# Are there downsides to using resistors with too high power ratings? [duplicate]

I'm hoping to measure the current draw to my ESP8266 with a scope and a shunt resistor. I think a 1ohm resistor might drop the voltage too much, so I'm thinking to use a 0.5ohm (or even a 0.1ohm) instead.

However, most 0.5ohm (and 0.1ohm) resistors tend to have at least 1W, 2W power ratings.

Are there issues with using resistors that have power ratings much higher than required?

The only usual downsides that come to mind are size, cost and sometimes resistors with a higher power rating use technologies such as wire wound that can have higher tolerance rating than some others. Other factors such as inductance that may be an issue with RF circuits won't be an issue for a current shunt.

So in general if you're happy with the size, cost and tolerance of the resistors you've found then there's no reason not to use a resistor with a higher power rating than you require.

Are there issues with using resistors that have power ratings much higher than required?

Bigger sized resistors may have higher self-inductance and this self-inductance in a component that is used to measure fast currents can lead to errors.

V across an inductor = L$\dfrac{di}{dt}$ and this is the problem.

Typically, a 3cm long piece of wire of 0.4mm diameter will have an inductance of 30nH (irrespective of its resistance) and if the current changes at 1 amp per microsecond (quite normal in a buck or boost converter), the volt drop due to di/dt will be 30mV. If the resistor is 3cm long and 0.03 Ohms, there will be a total volt drop of 60 mV for a sudden change of 1 amp flowing i.e. 50% bigger than what would be expected from a pure resistor.

Bigger sized resistors also have more self-capacitance (and capacitance to ground) and means another error when measuring high-speed signals. Here is a table from Vishay that shows tha parasitic inductance and capacitance of small SMT resistors - note the final columns: -

You can see that capacitance (parallel) and inductance (series) reduce with size and if you are operating above 1GHz resistor sizes become a big deal.

Also, as frequencies of RF transmissions rise there comes a point when the physical length of a resistor can cause standing wave problems. At 300MHz, RF has a wavelength of 1m and it is fairly unfeasible for anything significantly less than one-tenth of a meter will cause problems but what about wifi frequencies at 2.45 GHz - this has a wavelength of 12cm and conceivably, at this sort of frequency resistors that are about 1cm long will start to show problems.

WiFi is now occupying the 5 GHz band and some are starting to emerge at 60 GHz. 60 GHz has a wavelength of 5mm so realistically only 0201 resistors are not going to cause a significant problem.

• Although an interesting answer, it's not really relevant to the OP's question since he was looking for a shunt resistor in the sub-1Ω range and said nothing about high-frequencies or SMT devices. – tcrosley May 23 '15 at 19:26
• @tcrosley I believe you should rethink your comment. The title was "Are there downsides to using resistors with too high power ratings?" and the only question in the text was included in my answer. As the ESP8266 is a wifi module operating at 2.45 GHz I believe my answer to be totally suitable for him to make full conclusions AND general enough to cover other circuits. Are you suggesting maybe that I gave too detailed an answer? – Andy aka May 23 '15 at 20:33
• No, note I did say your answer was interesting. But the WiFi reference is not relevant, since he is only interested in measuring the current draw to the device. Note another answeer says "Other factors such as inductance that may be an issue with RF circuits won't be an issue for a current shunt". He got my upvote. – tcrosley May 23 '15 at 20:36
• @tcrosley I'm no expert on the ESP8266 but I would expect some part of the current consumption to have spectra at 2.45GHz - can you categorically say that a big power resistor isn't going to cause problems with the PA in the transmitter. The only questions raised were answered. – Andy aka May 23 '15 at 20:41
• ok, I give up. We're talking about two different things. – tcrosley May 23 '15 at 23:20

Usually larger resistors have higher ESL values, these are not listed for most resistors, but if your switching or using them for a DC to DC converter this could make a difference.

Larger resistors could have a different temperature coefficent.

Just a thought outside of the scope of the power question, are you sure that you mean to say a 1 ohm resistor? A one ohm resistor is an odd value to begin with, considering that below 1 ohms is essentially a conductor not a resistor. A 1 ohm resistor does not exactly limit current considering Ohm's Law: Voltage = Current*Resistance. So 1V/1ohm = 1 Amp which is a decent amount of current! Decent, meaning more than a typical circuit board would require or draw.

If you're measuring current it will most likely be in the mV range, and a kilo-ohm resistor would be better so that the voltage read out is on the order of ones. e.g. 5mA*1kilo-ohm = 5Volts

the power through this resistor would be 25mW. And yea you wouldn't need the higher power rating. Sorry for not answering the question, but I thought this was useful and relevant information, for someone considering taking current measurements.