Resistor tolerance

Homebrew IoTaWatt
1

I build my 3 IotaWatt with standard 5% tolerance through hole resistors. (i think 5% had them lying around in a big bag :wink: - could als be 1%)

I was wondering if changing them to 0.1% would improve it (not that i’m complaining :wink: when i made them i measured between 100 and 104W for a 102W lightbulb )

Since i am using SCT013-030 the resistors over the CT aren’t there.
So there are 4 locations left

  • the led: quite sure the tolerance doesn’t matter here
  • the voltage devider on the VT: i guess that it isn’t very important to have low tolerance here
  • the vref shunt: unsure, maybe this is important?
  • the LM358: unsure, maybe this is important?

The reason i’m asking is to know if the time and money (ok, 0.6euro per resistor isn’t that expensive) is worth it?

2

Correct, lower it if you want a brighter LED and visa-versa.

Right. As long as you calibrate the VT, it doesn’t matter. In the production units however, we do use 0.5% resistors and the result is that the standard issue VTs are usually within 1% without calibration.

Nope, but the tolerance of the shunt itself is critical. The production units use a 0.2% shunt. This is the whole enchilada. Everything is calibrated to that standard.

I assume you are talking about the 4.7K resistors in the voltage divider that feeds the LM358. That’s not critical. The bias is compensated dynamically, but I’d recommend at least 1% for that.

I used to do thru-hole, but found that SMT is actually a whole lot easier and better once you get a few basic tools. But this is particularly better with SMT. Resistors are pretty much 1% by default and .5% don’t really cost much more.

You’re right that the burden resistors need to be .5% or better to get consistent results.

3

Thanks for the clarification! My shunts are 1V and 0.35%, so a bit worse than yours, but still ok i guess :wink:
offtopic: i was thinking of trying some smd stuff on the next holyday :wink: