# power consumption System for electrical devices

#### -live wire-

Joined Dec 22, 2017
926
You need to define the requirements a bit better. The range of things listed in post #1 go from 3.6 Vdc to 240 Vac. No simple voltage or current sensor can cover that range. If you have devices groupls according to their input power requirements, that can be used to define a set of sensors for each group.

ak
Okay. assume one AC light bulb connected to AC main supply 230 vAC and I want to know how much power it consume for 1 hours using any microcontroller including ADC

I think I need only two sensors. current sensor to measure current throw bulb and voltage sensor to measure voltage drop across bulb

I am confused with voltage sensor if voltage drop is 9v AC (assume any device that consume 9v AC ) then how the microcontroller will know this voltage because it works only 5v DC
If there is any sort of inductance, including a transformer supply for LEDs, then you will have a power factor less than one. So you need to see if the current is out of phase, and figure out the power factor that way. You multiply the power factor by the apparent power to get real power. So if there is a power factor of .25, and you measure 1000 watts, that's 250 volt amps, and 750 volt-ampere reactives. If it a transformer with no load, you have a power factor of almost 0. But it may consume a lot of current. Just something to be aware of. You can see voltage leads current by 90 degrees.

#### Fanfire174

Joined Mar 13, 2018
240
Connect a small power transformer to the AC line in parallel with the light bulb. Something with a secondary voltage of 12 Vac or so.
Use a diode bridge and filter capacitor to convert that to a DC voltage.
Use two resistors to divide the DC voltage down to something less than 5 V.
Use the uC A/D to convert the DC voltage value to a digital value.
Measure the AC voltage directly with a meter.
That meter reading now equates to that digital value. This sets the scale for other readings.

There is an error introduced by the two diodes in the bridge, but that can be tweaked out with more software.

ak

#### AnalogKid

Joined Aug 1, 2013
9,478
The low side of the isolated voltage (bottom end of R2) must be connected to the measurement system ground (ground pin of uC).

Nice use of reference designators.

ak

#### -live wire-

Joined Dec 22, 2017
926
You don't really want a capacitor there. Just make sure your ADC is fast enough. Also, make sure you know the exact transformer turn ratio. And make sure your divider doesn’t draw too much current, or it could cause the voltage to drop a little.

#### Fanfire174

Joined Mar 13, 2018
240
The low side of the isolated voltage (bottom end of R2) must be connected to the measurement system ground (ground pin of uC).

Nice use of reference designators.

ak
I don't have any idea where to connect clamp sensor in block diagram. Do you have any idea where it will connect in circuit

#### AnalogKid

Joined Aug 1, 2013
9,478
The current sensor, whether a clamp-on current probe or a small current sense transformer, goes around one of the wires going only to the light bulb. In this wan it senses only the load current, not the current through the AC voltage transformer. Depending on what the currrent sensor is, the output might be either DC or AC.

ak

#### LesJones

Joined Jan 8, 2017
3,570
Hi Fanfire,
As you do not even know how to measure the current a device is consuming I don't think you will be capable of designing the interfaces to the ADC. Also if you want true RMS power readings I don't think you will understand how to measure instantaneous current and voltage values and calculate the RMS power. You could just buy a plug in power monitor such as this.

Les.

#### -live wire-

Joined Dec 22, 2017
926
Hi Fanfire,
As you do not even know how to measure the current a device is consuming I don't think you will be capable of designing the interfaces to the ADC. Also if you want true RMS power readings I don't think you will understand how to measure instantaneous current and voltage values and calculate the RMS power. You could just buy a plug in power monitor such as this.

Les.
Usually you can just get the right SPI/I2C library and interface easily with the ADC. It just required some very basic coding skills. Also, if you did it DIY, you could measure the power factor, and find out how much power you are actually paying for.