# Power meter + dimmer

This article describes a 230V AC power utility box containing a 6 function 20A LCD power meter, a 4000W rated phase control dimmer, fitted up as a 230V 10A device.

## Power meter

The power meter has displays 6 metrics:

• voltage
• current
• (real) power
• power factor
• energy (kWh)
• frequency

## Dimmer

The dimmer is rated at 230V 4000W (17.4A).

This is a low hysteresis dimmer. Though almost all dimmers are advertised as such, this has the necessary components to reduce hysteresis adequately.
See A comparo of two bare light dimmer modules for more discussion, this is a Type 2 module as described in the article.

Zoomed up, minimum conduction angle is about 1.5/10*180=27° Approximating the low conduction angle wave as a triangular wave, we can calculate Irms as Ipk*(t/T/3)^0.5=2.7*(1.5/10/3)^0.5=0.603A, and since Rload=53 in this test, P=I^2*R=19.3W, 2% of maximum output.

## Boxing it up

An inexpensive ABS Jiffy box was milled for the openings for the switches, power meter, cable strain reliefs and mounting screws for the dimmer module.

It is a tight fit and depends on grinding out some interior ribs and pillar.

Above, the finished box under test.

An explanatory note, the power meter is connected to the incoming mains, so it reads the incoming RMS voltage and RMS current. In this case, the load is dimmed by the phase controller and the current is 1.66A. The displayed PF is computed from the real power input and apparent power (being 238*1.66W), so it is well less than unity even though this is a resistive load. If the voltmeter was across the load, it would read the phase controlled voltage being 206.4/1.66=124Vrms (because it is a resistive load)… but connecting the power meter there would cause it to drop out at low output voltage.

Above is a view of the inside of the box.

Inside the power meter, current is sampled with a shunt comprising two R006 2512 2.5W metal current sense resistors. A quick calculation shows them good to 40A, so they might be destroyed in the even of a significant fault current, and failure might well mean the power meter module is destroyed.

I would not be using this on load with significant surge current above 10A, eg large induction motors, choke welders etc, the 100A power meter option uses a current transformer which provides more headroom, albeit at reduced resolution.

## On test

On test, the box works a treat.