Powering a Home Data Centre - Part 1b

 Part 1 - Sensing The Mains

b. Measuring Current

It is a slightly less than trivial matter to measure the voltage of the mains supply using a simple ammeter, since you need to insert the ammeter into the circuit, which means cutting off the supply for a while, and then hoping that the current being measured doesn't overload the instrument.

On the other hand, we can use a device that measures the magnetic field around a current-carrying conductor. For a handheld meter, this is achieved by using a clamp probe. This takes the form of a split ring of magnetic material that can be opened and then closed around a single, current-carrying conductor.

When connected to a meter, it will allow the meter to display a measure of the current in the conductor.

These accessories are both expensive and rather unwieldy for permanent installation into a fixed power monitor. An inexpensive and much more compact solution is to use a current transformer that is installed permanently into some part of the power distribution system.

These devices do require some additional circuitry in order to bring the output waveform into range for an ADC.

In addition, a current transformer is a step-up transformer (1000:1 is typical), which means that if left unconnected, the output could carry several thousand volts - a painful (but non-fatal) mistake that could spell the end of the ADC and any associated semiconductor devices. The solution is to connect a load across the transformer's terminals, and this is done in the supplied circuit using a simple resistor.

The current transformer has no split, so it must be threaded onto the insulated conductor - usually the line conductor.

Looking at the circuit, you will see that it is substantially the same as the circuit used for measuring the voltage of the output of the mains transformer in the previous part. Since the device is designed for use with electronic sensors, there is no need to pre-scale the transformer's output.

As before there is a midpoint bias for one side of the measured output, voltage limit diodes and a voltage follower circuit to buffer the high impedance output from the transformer.

The only other addition is a substantial resistor placed physically adjacent to the transformer.

The circuit board is large enough, at 100mm square, to include four copies of the above circuit - sufficient for four single-phase circuits or one three-phase circuit. 

This may seem excessive, but making the board smaller saves nothing in cost, and the components are cheap enough to make multiple channel sensing viable, even to the hobbyist - besides which, this is for monitoring a complex load's demands on multiple circuits.

The sensor heads are on sub-boards which may be broken out for remote installation (purple lines) and an additional rectangular break-out which is designed to allow this board to be installed immediately above the voltage sensor board in the previous part.

No hazardous voltages should ever appear on this board.

For clarity's sake: on this iteration of the design, there is no offset compensation on the voltage follower amplifiers, but it will be included in a future release.

As before, the tiny 1kΩ resistor is installed under each amplifier integrated circuit.

Gerbers and DipTrace files can be found here: [ https://github.com/AlyssonRowan/HomeDataCentrePower ]

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