# Difference between [W] and [WS] power rating of resistors

I see some datasheets of power resistors where power ratings are in Watts [W] and some in [WS] which I don't know what they mean.
What is the difference between them?

Example of datasheet

Some screenshots from datasheets:

• Perhaps "small"? i.e. the physical size is the same as the next rating down. Commented Oct 2, 2018 at 19:12
• @TomCarpenter Size is size, and power rating is power rating. Why to mix it together? Commented Oct 2, 2018 at 19:15
• I suspect Watt-seconds. I.e., you can give it 1/4W continuously, or you can blast it with 10W for 1/20th of a second. Commented Oct 2, 2018 at 19:17
• Not sure, other that many resistors from different manufacturers for a given power rating are similar in size. So it's plausible. Commented Oct 2, 2018 at 19:17
• @Chupacabras - I believe most Wattage sizes are "standard"/common. Same goes for SMD resistors. But they are only conventions and plenty break the mold.
– Bort
Commented Oct 2, 2018 at 19:17

TL;DR; It's my thinking that the S is simply relating to the package being smaller.

Resistors from different manufacturers for a given power rating tend to be similar in size - the power rating effectively becomes a de facto way of identifying package size and vice versa.

As such it's plausible that they are using the prefix S to indicate that it is the higher rated resistor, but with the physical size of the next power rating down. If you go through all of the tables in your question, you'll find this holds true.

To back this up, if for example you take the SR Passives example, the part number for say a 1k 0.25W resistor differs between "CF1/4WS-1K" for the smaller package 3.2mm long package, and "CF1/4W-1K" for the larger 6mm long package. Both resistors are rated for 1/4W.

If you look at the Tecla & Chiara example, they seem to separate the two in a similar way - the ones without the S suffix are under the label "MF", and those with the suffix are under the label "MFS", which are likely the series names.

If it were representing an energy - Watt-seconds as suggested - then it would be Ws, not WS per SI unit capitalisation. I could understand if one failed to make this distinction, but the fact that three fail makes me highly sceptical. As a specification it also doesn't make much sense without context.

For example, could I run a 1Ω "2WS" resistor with 500V across it for 2ms? That would not exceed 2 Watt-second, nor exceed the voltage rating, however it would result in a 500A surge through the resistor. Is this allowed? Would the "2W" version be able to withstand this? Could it withstand more? It would be a meaningless specification.

• +1 Tom for being correct, as I have to pay attention when buying 100 1% resistors at a time for maybe \$3 USD. I noticed the same values in the Digi-key listing had 2 sizes but the same 1/4 watt rating (through-hole). One is short like a 1/8th watt, the other looks like a 'standard' 1/4 watt package. I would say better metal film materials allow the manufacture to do this, along with a tougher coating. Resistors with a FP (flame proof) rating tend to be standard size.
– user105652
Commented Oct 2, 2018 at 21:10
• Yes, there is a slight difference in the long manufactures part number between the 2 body sizes. Also some 3 watt resistors are in a 2 watt package size.
– user105652
Commented Oct 2, 2018 at 22:55

They are both Watts. The "WS" has been used by some Chinese manufacturers for the power rating of miniature resistors. It probably started as a mistake made when copying some other manufacturer's datasheet, and the mistake has been copied to new datasheets and to the datasheets of other Chinese manufacturers that shared the same facilities, staff and technical data. Many of the higher-wattage rating miniature and high surge resistor lines end in "S" (e.g. MF/MFS, W/WS). I have seen this type of error propagated in other specifications and datasheets copied among the different names of the Chinese manufacturer.

Mayloon's (Sinloon's) datasheets for normal resistors use "W", while their datasheets for miniature resistors use "WS".

Bourns has a WS series resistor, which is a wire-wound, high-surge resistor. The datasheet has a standard and miniature version, and both use "W" for power.

MFR's metal film resistors datasheet states that the MFS (miniature) resistors have twice the continuous power rating as the standard (MF) resistors, as does KOA's. They use "W" for power rating in their miniature lines.

One rating (W) is average power over some poorly defined integration time, the other (Ws) is energy in a pulse application.

Note that voltage limits are also given and apply even in the pulse service. Basically as long as you stay within the average power, the thing will not overheat and if you have chosen a pulse rated part the energy limit is specified in the Ws figure.

You typically see pulse rated parts used for things like inrush limiting.

• +1. The datasheet's 'WS' reads as 'watt-siemens' which is nonsense. 'Ws' is correct. Capitals matter. Commented Oct 2, 2018 at 19:21
• @Transistor It depends whether "S" stands for some unit, or simply for "small", as mentioned Tom Carpenter in his comment Commented Oct 2, 2018 at 19:30
• something got lost in the translation for "Maximun" too. If Ws =2xW for small parts then the thermal time constant is 0.5s ( depending on air gap or better with thermal conductance) Commented Oct 2, 2018 at 19:32
• Another possibility is S stands for "surge". I know, for example, Vishay sells "surge-rated" resistors. Commented Oct 2, 2018 at 20:14