Using a larger current sensor (30A or 50A) on a 15A circuit - will it be accurate?

Thread Starter

RogueRose

Joined Oct 10, 2014
374
I'm working on an energy monitor and I'm getting the current sensors and I'd rather have some sensors that are larger than needed, say a 30A on a 15A or 20A circuit, if it will work close enough.

What I'm wondering is if the sensors read X% of the current, so if I'm pulling 75% on a 20A circuit (so 15A), will the 20A read 15A and a 30 read 22.5 (b/c 75% of 30A is 22.5A)? I'm guessing it won't do this, but want to be certain. I'm suspecting that the reading would just be less accurate like instead of reading 15.15 amps, it might read 15.2, or 5.56 vs 5.6 - because a larger scale is being used to measure a smaller amount of current.

Is this correct? Will I be alright with the larger sensors for the extra's that I want to ensure will cover the largest circuit but also be able to use them on smaller ones?
 

Wolframore

Joined Jan 21, 2019
1,314
In very general terms as your question has no details... yes.

What is sensor... exact parts
What is doing the measurement? Analog digital?
What are you measuring.?

So many questions.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
374
In very general terms as your question has no details... yes.

What is sensor... exact parts
What is doing the measurement? Analog digital?
What are you measuring.?

So many questions.
Sorry, I didn't think it would matter much if it was sensing current. If it matters as to what is reading it, it's a one-off arduino proto shield board connected to an arduino.
As far as what I'm measuring - current - mainly the how much, for how long, etc. like a kill-a-watt meter for kwH's.

https://www.ebay.com/itm/Wholesale-SCT-013-030-060-Non-Invasive-Split-Core-AC-Current-Sensor-Transformer/113678098796?hash=item1a77be216c:m:mbpE6ACKHEm3__9rLhejsoQ

I'm also trying to figure out what wattage these resistors are and I can't find any reference anyhwere - all i have is a picture for comparison in size. I'm guessing they are probably 1W or maybe even 3W.




From looking at the size of the 3.5mm jack, I'm guessing they are 1W or less. Is there any reason that using a larger resistor than necessary would hurt?
 
Last edited:

crutschow

Joined Mar 14, 2008
23,487
The error in reading the current is usually given as a percent of full-scale, so for the same percent full-scale error, it's apparent that a 50A sensor would have a larger measurement error at 10A as compared to a 20A sensor.
Is there any reason that using a larger resistor than necessary would hurt?
Larger in power rating would be fine.
Obviously the resistance value must be the same.
 

Wolframore

Joined Jan 21, 2019
1,314
Thanks RR, just makes it clearer for us... yes, in this case your eating up some of your 10 bit ADC resolution as well. I would rather have the system rated higher than your max reading. It is possible to maybe use a dual rate and auto-range if you're pegged on one. There's a lot you can do in the programming side. I hope this makes sense.
 

MisterBill2

Joined Jan 23, 2018
3,983
If the sensor, in this case an ammeter shunt, is accurate it will be accurate at all the points in it's range. The meter may or not be accurate, and if you are using an arduino I have no clue about the accuracy or error sources in the system. The big limitation would be electrical noise, because as the voltage from the shunt is smaller the noise is not any smaller.
 
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