ESPHome Power Monitor

Device meant to monitor power usage of a house or circuit. Supports both blinking LED (e.g. 800 blinks per kWh consumed) with a detector, or interfacing with a Current Transformer (CT) clamp.

Fits an ESP32Devkit1 board. Please carefully compare the markings on the board with the ESP32 board.

See [Releases] for fabrication files. I generally order from jlcpcb.com, so that's what the files are tailored for.

Hardware to bring yourself

2x Single row, 15 pin 2.54mm / 0.1" pitch PCB header sockets
JST PH angled connectors, 2mm pitch:
- 2 pin: For CT Clamp
- 3 or 4 pin: For pulse detector
- Cable assemblies to connect to these JST PH connectors
Any other 2.54mm / 0.1" pitch connector up to 7 pins for other side of the board, if required
Through-hole Burden resistor (see further down) Pitch: 7.62mm / 0.3", diameter: 2.5mm / ~0.1"
For pulse meters: a pulse detector providing 3.3V logic level pulses

Solderable jumpers

Some soldering is required to start using the board.

PWR IN

The PWR IN jumper connectos the "Vin" connector pin either to the "Vin" pin of the ESP32 board, or to the 3.3V pin of same.

If you are supplying power via the connector (Vin and GND), select 3.3V or 5V depending on your power source.

If you are powering the ESP32 board directly via a USB cable, then you might use Vin the other way around. In that case the jumper selection can be used to decide whether to supply 3.3V or 5V to that pin. In my case, I'm using this connector to feed a pulse detector 3.3V.

A0 SRC

The board features a 4-channel analog to digital converter, the ADS1115IDGS. Its first input A0 can be wired to the CT Clamp circuit, or to a connector on the other end of the board, AI0.

A1-A3

The remaining ADC channels are also made available. 3 Jumpers are provided to make the connections to the ADC chip by the bottom of the board. If they are left unconnected, these pads can be used for other purposes. Note that all of these inputs should be kept within a 0-3.3V range.

Pulsing meters

For using with a meter that blinks and LED or otherwise provides a pulse with every x amount of energy consumption, wire the pulse to "Blink" on the connector. It expects a 3.3V logic pulse, going directly to pin D27 on the ESP32.

CT Clamps

Current Transformer clamps clip around a conductor carrying an alternating current and induce a smaller current themselves as a result.

See this introduction to using CT Sensors

The important part is that you need to provide your own resistor to connect across the CT Clamp. Through ohm's law, the current through that resistor, multiplied by the resistance value results in a measurable voltage. This resistor is called the burden resistor and I'll talk more about it shortly.

ADC bias calibration

Trimming potentiometer is used to set 50% midpoint at 1.65 V, to centre the CT Clamp signal around. Without a CT-Clamp connected, but with the board powered up, use a multimeter to measure the DC voltage from GND to the CT clamp pin furthest from the board edge. Tweak RV1 until this voltage is 1.65 V.

Preselected burden resistor values

Use this table to select a resistor to use as burden resistor for your CT Clamp.

Info you'll need:

$I_{live_{RMS}}$

The maximum current you expect to go through the conductor you put the clamp around
$n$

Number of windings of the current transformer in the CT Clamp. For example: 100 A : 50mA -> 2000

In the table, $R$ is the approximate ideal resistance value to get maximum range out of it, and E12/E24/E48 give a resistor value available in the E-series of preferred numbers - that is, a resistor you can actually buy.

$I_{live_{RMS}}$	$n$	$R$	E12	E24	E48
200 A	2000	11.67 Ω	10 Ω	11 Ω	11.5 Ω
100 A	2000	23.33 Ω	22 Ω	22 Ω	22.6 Ω
60 A	2000	38.89 Ω	33 Ω	36 Ω	38.3 Ω
50 A	2000	46.67 Ω	39 Ω	43 Ω	46.4 Ω
32 A	2000	72.92 Ω	68 Ω	68 Ω	71.5 Ω
16 A	2000	145.84 Ω	120 Ω	130 Ω	140 Ω
10 A	2000	233.35 Ω	220 Ω	220 Ω	226 Ω
5 A	2000	466.69 Ω	390 Ω	430 Ω	464 Ω
2 A	2000	1,166.73 Ω	1 kΩ	1.1 kΩ	1.15 kΩ
1 A	2000	2,333.45 Ω	2.2 kΩ	2.2 kΩ	2.26 kΩ
200 A	1000	5.83 Ω	5.6 Ω	5.6 Ω	5.62 Ω
100 A	1000	11.67 Ω	10 Ω	11 Ω	11.5 Ω
60 A	1000	19.45 Ω	18 Ω	18 Ω	18.7 Ω
50 A	1000	23.33 Ω	22 Ω	22 Ω	22.6 Ω
32 A	1000	36.46 Ω	33 Ω	36 Ω	34.8 Ω
16 A	1000	72.92 Ω	68 Ω	68 Ω	71.5 Ω
10 A	1000	116.67 Ω	100 Ω	110 Ω	115 Ω
5 A	1000	233.35 Ω	220 Ω	220 Ω	226 Ω
2 A	1000	583.36 Ω	560 Ω	560 Ω	562 Ω
1 A	1000	1,166.73 Ω	1 kΩ	1.1 kΩ	1.15 kΩ

Selecting a burden resistor

These are more details about how this burden resistance is selected.

There are three factors at play to select the burden resistor.

Desired output voltage range
Maximum expected current through the live conductor the CT clamp clips around
Number of windings of the current transformer (or, its ratio).

The last two combine into the maximum expected current through the current transformer winding and thus through the burden resistor.

Desired output voltage range: fixed

The desired output voltage range happens to be fixed. Based on the operating voltage of the ESP32 and the analog to digital converter (ADC) used in this design, the DC output voltage should never be higher than 3.3 V.

The voltage across the burden resistor will be in the form of a sine wave, so to remain between 0V and 3.3V, it is superimposed on a 1.65 V offset. From that, the peak voltage across the burden resistor ($V_{pk}$) should be 1.65 V.

Maximum expected current through live conductor

To play it safe, this should probably match or exceed the current rating of the breaker on this circuit, or any lower-rated fuse that's in line with it.

For example, this could be 32 A.

Ratio of the current transformer

A CT Clamp will typically have a marking like 100 A : 50 mA or something similar. Divide those by one another to get the ratio or number of windings. In this example that's

$$n = \frac{100 A}{0.05 A} = 2000$$

Max expected current through burden resistor

Our max expected current through the burden resistor becomes the maximum expected current through the live conductor, divided by the number of windings. Taking our example:

$$I_{burden_{rms}} = 32 A \div 2000 = 16 mA$$

This 32 A rating is typically an RMS value, that is, not the actual peak value. To get the peak value, multiply by $\sqrt2$:

$$I_{burden_{pk}} = 16 mA \times \sqrt{2} \approx 22.63 mA$$

Putting it all together

Now we can pick a burden resistance value:

$$R_{burden} = V_{burden_{pk}} \div I_{burden_{pk}}$$

Example:

$$R_{burden} = 1.65 V \div 22.63 mA \approx 72.91 \ohm$$

For convenience, here's the equation from base values:

$$R_{burden} = V_{burden_{pk}} \div \frac{I_{live_{RMS}} \times \sqrt{2}}{n}$$

Where $V_{burden_{pk}} = 1.65V$.

You'll then want to pick a resistor that actually exists. Round the value down, since rounding up will result in higher voltages than those calculated.

Playground and connections

All pins of the ESP32 have an additional open pad next to them to directly wire things to them as desired. All ADC inputs have a directly connected pad, near the center of the board (A0-A3). Each connector on the board has duplicate pads to wire directly to them.

Lastly, there is a "playground" area with 5 pads for the 5V*, 3.3V, and GND rails. A set of further pads, all spaced 2.54mm (0.1") apart is available as a sort of mini proto-board.These pads are not connected to anything unless marked otherwise on the board.

ESPHome configuration

TODO.

Pertinent info if you already know what you're doing:

Pulse input pin: 27

ADC:

domain: ads1115
i2c scl pin: 13
i2c sda pin: 12
i2c address: 0x48
Multiplexers available:
- A0_GND (CT Clamp or AI0)
- A1_GND
- A2_GND
- A3_GND

Helpful esphome stuff:

CT Clamp
Pulse Meter Sensor
Custom Pulse Meter component by stevebaxter - This uses time between pulses to get a more precise idea of present power consumption

michd / esphome-powermon