How many (m)W should 120 ohm CAN bus terminators have?

When I started with Arduino I bought a set of 600 resistors (30 times 20 values). However, I found out 120 ohm needed for CAN bus is not among them.

So I need to buy 120 ohm resistors (I don't want to combine multiple). However, I was wondering how many watts they should handle.

I want to connect two or later maybe three devices, all running on USB power (5V). But I cannot find how many (m)A CAN is using.

My calculations:

V = I * R <=> 5 = I * 120 <=> I = 5 / 120 = 41,7 mA

P = I * V = 0,041666 * 5 = 0.208W

or:

P = I^2 * R = 0.041666 0.208W

So I would be safe with default (0.25W) 120 ohm resistors?

• Your assumption that there is going to be 5 V across these resistors is incorrect I believe. According to the CAN Bus page in Wikipedia the voltages are half of that so 2.5 V maximum. So then you'd be well within limits to use 0.25 W resistors assuming you'd be terminating to the 2.5 V which is the recessive ("one") voltage of the datalines. Aug 22 '17 at 9:51
• this might help: e2e.ti.com/support/interface/industrial_interface/f/142/t/… Aug 22 '17 at 9:53
• Who says you need to generate 12 V ? Looking at this graph en.wikipedia.org/wiki/CAN_bus#/media/File:ISO11898-2.svg you can generate it from 5 V. That +/- 12 V of common has more to do with absolute maxima, you must be able to handle that (use input voltage protection) but no need to generate that. I think that is to make the system immune to when a broken circuit puts 12 V on the line or the car battery is connected in reverse. Aug 22 '17 at 9:57
• OK, you're not going to use it in a car and no one is going to check if you meet all the CAN Bus requirements then for sure you do not need input protection. Aug 22 '17 at 10:06
• The resitors will have 5V across them when the bus is in the dominant state and 0 V when it is in the recessive state. Since the bus will when active spend around 50% of the time in each state it's reasonable to use a rating that is assuming half way in between the two. which gives a power of V^2/R = 50 mW. Aug 22 '17 at 10:14 