# Which wattage rating to get when buying resistors

Hello forum,

I'm using a 3.3V pro-mini to read pressure values coming from a 4-20mA transducer. I'm using the internal 1.1V reference after pushing the mA through a 47 ohm resistor, 47 because it's what I had available.

Now I'm looking at getting some 55 ohm resistors which will allow the 20mA to be converted much closer to 1.1V, which my inexperienced mind tells me I'd be getting the full range of 1024 steps out of the 1.1V, as opposed to the roughly 875 steps I can measure when using a 47 Ohm resistor, which will only give around 940mV at 20mA.

1. Am I correct in saying this? Would a 55ohm be better and safe to use?

2. If no, what value would be the closest to this that I should use?

3. If yes, what wattage would be best to get? On Mouser I see 1, 2 and 5W.

Thanks,
Hein

The power dissipation of a resistor = I2R = (.02)2*55 = 0.022 Watts.

Don’t forget to take into account the tolerance of both the resistor and the 1.1V reference. The latter is around 10% for the ATmega328, so the actual value of the voltage reference could be as low as 1.0V. In that case, the 47 Ohm resistor is the best choice.

Why don't you start by calculating the power the resistor has to dissipate?

Or just don't worry and get a bog standard 1/4W resistor... as that'll do perfectly fine for the 5-22 mW it has to dissipate.

jremington:
The power dissipation of a resistor = I2R = (.02)2*55 = 0.022 Watts.

Don’t forget to take into account the tolerance of both the resistor and the 1.1V reference. The latter is around 10% for the ATmega328, so the actual value of the voltage reference could be as low as 1.0V. In that case, the 47 Ohm resistor is the best choice.

Thank you very much @jremington.

heinburgh:
Now I'm looking at getting some 55 ohm resistors which will allow the 20mA to be converted much closer to 1.1V, which my inexperienced mind tells me I'd be getting the full range of 1024 steps out of the 1.1V, as opposed to the roughly 875 steps I can measure when using a 47 Ohm resistor, which will only give around 940mV at 20mA.

51 ohm is a standard 1% E24 metalfilm value, and comes the closest to reading 4-20mA.

55ohm might be too high for some ~1.1volt Aref.

0.25 watt (sometimes called 0.4watt) is the default through-hole size, and perfectly ok for 20mA.

A couple of notes & comments -

The power rating is the “absolute maximum” so standard practice is to de-rate the resistor. i.e. If you are dissipating 1/4W, use a 1/2W resistor, etc.

3 Ways to calculate power:

Power (Watts) = Current x Voltage (This is the basic power formula. The others can be derived using Ohm’s Law)
Power = V2/R (This is the one I use most often because we usually know voltage & resistance)
Power = Current2 x R

“Typically” with microprocessors we are dealing with low voltages and resistors in the K-Ohms (or higher) and we don’t have to think about it!

Thanks for all the great advice!

Wawa:
0.25 watt (sometimes called 0.4watt)

Only by those who know no better Perhaps you mean “1/4” which is totally different from “0.4”

I mean that some (carbon?) resistors are rated 0.25watt (1/4watt) while others of the same size (metalfilm?) could be rated 400mW (0.4watt). Through-hole resistor rating is a grey area.
Leo..

Rating is rating and nothing to do with size. If the manufacturer rates is at 0.25W then that’s what it is.
Please don’t confuse the issue with personal opinions as they don’t really help those who are seeking assistance.

Catfight! jackrae:
Rating is rating and nothing to do with size. If the manufacturer rates is at 0.25W then that's what it is.

I don't understand this comment. Nobody ever even suggested it would be different.