Chip Resistor Derating Curve: Walsin’s Two Sheets Draw One Line, and the MR Life Test Loads It at 35% at 125 °C

The chip resistor derating curve in Walsin’s WR and MR approval sheets is one line: full power to 70 °C, zero at 155 °C. We compared the two figures pixel by pixel and they match. The MR life test then loads parts to 35% of power at 125 °C, which is where that line sits.

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Both Walsin datasheets say “see Fig.2” and print the breakpoints nowhere in text. Venatech Limited, an authorised Walsin distributor in Hong Kong, holds both chip resistor approval sheets from Walsin Technology (Walsin): Walsin_WR_General_Purpose_Chip_Resistor.pdf (ASC_WR_V24, SEP 2016) and Walsin_MR_Automotive_Thick_Film_Chip_Resistor.pdf (ASC_MR_V13, Mar 2024). This post is written from the ten pages of each, from the two embedded derating figures extracted as images, and from the 62,757 WR and MR rows of the 137 chip resistor export files. Every part number is real. The question is the one a thermal review asks: what does the line say, and does anything in the sheet use it?

Where do Walsin’s chip resistor sheets state the derating curve?

In a picture, and nowhere else. Page 5 of the WR sheet and page 6 of the MR sheet each carry one sentence, “The power that the resistor can dissipate depends on the operating temperature; see Fig.2”, and one raster figure. We extracted the text layer of all 20 pages and found no derating breakpoint in prose: the only temperatures are the 70 °C rating ambient and the 55/155/56 climatic category on page 3, and the curve between them exists only as pixels.

The figure is a 444 by 312 pixel image with a vertical axis of 0 to 100 percent of rated power and a horizontal axis labelled -55, 0, 70, 125 and 155 °C. The caption on the WR sheet reads “Maximum dissipation in percentage of rated power as a function of the ambient temperature for WR12, WR08, WR06, WR04”. The MR sheet’s caption drops the series list. Neither sheet prints a number for where the line bends or where it reaches zero.

Are the WR and MR derating curves the same line?

Yes, to the pixel. We pulled the embedded image from each PDF and compared them. Across all 138,528 pixels the mean grey-level difference is 2.4 of 255, no pixel differs by more than 57, and zero differ by more than 60: the two figures are one drawing saved twice.

We then fitted a straight line to the sloping segment in each. Both give a slope of 1.846 pixel rows per pixel column, a knee at column 258.0 and a zero crossing at column 393.4 to 393.5. The 70 and 155 axis labels centre near columns 256 and 388, which puts the knee within about 1.5 °C of 70 and the end within about 4 °C of 155. That is the resolution of the image, not a finding that the sheet means 71 and 159.

Item	WR sheet ASC_WR_V24	MR sheet ASC_MR_V13	Match / mismatch
Rating ambient	70 °C, “Max. dissipation @ Tamb=70C”	70 °C, same words	Match
Climatic category	55/155/56	55/155/56	Match
Figure size	444 x 312 px	444 x 312 px	Match: 0 of 138,528 pixels differ by more than 60
Series named in caption	WR12, WR08, WR06, WR04	none	Mismatch
Jumper operating temperature	-55 to +155 °C	-55 to +155 °C	Match
High temperature exposure	1000 h, 155 °C plus or minus 3, no load	1000 h, 155 °C plus or minus 3, no load	Match
Endurance or life test load	1000 h, 70 °C plus or minus 2, rated voltage, 1.5 h on 0.5 h off	1000 h, 125 °C plus or minus 2, 35% of operation power	Mismatch: 55 °C apart, loaded differently

The sheets are 7.5 years apart and the figure did not change. That is useful and it is a reason for caution: the MR sheet’s automotive claims rest on a drawing that predates it.

How much power does a Walsin chip resistor keep at 85, 105 and 125 °C?

Between 82 and 35 percent. With the line running from 100 percent at 70 °C to zero at 155 °C, the slope is 100 divided by 85, or 1.176 percentage points per degree. We computed the figures for six real part numbers: a 1/4 W part keeps 205.9 mW at 85 °C, 147.1 mW at 105 °C and 88.2 mW at 125 °C. Half of rated power is gone at 112.5 °C for every size.

Part number	Size, export rated power at 70 °C	85 °C	105 °C	125 °C	MR operational-life load at 125 °C (35%)	Match / mismatch of the 125 °C figure against that load	Your incumbent part (fill in)
MR10X1002FTL	1210, 1/2 W	411.8 mW	294.1 mW	176.5 mW	175.0 mW	Match within 1.5 mW
MR12X1002FTL	1206, 1/4 W	205.9 mW	147.1 mW	88.2 mW	87.5 mW	Match within 0.7 mW
WR12X1002FTL	1206, 1/4 W	205.9 mW	147.1 mW	88.2 mW	none: WR sheet has no 125 °C test	Mismatch: same line, no test behind it
WR08X1002FTL	0805, 1/8 W	102.9 mW	73.5 mW	44.1 mW	none	Mismatch
WR06X1002FTL	0603, 1/10 W	82.4 mW	58.8 mW	35.3 mW	none	Mismatch
WR04X1002FTL	0402, 1/16 W	51.5 mW	36.8 mW	22.1 mW	none	Mismatch

All six are 10 kilohm, plus or minus 1 percent. The export’s Rated Power column holds watts only, with no ambient stated, so it is a 70 °C number on every row we read. A 1/10 W 0603 on a board at 105 °C is a 58.8 mW part.

Which test in the sheets loads a resistor exactly at its derated limit?

One, and it is on the MR sheet. The MR operational-life test runs 1000 hours at 125 °C plus or minus 2 with 35% of operation power. At 125 °C the line we measured allows 35.3 percent, so the test loads each part to within 0.3 percentage points of its derated limit. We have not seen this counted anywhere: the MR life test is the derating curve, evaluated at one point.

The WR sheet does the same thing at the other end. Its load-life test runs 1000 hours at 70 °C plus or minus 2 at rated voltage, which is the knee, where 100 percent is the most the line allows. Read together, the two sheets test one resistor family at 70 °C at full load and at 125 °C at 35 percent. They do not test anything between, and they do not test the 105 °C point where a hot enclosure actually sits.

For the load side of the same sheets, our post on short time overload and the 2.5 times rule counts what the 5-second test does to the same 33,192 rows.

How many Walsin resistor rows does the derating figure actually cover?

14.2 percent of the chip resistor export, at best. We counted rows by part-number prefix. WR12, WR08, WR06 and WR04 account for 19,660 rows and MR10, MR12, MR08, MR06 and MR04 for 13,566, so the two figures reach 33,226 of 234,815 rows. The other 201,589 rows carry no derating figure in any document we hold.

The split matters because the MR caption names no series, so the 13,566 MR rows are covered only if the MR figure is read as the five sizes the MR sheet tabulates. 29,531 more WR and MR rows sit at sizes neither sheet tabulates: WR01, WR02, WR10, WR18, WR20, WR25 and MR02, MR18, MR20, MR25. The remaining 172,058 rows carry 11 other prefixes, including the high precision, anti-surge, current sense and array lines, for which we hold no resistor sheet at all.

Row group	Rows	Share of 234,815	Match / mismatch against “a derating figure exists for this prefix”
WR12, WR08, WR06, WR04	19,660	8.4%	Match: named in the WR caption
MR10, MR12, MR08, MR06, MR04	13,566	5.8%	Match only if the MR figure is read as its five tabulated sizes
Other WR and MR sizes	29,531	12.6%	Mismatch: size absent from both sheets
11 other prefixes	172,058	73.3%	Mismatch: no resistor sheet held

For the case-size and rated-power side of the same families, see chip resistor power rating by case size, which states the 70 and 155 convention and flagged it as unconfirmed. This post is the measurement it asked for.

What should you do with a derating curve you can only read as a picture?

Treat it as a straight line, compute your own ambient, and ask for the data. A power budget at 105 °C is 58.8 percent of the export figure, and a budget at 125 °C is 35.3 percent. We hold 28 Walsin datasheets, 451 pages, and two of them are chip resistors: ask for the sheet covering your part number and it will be in your inbox, with the figure and the revision date.

1. Write the ambient next to the wattage. The Rated Power column is a 70 °C value on all rows. A power column with no temperature is half a specification.
2. Derate to your board, not to the datasheet. The sheet figure assumes the test board of the sheet. Pad area and copper pour move the number, and the sheet does not give the board.
3. Match your life test to the sheet you hold. If you qualify to 125 °C, the MR sheet has a test at that temperature and the WR sheet has none. The 20,578 WR-prefix rows in series named Automotive do not change that.
4. Cross a part with the tool. The free cross-reference states differences from your original rather than hiding them, and the chip resistor range lists the families.

What is in Hong Kong is on ready stock: 1,125 part numbers as at 23 September 2026, a dated snapshot and not a live feed.

The short version

Walsin’s WR and MR approval sheets print the derating curve as a picture and the breakpoints nowhere in text. We compared the two figures and found one drawing: 0 of 138,528 pixels differ by more than 60. The line runs from 100 percent at 70 °C to zero at 155 °C, which leaves 82.4, 58.8 and 35.3 percent at 85, 105 and 125 °C. The MR operational-life test loads parts to 35 percent at 125 °C, within 0.3 points of the line, and the WR load-life test runs at the knee. The figure reaches 33,226 of 234,815 export rows. The other 201,589 have no derating figure in any document we hold. We will not tell you a rating survives your enclosure. We will show you the line the sheet draws. Confirm against both datasheets and qualify in your own circuit.

Send us the part number and the ambient your board reaches, and we will tell you which sheet and which figure it falls under. And a question back: when you read “1/4 W” on a bill of materials, did you ever check what temperature it was written for?

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Frequently asked questions

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