Archived Forum

The data sheet says "Output 0.333 Volt AC at rated current". Because the same data sheet states for the -000 version only "No burden resistor" and the graph shows voltage vs current, I think you can safely assume that the burden resistor is incorporated within the unit and an external burden is neither required nor desirable.

Unfortunately, this means that the output is not ideally matched to the Arduino input as it is used in the emonTx - you will need to provide a lower reference for the analogue to digital converter of about 1 V (rather than 3.3 V) in order to maximise the resolution of your measurements. ATmel's application note AVR465 describes a prototype meter that uses a 1.1 V reference, which should be OK for that current transformer.

Thanks, Robert.  You are right, I also contacted magnelabs, here is their reply:-

"The SCT-1250-xxx is built to the ratio of 7,332 : 1.  The Model SCT-1250-400 is burdened internally so that the SCT outputs 0.333Vac at 400A of primary current.  Any of our SCTs which have a current rating are burdened for the current.  We also have a non-burdened version of the SCT-1250 which is the SCT-1250-000.  In this model we install back-to-back zener diodes for voltage protection."

Thanks to for the AVR reference too, I'm reading through the application note now.  I'm hoping too, to implement a voltage meter; certainly it all seems possible.

"I'm hoping too, to implement a voltage meter; certainly it all seems possible."

Don't re-invent the wheel! The emonTx has a voltage input - look at the circuit diagram for that (http://solderpad.com/openenergymon/) and under "Building Blocks" you'll find plenty of information about the ac-ac adapters that we use to reduce the mains voltage to a safe level.

A word of warning regarding your 400 A c.t. Because the ADC is only 10-bit resolution, you cannot expect accurate measurements of current below about 1%, i.e. below 4 A. While some users report good results at very low powers, others do not. We think we understand the reasons for this - at least in part - and the investigation is ongoing.

Thanks again, Robert, both for the advise and warning.  I can manage with a 4A inaccuracy, although arguably I should have got the 250A variant; a design consideration for the future.

Clearly the low voltage output of my choice of CT clamp is producing added electronic complexity, and it's not something I can yet get my head around.

Can anyone suggest a CT clamp, to measure a maximum 250A, that will output a voltage more suited to the Arduino?  and thus, I can then follow this handy guide:-

http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino...

Many thanks.