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power constantly applied on resistor.

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alphacat

New Member
Hey,

In my product, there's a resistor which there's a 0.03W applied on it constantly (its part of a voltage divider that is fed from the AC mains).

Up to now, I used a resistor which its power rating was 1.25W.
Now, I'd like to pick a resistor with smaller dimensions.
Will a resistor which is 0.06W rated, work fine?
My fear derives from the fact that this resistor constatnly consumes 0.03W, 365 days a year.

Thanks a lot.

PS
I'm using thick film SMD resistors.
 
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ericgibbs

Well-Known Member
Most Helpful Member
Hey,

In my product, there's a resistor which there's a 0.03W applied on it constantly (its part of a voltage divider that is fed from the AC mains).

Up to now, I used a resistor which its power rating was 1.25W.
Now, I'd like to pick a resistor with smaller dimensions.
Will a resistor which is 0.06W rated, work fine?
My fear derives from the fact that this resistor constatnly consumes 0.03W, 365 days a year.

Thanks a lot.

PS
I'm using thick film SMD resistors.
hi,
Resistors are power rated for continuous use, so a 0.06W should work ok at 0.03W continuous dissipation.:)
 
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EN0

Member
You could still lower it to 0.03W, since that is the maximum amount of power necessary to be dissipated. Although, you could go as high as 3W but due to size constraints you usually want the lowest needed. So, you can go higher than 0.03W but you can't go any lower, or else the part would be ruined.
 

audioguru

Well-Known Member
Most Helpful Member
Your tiny surface-mount resistor might not be rated for such a high voltage. The peak voltage of the mains is 1.414 times higher than its RMS voltage.
 

tcmtech

Banned
Most Helpful Member
The smaller resistor may be well within its wattage limit but the higher temperature coming off a smaller resistor can cause circuit board breakdown around it fairly fast.
I am sure you have seen many circuit boards cooked black and brittle with the traces pealing off around resistors, switching devices, and some IC's that run at a hot but still touchable temperature.
Most line level signal sensing I do uses a resistor with a 10x wattage over its working power level. That keeps the heat down and also greatly improves surge and voltage spike capacity.
 

EN0

Member
The smaller resistor may be well within its wattage limit but the higher temperature coming off a smaller resistor can cause circuit board breakdown around it fairly fast.
I am sure you have seen many circuit boards cooked black and brittle with the traces pealing off around resistors, switching devices, and some IC's that run at a hot but still touchable temperature.
Most line level signal sensing I do uses a resistor with a 10x wattage over its working power level. That keeps the heat down and also greatly improves surge and voltage spike capacity.
I doubt that 0.03W would overheat an SMT resistor.
 

RadioRon

Well-Known Member
Hey,

In my product, there's a resistor which there's a 0.03W applied on it constantly (its part of a voltage divider that is fed from the AC mains).

Up to now, I used a resistor which its power rating was 1.25W.
Now, I'd like to pick a resistor with smaller dimensions.
Will a resistor which is 0.06W rated, work fine?
My fear derives from the fact that this resistor constatnly consumes 0.03W, 365 days a year.

Thanks a lot.

PS
I'm using thick film SMD resistors.
A quick look at a data sheet for a typical thick film SMD resistor shows that an 0402 size resistor (0.062 watts) has an insulation voltage limit of only 75 volts (ref: Vishay Dale Dxx/CRCWxx data sheet http://www.electro-tech-online.com/custompdfs/2009/09/dcrcwe3.pdf ).

Assuming this value is fairly typical of many manufacturers, and not knowing what your voltage divider is doing, I would want to specify a rating well above the theoretical peak of 170 volts to account for powerful transients and distortion that may occur on your line input. Scanning down the data sheet, I see that the 1206 size is the one with an insulation voltage higer than 300V. This seems like a better choice than 0402.

Another look at a different manufacturers data sheet, this time from Panasonic, shows a "limiting element voltage" maximum for 0402 size of only 50 volts. I'm not exactly sure what determines this limit but I would heed this spec and move up to a larger size. The Vishay data sheet also has this specification but I see that the size 2010 resistor from them has a limiting voltage of 300V which is a more comfortable value.

I also recommend that you read the applicable standards that define suitable minimum spacings for line voltage connections on a pcb. I think that the agencies with this sort of information might be IPC, CSA, and perhaps UL. A product that must meet some sort of safety standard approval, like CSA (Canadian Standards Association) for example, usually has to comply with these spacing rules.
 
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Diver300

Well-Known Member
Most Helpful Member
A quick look at a data sheet for a typical thick film SMD resistor shows that an 0402 size resistor (0.062 watts) has an insulation voltage limit of only 75 volts (ref: Vishay Dale Dxx/CRCWxx data sheet http://www.electro-tech-online.com/custompdfs/2009/09/dcrcwe3-1.pdf ).

Assuming this value is fairly typical of many manufacturers, and not knowing what your voltage divider is doing, I would want to specify a rating well above the theoretical peak of 170 volts to account for powerful transients and distortion that may occur on your line input.
The resistor in question might not have full mains voltage supplied to it, so this might no apply. It is part of a voltage divider, so it will only have part of the the main voltage. We haven't been told if it is most or hardly anything.

The OP is in France, where the voltage is 230 rms, or 325 peak. The voltage rating is going to be the most important thing to consider when choosing the size (not value) of the resistor.
 

alphacat

New Member
Hey guys.
Thank you very much, you have no idea how much you helped me out on this.

I wanted please to make sure something about sizes and power rating of components.

In the datasheet of an IC (CS5463) i use, there was its recommended layout and a table which contains the recommended components (size, power rating, tolerance, etc.)
This IC receives 5V and 3.3V in order to function.

I wonder, why did they recommend on such large and high power-rated resistors?

Most of the resistors in the table are 1/8W or 1/3W power-rated, but since their resistances are in 1kΩ (order of magnitude), than their power consumptions are not larger than
(5V)² / kΩ = 25mW. (1/8W is 5 times larger than that).
 

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Diver300

Well-Known Member
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The 1/8th Watt resistors are 0805 size. It seems to be the size that has been standardised on for that design. There is hardly any cost advantage in going to smaller resistors, and they become more difficult to handle.

We pay about 0.15 cents for an 0805 resistor.

It is also much easier to assemble stuff and obtain all the components if it has lots of the same component. It certainly isn't worth changing resistor type just because one resistor dissipates a bit less power, unless size is really critical.

There is likely to be some reason that four of the 1k resistors are 1210 size. It could be power rating, voltage rating, or they could be larger to be easier to change by hand or to allow more tracks underneath them on that side of the circuit board. The design also calls for 0805 1k resistors, so those would have been used if they could have been.
 

alphacat

New Member
Hey,
Great comment, thanks :)

Nowadays, everyone wants their products to be as smaller as possible, so there's a great advantage in picking up the smallest components as possible.

Thats why i'm looking to decrease the size of the components which allow it.
 

mneary

New Member
The CS5463 (the bill of materials is actually for the CDB5463U development board) (please make it easier to find next time :mad:) is a power monitor and is not strictly 3.3V-5V. :p

You neglected to observe that the resistors in question are used to attenuate the AC mains voltages to levels that the CS5463 chip can tolerate.

:rolleyes: Therefore several of the resistors receive much more than 5V. :eek: They must be 1206 sometimes to have a 1/3W power rating, and other times to have clearance for the voltage.
 
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