How to Calculate Power (P) from Voltage and Current Values in Node-RED?

General
1

Hello,

I'm working on a project in Node-RED where I receive voltage from a ADS1115 and current from a ina219. I want to calculate the power (P) using these values, but i dont know how.

However, when I try to add both inputs into a Function node and write the following script:

var voltage = msg.payload;
var current = msg.payload;
var power = voltage * current;
var newMsg = { payload: power };
return newMsg;

I get some weird values, and If I rename the payloads to msg.payload.voltage and msg.payload.current as suggested in some examples, I encounter error messages "NaN".

Could someone please provide guidance on how to accomplish this? I need to perform the calculation within the Function node without changing the message structure. Any examples or suggestions would be greatly appreciated. Thank you!

2

Doesn't the INA219 provide all the data you need without a ADS1115?

3

Usually, an INA219 measures both voltage and current, that's correct. However, there's an issue (at least with the pre-built Node-RED flow) where values above approximately 16V are only displayed as mismatched negative values. It's a known problem, which is why I've opted to use an ADS1115 for voltage measurement instead.

4

If you can post the flow and message structure you will probably get some help.

5

Voltage and Current cannot have the same property (msg.payload). So i assume the payloads are in separate messages. You will have to use a join node to join the 2 messages before the function node. Or save the values to context.

6

Thanks for the quick answer.

Here is the structure:

111
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[
    {
        "id": "d0b817e64cb5e8f3",
        "type": "tab",
        "label": "Flow 3",
        "disabled": false,
        "info": "",
        "env": []
    },
    {
        "id": "5a1a25108a44a4b7",
        "type": "ina-sensor",
        "z": "d0b817e64cb5e8f3",
        "name": "",
        "handle": "68a7603e9b0ab180",
        "x": 270,
        "y": 240,
        "wires": [
            [],
            [
                "6bdf32801ece1206"
            ]
        ]
    },
    {
        "id": "003214632c94b6b5",
        "type": "mqtt in",
        "z": "d0b817e64cb5e8f3",
        "name": "",
        "topic": "ina_219_1",
        "qos": "2",
        "datatype": "auto-detect",
        "broker": "4d29b73341f90358",
        "nl": false,
        "rap": true,
        "rh": 0,
        "inputs": 0,
        "x": 100,
        "y": 240,
        "wires": [
            [
                "5a1a25108a44a4b7"
            ]
        ]
    },
    {
        "id": "ae5ce491679590df",
        "type": "inject",
        "z": "d0b817e64cb5e8f3",
        "name": "",
        "props": [
            {
                "p": "payload"
            },
            {
                "p": "topic",
                "vt": "str"
            }
        ],
        "repeat": "3",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "",
        "payloadType": "date",
        "x": 110,
        "y": 300,
        "wires": [
            [
                "8fe81bc608d61bec"
            ]
        ]
    },
    {
        "id": "8fe81bc608d61bec",
        "type": "ads1x15_i2c",
        "z": "d0b817e64cb5e8f3",
        "property": "payload",
        "name": "ADS1115",
        "i2c_device_number": 1,
        "chip": "0x01",
        "i2c_address": "0x48",
        "inputsForChannel": "singleEnded",
        "singleEndedChannel0": true,
        "singleEndedChannel1": false,
        "singleEndedChannel2": false,
        "singleEndedChannel3": false,
        "differentialChannel0_1": true,
        "differentialChannel0_3": false,
        "differentialChannel1_3": false,
        "differentialChannel2_3": false,
        "samplesPerSecond0": "920",
        "samplesPerSecond1": "128",
        "progGainAmp": "6144",
        "x": 260,
        "y": 300,
        "wires": [
            [
                "d3269ae4faf888f3"
            ]
        ]
    },
    {
        "id": "d3269ae4faf888f3",
        "type": "change",
        "z": "d0b817e64cb5e8f3",
        "name": "extract_value_from_Channel_A0",
        "rules": [
            {
                "t": "set",
                "p": "payload",
                "pt": "msg",
                "to": "payload[\"/dev/i2c-1\"].a1115[\"0x4ds8\"].singleEnded.channel_1.Volts",
                "tot": "msg"
            }
        ],
        "action": "",
        "property": "",
        "from": "",
        "to": "",
        "reg": false,
        "x": 480,
        "y": 300,
        "wires": [
            [
                "d33f98b9d8ab1426"
            ]
        ]
    },
    {
        "id": "d33f98b9d8ab1426",
        "type": "voltage_undivider",
        "z": "d0b817e64cb5e8f3",
        "name": "Voltagedivider",
        "property": "payload",
        "Voltage_Input": "V-Volts",
        "Resistor_1": "2200",
        "Resistor_1Types": "Ω-Ohm",
        "Resistor_2": "200",
        "Resistor_2Types": "Ω-Ohm",
        "Voltage_Output": "V-Volts",
        "Round_Output": "Thousandths",
        "x": 740,
        "y": 300,
        "wires": [
            []
        ]
    },
    {
        "id": "6bdf32801ece1206",
        "type": "smooth",
        "z": "d0b817e64cb5e8f3",
        "name": "",
        "property": "payload",
        "action": "mean",
        "count": "3",
        "round": "3",
        "mult": "single",
        "reduce": false,
        "x": 720,
        "y": 240,
        "wires": [
            []
        ]
    },
    {
        "id": "68a7603e9b0ab180",
        "type": "ina-sensor-manager",
        "address": "0x40",
        "delay": "250",
        "ohms": "0.01",
        "customResistor": false
    },
    {
        "id": "4d29b73341f90358",
        "type": "mqtt-broker",
        "name": "Mqtt INPUT",
        "broker": "127.0.0.1",
        "port": "1883",
        "clientid": "",
        "autoConnect": true,
        "usetls": false,
        "protocolVersion": "4",
        "keepalive": "60",
        "cleansession": true,
        "autoUnsubscribe": true,
        "birthTopic": "",
        "birthQos": "0",
        "birthRetain": "false",
        "birthPayload": "",
        "birthMsg": {},
        "closeTopic": "",
        "closeQos": "0",
        "closeRetain": "false",
        "closePayload": "",
        "closeMsg": {},
        "willTopic": "",
        "willQos": "0",
        "willRetain": "false",
        "willPayload": "",
        "willMsg": {},
        "userProps": "",
        "sessionExpiry": ""
    }
]```
7

See this article in the cookbook for an example of how to join messages into one object.

8

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