say PVC insulated wiring touching the resistor, running it to modern surface temperature allowances will damage the insulation. David
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1st Apr 2020, 10:55 pm | #7 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
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Re: Resistor power rating
They could be wirewound, in whcih case they probably have a higher rating than if they were film resistors.
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1st Apr 2020, 11:26 pm | #8 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: Resistor power rating
I would say that the pictured item is a metal glaze 2 watt resistor. I have many that are identical.
Joe |
1st Apr 2020, 11:51 pm | #9 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Resistor power rating
Maarten, I believe it's a metal film type.
I'll assume it's 1W which seems to be the consensus, thanks guys crossed post Joe! |
2nd Apr 2020, 1:27 am | #10 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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Re: Resistor power rating
12.5mm long (or 1/2" in proper measurements ) according to Vishay's data on several different modern resistor series, would be 2W.
But probably better to rate it as you have at 1W without confirmed make/manufacturer data. |
2nd Apr 2020, 7:14 am | #11 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
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Re: Resistor power rating
The max dissipation of any resistor, taken from a datasheet, is in free air. The big mistake is putting the resistor in hard contact to a circuit board. Even Tektronix have made that mistake from time to time.
At best it will discolour the board. At worst the resistor will burn out. Space resistors being used close to their rated power rating off the board by a few mm at least. Craig |
2nd Apr 2020, 10:42 am | #12 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: Resistor power rating
^^ Wot he say ^^
Ceramic spacers (sometimes referred to as fish-spine beads) and tubes can be handy here, some manufacturers rivet tinned rolled copper tubes to PCBs and thread power resistors through them to maintain clearance. When possible, some put small holes in the PCB directly under and around hot components. |
2nd Apr 2020, 11:40 am | #13 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: Resistor power rating
Or put a small kink in each lead using fine nose pliers!
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4th Apr 2020, 8:57 pm | #14 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Resistor power rating
They're about the size of PR02 resistors, which were good for 2W. But if you were planning on having them dissipating power continuously, you might want to limit that to 1W.
Resistors get hot, and all that's different about modern, smaller ones is they can withstand higher temperatures. The number of Watts you can transport through a square metre of surface area for 1 degree rise is the same as it ever was.
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