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380mW of dissipation in a 2010 size resistor

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treez

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Hello,
The Resistor shown (R82) is a 2010 size current sense resistor of a 60W PFC’d Flyback LED driver.
It dissipates 380mW.
The enclosure is plastic and has no vent holes.

The PCB layout guy said he had to use the thermal reliefs as shown, rather than solidly connecting the left hand pad to the copper plane. Is this right?...i thought for 2010 resistors, they could be soldered in without such aggressive thermal reliefing?

Anyway, we have actually found an all metal, 2010 resistor which looks like it can handle up to 275degC..
WSLT2010R2200FEA18
http://www.vishay.com/docs/30138/wslt2010.pdf

…do you agree that the WSLT series resistor would even be able to handle the 380mW of dissipation even with just being on a minimal pad, without even the thermal reliefs?
 

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barry

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If you read the data sheet, you'll see that at 275 degrees the device is derated 100%. That "275 degrees" in the Data Sheet title is 100%, grade A marketing BS.
 

FvM

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It may be no problem for this design, but I don't understand the purpose of thermal reliefs here. I won't use it.

High resistor element working temperature of 275°C isn't useless, because it involves higher power rating at elevated ambient temperatures. The resistor might however drop off the board before reaching it's rated temperature…

I don't think that the current sense resistor is a bottleneck. I would rather worry about overall board temperature, and possibly some junction temperatures.
 
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barry

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I agree that the thermal relief may not be needed if this going to be reflowed; if it's hand-soldered it could be very difficult without relief. But I disagree about that 275 degrees; it is very misleading. The only way this device will work at 275 degrees is if it's dissipating zero power.

But, still, there's a lot you're not saying about this design. If this resistor is dissipating 1W, where is that heat going? Is it just building up inside your plastic box? Is your plastic going to melt (most plastics melt below 250 deg. C)? As FvM points out, there's a bunch of other stuff besides this resistor to consider.
 
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KlausST

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Hi,

Why not using a lower value resistor? For current measurement it should work.

Usally current sense resistors need kelvin wiring. I don't see this in your picture.

Klaus
 
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treez

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Thanks
This resistor datasheet says its of "solid metal...element",
but stops short of saying its a solid piece of metal.
Do you know?

Its difficult to tell whether its of "weak" ceramic construction, interwoven with metal, or whether its all metal.......
If its all metal then i can expect that it could operate at 120degc and dissipate 1W....and still not blow up...though i dare say its resistance may change by 5% or so...but that's not as bad as blowing up.
So may i ask is at "all metal".....

http://www.vishay.com/docs/30138/wslt2010.pdf
 

d123

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Hi,

I'm not sure I agree with you, on the first page, top right-hand side, it says: "Solid metal nickel-chrome alloy resistive element with low TCR"

Have you seen any of the related material? The pictures show predominantly what appears to be a solid metal or three pieces of metal interior. Not sure if you've come across this stuff. I wonder if WSHM/WSHP are the same construction (not just family) as the WSLT2010. You could contact Vishay to ask, bottom of page 1 has a contact email for technical questions ("How many PCBs are your company going to produce with this product?" being question number one, as you must know by now!) in POWER METAL STRIP® RESISTORS VMN-PL0022-1611.

The three-minute video, Vishay Dale Power Metal Strip® - WSHM/WSHP, linked to by Vishay on the power metal strip resistor page shows the construction/composition.

You could register with Vishay and ask a question via the form on the WSLT2010...18 PRODUCT INFORMATION Technical Questions page.

In my position as bungling, clueless hobbyist, all I can say is that from a brief search they look pretty darned solid metally to me, treez :).
 
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