FAQ

This Wiki is a working living document and will evolve as questions are asked, in order to hopefully help others.

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Which way should the current clamp be clipped on the Live tail?

The Arrow on the CT Clamp face typically should be the direction of current flow. So the arrow facing the load or consumer unit.

However, sometimes the wiring from the clamp maybe connected the wrong way to the plug, so simply reversing the clamp should fix it. If you have just wires from the CT Clamp, you could simply swap the clamp wires over to do the same thing.

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The Active Power displays Negative?

Example: Active power (Pmean 0x4A): -185.00 W

This could be caused when the secondary AC output from the transformer to the GTEM, is the wrong way round. It is because the AC sinewave from the transformer is 180° out, so also out of phase with the CT Clamp. Simply swap the AC input and the problem should be resolved.

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I already have a 9V AC supply -- can I use that?

The 9V AC supply will be fine. Off load it is probably be around 12-13V RMS, which is OK.

The GTEM board will work as low as 4V RMS but that’s an extreme – not the ideal!.

You will need to alter with the UGain value in the formula found in the Excel sheet. The test code in GitHub will at least work in order to prove the concept, even if it’s the wrong measurements that are displayed.

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Can I safely leave the USB cable connected to the GTEM, whilst powering the board from the AC transformer?

Absolutely not a problem. You can have both safely connected. This is also good for debugging and tweaking the register values.

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My CT Clamp is a different ratio than the YHDC SCT-013-000, will it still work?

The clamp will still work, although depending on the clamp voltage output for a given current, you may need to adjust the formulas, gain, Igain.

As the output voltage derived from the CT Clamp is proportional to the Burden resistor value (on GTEM it is 12 Ohms ±1%), based on the amount of current flowing through it, you may need to check to see if the clamp has a built in Burden resistor - using an ohm (resistance), meter.

Typically, the YHDC SCT-013-000 is around 100 ohms @ 1:2000 turns ratio, open circuit. It can produce 50mA for 100A current flow = approximately into 12R Burden = 600mV (which is the maximum for the ATM90E26 VP Input). This means the Burden resistance would ideally be around 10 to 12 Ohms maximum.

An online calculator which may assist can be found: Burden Calculator

If your CT is in free air, has a lower resistance open circuit, say 10-20 ohms across the terminals, then it is possible it has a built in Burden resistor.

You may then need to open (disable), the Burden solder link on the GTEM board bottom, or you may overload the current sensor as the CT Burden resistor would effectively be in parallel with the GTEM Burden, so not getting the desired output and lead to saturation.

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The board has an I2N and I2P input. Do I connect anything to this?

In the short term, no, leave it unconnected. It can be used for example, current sampling, but you are using the I1N and I1P inputs already for this, via the CT Clamp.

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Why is the CT Clamp on the I1N and I1P inputs?

The reason for using this input, is that it has a PGA (Programmable Gate Array), which allows the input gain to be tweaked between 1 and 24, before it goes into the ADC.

For information, the I1 differential inputs have a potential range of 5uV to 600mV, although this stepped in ranges 1, 4, 8, 16 and 24, via the MMode register 2BH.

So 5uV to 25mV RMS when Gain is 24, and the extreme, 120uV to 600mV RMS when Gain is set to 1. The gain is ideally left at 1.

More details on the MMode register 2BH can be seen on the Datasheet page 31.

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Can I use a Current Shunt, rather than a Current Clamp?

You certainly could in theory add extra circuitry BUT this would REMOVE Mains Isolation to the GTEM board - which is NOT safe!

Would recommend ONLY using the Current Clamp so you have a physical safety barrier to High Voltage Mains.

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Have updated values in the Set Point Calculator, but the readings are out. Do I have to update the checksums, every time I change a value?

Yes, you must update the checksums, or you will get erroneous readings.

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Would like to use the board on ESPHome, do you have an example code for this?

Unfortunately ESPHome is not currently used here in my lab, only Domoticz - so have not tested but certainly no reason for it not to work.

An ATM90E32 Power Sensor version is provided on the below ATM90E32 link - with additional examples for single phase. This may work and certainly worth a try - feedback welcomed!.

https://esphome.io/components/sensor/atm90e32.html