er.. Another CT question

Hi all, Great site and community, had visited a few months back but felt overwelmed by the amount of into. 


Have now returned for a brave foray into trying the basic setup with an arduino to begin with.

I have searched the site for info about my plan of using a CT from my old owl monitor cm113, and there are several responses with promises of more info when their projects are complete etc, but without follow up.. and some of the other info seems contradictory to my meager understanding.

So.. What i would like to do is to use the CT as stated, i have opened it up and found that i have as others have a 100ohm 1% resistor, i still have the working owl, so i have thought that i could compare the output voltages measured with a multimeter with the readings displayed by the owl to make some assumptions about the calculation needed, but for future upgrades i would prefer to follow the methodology set out in the basic setup page (ct interfacing with arduino).

So far so good, but here is where i hit a wall, i am not sure that i can figure out the values that i need and would really like some pointers to help me figure out the resistor values and capacitor that i would need.

any help would be really appreciated



Robert Wall's picture

Re: er.. Another CT question

You can use the resistor values in "CT sensors - Interfacing with an Arduino" in Building Blocks. If you're running on mains power the two equal resistors can be any value within the range stated, if you're running on battery they should be near the high end to minimise battery drain. Within that range and given those constraints, the actual value is not at all critical. Ignore the 33 Ω burden resistor, you have yours inside your CT. C1 should be about 10 µF.  

Depending on the supply voltage you want to use, you might need to replace (or put another resistor in parallel with) your existing burden resistor so that in the worst case, the peak-peak voltage out of your CT is contained wholly within the supply rails. As I posted elsewhere today, that means with about 5% in hand. So for a 5 V supply, aim for not more than 1.66 V rms out at maximum current, and for a 3.3 V supply, 1.1 V rms out. 

When you think it's working, you might find that one of Robin's test sketches is useful to look at and confirm the waveform that the sketch is seeing.

gambituk's picture

Re: er.. Another CT question

Ok thanks for that, i should be able to get a handle on that! It might seem simplistic, but i haven't done much of this for a very long time. 

I appreciate your time

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