I have recently migrated my energy monitoring from an Arduino board with the smartenergygroups v1 shield to a Sparkfun Photon Redboard (with the Particle Photon) and using an emonTX shield from the shop.

My current clamps are the 30A voltage style clamps, and I have read that I don’t need the burden resistor on the emonTX.

As the emonTX came with the SMT resistors already mounted, rather than remove them, can the maths be reworked to work out an appropriate current calibration coefficient?

I assume that the onboard 33R resistor is in parallel with the onboard 62R burden in the clamp, so doesn’t that make an effective resistance of around 22R?

Using the table, should the coefficient be 90.9 for 22R?

At the moment I have erroneously used a coefficient of 111.1. I have been running this for about 10 days now just using the current only option and have just compared it to my electricity company’s data. Unfortunately I can only get the kWh values from them, so I can’t easily verify the instantaneous wattage from that, but the derived kWh I am measuring seems to be about 40% too high.

So, should I remove the shop fitted burden resistors from the emonTX shield, or change the calibration coefficient as I have outlined?

I eventually want to add the AC power supply, so I’ll need to revisit the calibration aspects when I get to do that.


That’s correct.

Yes, the downside is you end up with, as you say, an equivalent parallel resistance that results in a much lower voltage appearing at the ADC input, so doing as you suggest actually concerts your 30 A c.t. into a 100 A c.t. so using it only up to 30 A means that you lose three quarters of the available resolution, effectively raising the lowest current that you can measure accurately by a factor of 4. If you do the sums, the burden value you need for 30 A with an emonTx Shield is a little over 100 Ω.
(You get 15 mA out with no burden @ 30 A, the Shield wants 1.6 V maximum.)

For best accuracy, what you really want to do is remove both burden resistors, and put a 100 Ω either inside the c.t. or on the Shield p.c.b. The full explanation for calibration is in ‘Learn’.

N.B. When you write “clamp”, I invariably think of busbar clamps, never current transformers.

Thanks for the quick reply Robert.

It might be difficult I suspect to remove the burden from the c.t., so I’ll remove the additional burden resistor from the emonTX shield, and reset the calibration co-efficient to 30 as one of the old forum posts suggests and see how that goes. This should also improve the low wattage performance of the solar feed in particular as that is usually showing about 50W over night.

No, it’s very easy. Open the c.t, and you’ll see two blue plastic clips holding the bobbin in place. Push the cable into the body, and at the same time ease the clips back. (That needs 4 hands, or a clamp to hold the body, long-nosed pliers - one jaw to push each clip back - and two hands.) The bottom core, bobbin and a small p.c.b. should all slide out in one piece. Just make certain you don’t snap the secondary winding wire when it’s out. There is said to be a TVS diode in there as well as the resistor and in parallel with it, but there’s no need while the burden resistor is present. If there’s no TVS diode, I suggest for safety soldering your new burden resistor in there, not in the emonTx Shield.

Mr. Wall’s observations about accessing the burden are accurate. I just did a burdenectomy on ten various sct013-030 and set-050 that I had laying around. I had acquired them from various sources. Couple of things:

All of the units I reworked did indeed have TVS diodes in addition to the burden resistors.

More importantly, they are not just sct013-000 with burden resistors. The turns ratio is 1860 rather than 2000 as in the sct013-000. If you do the math from their specs: 62 ohm burden, 30A = 1V, then 30 x 62 = 1860. I tested all of them and that was the ratio. Not a big deal, but the lower ratio actually means they might be slightly better suited to the lower current range they were originally burdened for.