Multi input CT board

I want to build an arduino board to measure current only with 3 or 4 CT’s on seperate circuits. Does each CT need it own Aref/2 resistors (10K) and a 10 uF capacitor or is one DC bias circuit OK to feed all the CT’s. Each CT will have its own analogue input and indivdual burden resistors.

If you’re using the emonTx 3 trick of grounding the ADC input to indicate the absence of a plug, then of course you can’t share the bias supply.

If you do share the bias supply, we recommend that you provide a much lower impedance source, e.g. with an op.amp buffer providing the bias (see ‘Learn’), otherwise you are likely to get crosstalk between the various inputs.

Just explain how you’d do it any other way :exploding_head:

@bobcroft This might be useful …

The OEM shop sells an ‘emonTx Arduino Shield SMT’. This is a shield for an Arduino Uno which has four current CT inputs and one AC voltage reference input (for real power measurements in conjunction with the CT inputs). It also can be fitted with a RFM69 wireless module to transmit date back to an emonBase or emonPi. The shield is available as a complete kit or bare board.

I think you might have to customise the various pieces of available OEM firmware to suit your needs - for example if you add an RFM69 module you will need to choose between Jeelabs transmission format (for existing hardware compatibility e.g. emonTx3/old emonPi/emonTH) or standard format (RF encryption e.g. emonTx4/new emonPi).

For completeness, I believe firmware is becoming available to update existing hardware to standard format.

Thank you for sharing that information. I couldn’t use it for my present project without changing the burden resistors because I am using HMCT103C CT’s. I will keep it in mind for project where I plan to use the 100A CT that comes with the stereo jack. It does look to be a good value for money board.

I was not aware of the emonTx 3 ‘trick’ nor do I intend to use it. I have also used the Robin Emley op-amp circuit on a single CT design to compare a basic resistor divider versus the op-amp version.
From your reply I take it that it is better to use a bias circuit for each CT, that is all I wanted to know and there is no reason why I can’t do that.
Regarding your last comment I didn’t imply I could do it any other way. I sometimes find it hard to know what information to put into a post so as to make my question clear, so if I hadn’t mentioned I would use individual burden resistors, would you have mentioned that I should? I have seen a recent post where you suggested an option where two CT’s could share a burden resistor, albiet on the same analog input. So, I thought I would clarify the fact I was using a conventional burden resistor set up. Nothing more.

What I wondered was, were you intending to use something like analogue switches or even reed relays to select the c.t., rather than relying on the inbuilt multiplexer. And maybe share a burden - which wouldn’t be good for the unburdened c.t. And I couldn’t see the point of that.

And the reason I mentioned not sharing the bias supply was some considerable time ago, someone posted that they’d copied exactly the emonTx V3 input arrangement, but also used the op.amp bias, not realising the effect that the switch inside the socket grounded the input and the effect it would have (provide a zero counts input from the ADC, but almost nothing elsewhere with the individual bias, but as they found, catastrophic when they grounded the common op.amp. output). I wanted you to avoid that error.

You can remove the SMT burden on the emonTx Shield and replace it with your own choice of burden - holes are provided for a wire-ended resistor.

Robert, thank you, yet again invaluluable imformation. Actually until rupert mentioned the emon Tx shield I hadn’t looked at the schematic diagram. if I had, I would have known to use seperate bias resistors.
It is good to know the shield can be re-configured with wire ended resistors as I have load of those but no SMT parts yet!

That applies to the non-shield version of the EmonTx v3, as well.