Achieving good precision with currents of less than 5 A

 I'm considering building an energy monitor based on the Open Energy Monitor design for the purposes of determining power dissipation of some computers in a cabinet. The power information would then be used in a feed-forward control system to predict how much cooling effort is required - as opposed to a feed-back control system where the system waits for it to get too hot before applying more cooling effort.

For my requirements I think I need good resolution up to 4 A (about 1000 W) so I was considering using a more suitable CT than the usually recommended 30 A CTs.  At the moment I'm looking at an AC1005 5A current transformer <>.  Do you think this can be made to work with the Open Energy Monitor design given appropriate adjustments to the value of the burden resistor?  I realise I need to break into the mains circuit to do that since this isn't a clip-on CT, but I'm perfectly comfortable with mains voltage work.

Alternatively, does anybody have experience with using a current transformer such as the SCT-013-030 which is good for up to 30 A, but  with multiple turns on the primary in order to increase sensitivity. By looping the primary through six times can I use the SCT-013-030 as a 0 to 5 A transformer rather than a 0 to 30 A transformer?




glyn.hudson's picture

Re: Achieving good precision with currents of less than 5 A

 Hi Rob,

Sounds like an interesting project. You are already thinking along the right lines, you will get better precision using a CT designed for lower power levels. 

Have you seen this page: ? The calculations at the bottom of the page runs through the process of calculating a value for the burden resistor depending on how many turns the CT has an over what range you wish to measure. 

The AC1005 CT should work fine. The SCT-013-030 CT has actually got a built in burden resistor and gives a voltage output. See datasheet here:

Hope this is helpful, let us know how you get on

All the best, Glyn. 

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