I’m sorry if this questions has been answered or is in the docs. I’ve been reading a lot of the docs, and don’t remember seeing this. On the emonPi (and probably others like the emonTx, etc) there are two burden resistors that are both 22ohm, but everything else just references having one 22ohm or 33ohm. The sketch seems to show two, but then the PCB design only shows one? Am I just confused? Also, is that a 1/2W, 1/4W, 1/10W? I don’t seem to see reference to that anywhere. I’m assuming they are all SMD resistors, but I don’t have an actual emonPi, I’m building my own physical device. Second question, that 1k ohm resister on the way to the analog input, what is that for? Again, not referenced in the do it your self section of the web guide, but definitely in the schema for the product. Thanks in advance for any thoughts on these two questions.
No, there is provision for two burden resistors, only one is fitted and that is the SMT one. The other is to provide holes for a wire-ended component for when you require a different value of burden (as those of you in the USA are likely to need).
You can calculate the power rating of your burden resistor easily enough because you know the maximum rms voltage (~1.1 V for a 3.3 V ADC input range, ~1.6 V for a 5V range) and the value of the burden.
It’s an attempt to limit the current flowing in the protection diodes (in the ADC multiplexer front end) in the event that the input is seriously overloaded.
On other thing, I think there is an error on this page:
The notes read:
Using 22R burden and YHDC SCT-013-00 with 22 turns
But I think that should read 2000 turns for an SCT-013, not 22.
Well spotted, I’ll get it changed. It should read SCT-013-000 (there isn’t a -00) and the secondary turns is irrelevant - it’s the current ratio that specifies a current transformer. In fact, the manufacturer may well tweak the turns ratio by a small number to best meet the linearity specification.
Thanks for the response. Yes, I calculated mine out to be 33ohm for the 100A and 100ohm for the 30A. So, that’s what I’m running, I just wanted to make sure I wasn’t missing something. I’m still tweaking and learning, but I have the whole system up and running now. I have two solar systems and my mains monitored, and once I get the rest built out I’ll have monitoring on around 12 more circuits like our electric car, mini-splits, dryer, etc. Fantastic project here, I really appreciate it. I did a few modifications to target my use case. Once I’m done, I hope to write up my notes and contribute them back to this project (mostly for other folks in the USA). I see a number of other folks have done their own projects based on this work, and then published their work, so I figured I would do the same if it helps anyway. Good stuff here.