Powering a Home Data Centre - Part 1

Part 1 - Sensing The Mains

In this first part, I will be discussing how to detect and to measure mains voltage and current. Whilst detection and current measurement may be carried out indirectly (i.e. without connecting to the mains circuit), there is no convenient way in which to do this  for voltage measurement.

Why measure the mains?

There are two main reasons for measuring the mains - 

●  Monitoring the power usage
●  Ensuring that the electrical supply circuit is not being overloaded

There are two measurements used for assessing overload - 

●  Ensuring that the current drawn is within the specification for the circuit
●  Ensuring that the voltage drop on load due to the length and size of the cable is less than that allowed.

The first of these is less critical if an appropriate protective device (a fuse or similar) is used in the circuit.

The second impacts both the efficiency of the device(s) being supplied, and the amount of heat being generated in the cable. Overheating cables lead to short circuits, to overheated flammable materials, and to fires in both cases.

Here in Britain, the incoming mains is pretty stable, so we don't need to measure that voltage, so measuring the voltage at the point of demand will be sufficient. The 18th Ed. wiring regulations say that we are allowed a 5% voltage drop on non-lighting circuits - which comes to 11.5V (Lighting circuits are allowed 3%, or 6.9V).

The amount of voltage drop is related to the amount of current being consumed, so there is no point in measuring this with no load connected.

The amount of power being consumed by a circuit in Watts gives a measure of how much the circuit is costing to run.

Finally, detecting the presence of a mains voltage on a the line side of a circuit tells you whether the circuit is energised or not, an important factor when using digital control systems.

Isolation - keeping the voltages apart.

Digital systems operate on a very low voltage - usually 3.3V or 5V DC. If we connect the mains to a digital control board, there will be a loud bang, smoke, bits of circuitry flying and complaints about the waste of (expensive) parts.

In order to prevent this, especially when interacting with the mains from control circuits, we use various forms of isolation. This may involve the use of transformers and/or optical links of various sorts. It also requires particular care when designing printed circuit boards. Slots (air-gaps), surface spacing and earth barriers all have their place in the design.

Where possible, non-contact sensing should be used - where the sensor itself is fully insulated from the mains conductors - usually by passing the insulated wire through an aperture in the sensor head itself.

Currently, there is no convenient or accurate means of measuring the voltage on an AC conductor without making electrical contact with that conductor - therefore, that circuit will use a small, encapsulated isolating transformer to effect the connection.

 

The following parts will cover

1. Voltage Measurement circuit
2. Current Measurement circuit
3. Supply Detection circuit by both live connection and non-contact methods
4. Ancillary circuits
5. Microcontroller (Arduino) hardware and example firmware.

 

A note about the PCBs

 Some of the published PCBs may seem to be rather underpopulated, but there is a method to my madness.

The PCB production service that I use has a minimum charge which covers PCBs up to 100mm square.

By using a templated PCB footprint, it has been possible to design these modules so that they can be conveniently stacked using nylon standoffs. The current sensor board is designed so that by breaking out the sensor head sub-boards, a suitable shape is left that will sit above the voltage sensor board, with the transformer filling the gap.

The files for the project (images, circuit schematics and circuit board sources, gerber files) can be found on my GitHub project repository, which may be found here: https://github.com/AlyssonRowan/HomeDataCentrePower

 

More information on capturing data about your mains supply can be found at:

The Open Energy Monitor website  https://openenergymonitor.org/ 
and particularly at https://learn.openenergymonitor.org/

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