1

So now that I feel comfortable with using ADCs, I am attempting to measure the AC current with the SCT013 clamp with AC current sensor.

Setup: I clamp the SCT013 around the HOT wire of a table lamp I have set up. I plug the sound-jack like end of the SCT013 into the AC current sensor. I then plug the Analog out of the AC sensor into one of the analog ins on the MCP3008, one wire into 5V, and another into the ground. I then use this code and module to read the digital values from the MCP.

Weird Results: While the grounded pins on the MCP hover at 18 (on a 10 bit scale 0-1023), the channel where the current sensor is plugged in hovers at 20 when the table lamp is on, 18 when not. Why is the difference so small? Another weird observation is that when I plug in the lamp, there is a large spike in readings all the way up to 60-80 sometimes, then it drops back to 20.

I know the MCP3008 is working because I have hooked up a photoresistor which is giving accurate readings.

Anyone care the shed any light into this? Thank you!

Improve this question

1 Answer 1

Reset to default
2

According to this datasheet for the STC-013, the output voltage ranges between 0 and 50 mV for 0 to 100A. An increase of 2/1023*5 gives a measured voltage of around 0.009 V, or 9 mV, which would suggest a current of around 0.9 A. The readings you're getting seem reasonable, just not terribly helpful.

In this sort of situation, I believe you can use an op-amp is used to boost the incoming voltage. How to use an op-amp to boost sensor question however is a question more suitable for https://electronics.stackexchange.com/.

Improve this answer
9
  • 1
    Or instead use STC-12345 with a input range of 0~5A. Op-amp is newbie scary. Let me see if I can find more newbie friendly solutions.
    – tlfong01
    Sep 27, 2019 at 0:44
  • I searched AliExpress and found ac current detector with range 0 ~ 5A. If you do not insist on a "clamp" type, there are detectors of even smaller ranges.
    – tlfong01
    Sep 27, 2019 at 1:18
  • Okay ! This is very useful. My end goal is to use it to measure current at my waterheater. So I think the clamp I have now will work okay. I am going to next make sense of the electrical conversions and equations above. (I am not knowledgable in these matters). But why does the reading 'spike' up to 60-80 during the instant I turn on the lamp? Thanks!
    – Tuomas Talvitie
    Sep 27, 2019 at 2:18
  • 2
    @TuomasTalvitie A spike in current is expected when turning the lamp on, this is the inrush current. When first turned on must devices take a bit more power to charge capacitors, heat up coils, etc.. This can be 1.5-10x more current than the rated steady state current. How much more depends exactly on what type of bulb for a lamp. If your reading spiked to, e.g., 70, that would be an increase over the base reading of 18 of 52, and then from 52/1023*100=5.1 we can see this reading suggests an inrush current of 5 A, which falls in the expected range. This should only last a few milliseconds.
    – Fred
    Sep 27, 2019 at 9:10
  • 2
    A bit more about inrush current for LED bulbs: thelia.org.uk/sites/default/files/resources/… and a bit more about inrush current in general: blog.1000bulbs.com/home/what-is-inrush-current.
    – Fred
    Sep 27, 2019 at 9:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.