Using CT4 on emonTX for loads greater than 19A


I’d like to take advantage of the extra CT4 input on the emonTx. The load I want to measure is likely to always be less than 19A but I can’t 100% guarantee that will always be the case, as it’s currently protected with a 63A MCB.

I have a couple of questions:

  • If I connect up a YHDC SCT013 to the input and the current exceeds 19A on the primary, is there any risk of damage to the emonTx, or will I just get a maximum reading?

  • I found a very useful post CT4 as 100A which discusses adding a 27 Ohm resistor in parallel to change the input to 100A. I’d prefer not to alter the emonTX board, so could I add the resistor in parallel, external to the emonTx i.e. use an additional connector and wire it across the SCT013 output?

Many thanks for your help,


Welcome to the OEM forum, Tim.

Damage is not likely, but it is possible. There is some protection on the input, but I wouldn’t like to guarantee that it will be adequate under all conceivable conditions.

Personally, it’s probably worth the risk, because we’ve never heard of an input being damaged in that way. That doesn’t mean to say it hasn’t happened though.

Yes, you could do that. But I think it would be more reliable and a lot easier to add the resistor to the p.c.b - there are holes ready and waiting for it. If you do add the resistor externally, it should be connected between tip and sleeve.

Hi Robert,

Many thanks for the advice, that’s much appreciated. Looking forward to getting it all going.



1 Like