12V DC Power

Why have a separate 12V dc power network?

  1. The main reason for doing this is that a lot of the home automation and security systems we have built use a 12V DC power supply. This includes cameras, PIR sensors and the core of my Home Control System (HCS).
  2. In addition, there are a huge range of devices designed for car and caravan use and many entertainment and gaming devices also come with adaptors to allow use in a car.
  3. Low voltages are also much easier to work with and it is easy to store electricity for emergency use in 12V batteries. A 12V uninterruptable power supply (UPS) is much easier and cheaper to build than one for mains voltages.
  4. Emergency lighting can easily be run from 12V dc and you can now easily obtain very energy efficient, compact flourescent bulbs and 12V LED lighting. In fact we plan to mainly use 12V lighting in our home any way.
  5. A 12V dc supply network is resilient to interference and also generates very little itself. This means that power cables can be ducted alongside communications and networking cables.

There are a few things to consider with a 12V dc power network:

  • 12V is not always exactly 12V. A '12V' solar cell can generate more than 21V when connected to a low load.
  • The output from a 12V lead-acid battery can exceed 13.5V when fully charged and if connected, a trickle charger may well exceed 18V.

For these reasons, we assume the following types of 12V dc power exist in our home and we use the following designations to be ensure we connect devices to the right 'type' of 12V power:

12V
This is a raw feed from our battery bank. The battery bank will also have charging devices (trickle charger, solar panel, wind turbine, etc.) connected to it. The voltage is expected to be between 12V and 21V. This assumes that the battery bank is not discharged to a level where the the voltage drops below 12V.
12V [B]
This is a direct 12V connection to a 12V lead-acid battery and it will not be connected to battery charging devices. The voltage can be expected to reach as much as 13.5V. A lot of devices and some 12V LED bulbs are designed to handle these kinds of voltages with ease. You can also expect some voltage drops on longer runs of wire. A 12V LDO regulator used remotely can overcome any problems associated with both wiring drops and excessive supply voltages.
12V [U]
This is a mains power derived 12V dc power source that is not protected from mains power failures. Further designations will determine whether this is regulated or unregulated. Typically these will be used for non-essential lighting using a 12V transformer.
12V [P]
This is a mains power derived 12V power source that is protected from mains power failures, using a battery or UPS to maintain power. This power is prioritised for essential automation and security elements of our home.
12V [R]
This is a 12V power source that is regulated using a voltage regulator. Typically these will be used for devices that are sensitive to voltage variations such as delicate electronics.

So to summarise, a 12V DC [PR] power supply is required to power our Home Control System (HCS) mini-ITX PC and is typically used for all home automation and security systems. One of our projects shows how this is currently implemented.

There is one other type of 12V power in our home, but we are working to eliminate this completely. We have some lighting transformers that provide 12V ac power.

Design

12V design overview
We have used standard automotive blade fuses throughout our 12V network and systems as there are available in a wide range of values. You have to go to a distributor for 1A and 2A rated fuses though, as most cars use fuses rated at 3A or higher. The cable for the common earth is rated at a value twice the sum of all the fuses shown in this diagram.

Raw Feed - 12V

Our current design assumes a single battery bank and this currently also acts as the store for our 12V UPS. The UPS is responsible for charging up the batteries and can deliver up to 2A to achieve this via a mains power adaptor. The plan is to replace this power source with photo-voltaic cells and a wind turbine in our next house. The battery bank capacity will also be significantly increased.

Some direct feeds (via fuses) are taken from the battery bank, to power switched devices such as battery chargers, task lighting and other 12V devices. Typically, all of these will have a regulator between them and the battery bank to protect them from voltages over 12V.

Regulated Feed - 12V [R]

This is used for low current components of our Home Control System (HCS) and these are permanently connected to the battery bank.

UPS Feed - 12V [PR]

This is used for high current components of our Home Control System (HCS) and is provided using a switched mode power supply. All of the loads connected to it can be automatically shutdown by the Home Control System (HCS). With our current battery bank, the UPS can continue to power all of the connected devices for over 3 days. The Home Control System (HCS) is also monitoring the state of our battery bank, in order to determine when to shut components down.

Voltage Regulators

A voltage regulator is a device which takes an input voltage and outputs a stable fixed voltage. Variable voltage regulators allow you to set the output to any required voltage (typically between 1.5 to 30V). For the more commonly required voltages, there are fixed voltage regulators which can be used to output just one voltage.

Unfortunately these regulators typically have quite a high voltage drop of around 3V. Therefore, to obtain a reliable 12V output voltage, an input of 15V or more is required, which is more than a 12V lead-acid battery produces even when under charge. To get around this problem 'low drop-out' (LDO) regulators were developed.

A low drop-out regulator supplies a fixed output voltage with a typical voltage drop of just 0.5 Volts. This makes them perfect for use with 12V lead-acid batteries. Ideally a lead-acid battery should never be allowed to get below 12.6 Volts (40% charge), and since 12.6 - 0.5 is still over 12V, a low dropout 12V regulator will be able to reliably supply a 12 Volt output whenever the battery has sufficient charge.

In our home we are using the LT1083CP-12 voltage regulator for high current applications. This device is used in our 12V monitor project.

Switched Mode Power Supplies

Another way to achieve accurate voltage regulation is to use a switched mode or dc to dc power supply. This is used in our 12V UPS project.

Wiring & Distribution

Most of the 12V wiring we use has been specifically developed for automotive use and the main supplier we use is Vehicle Wiring Products. Getting the distribution of your 12V power right is important. Not only will it make life simpler but, it will also prove safer. To make our life easier, we have adopted a colour coding convention.

Fuses

Automotive 8-way fuse holder
Our 12V battery bank can supply a huge current. For added protection, all feeds from the battery are via a fuse. We have used automotive technology and components as they are readily available and reasonably priced.

Automotive blade fuses
We have used standard automotive blade fuses throughout our 12V network and systems as there are available in a wide range of values. You have to go to a distributor for 1A and 2A rated fuses though, as most cars use fuses rated at 3A or higher.

Wiring

All wiring has some voltage drop associated with it, due to the resistance of the wiring itself. With 12V systems this can be a real issue, especially where high currents are involved. The trick is to minimise the load by using better technologies. For lighting, this means 3W LED spotlight bulbs instead of 35W halogen spotlight bulbs.

Earthing block
We the approach we have taken you need to connect many items to the earth (battery bank -ve terminal). This is achieved using a high current earthing block.

6-Core Alarm Cable

6-core alarm cable
6-core alarm cable is the standard cable for most burglar alarm installations. it is normally divided in to 3 pairs, 12v power, zone and tamper.

Ducting & Trunking

To keep the wiring tidy in our home we have used ducting to contain 12V dc power cables, IP network cables and other wiring used in the security elements. We have standardised on a MK 40mm x 25mm plastic mini-trunking. This also ensure these low voltage cables are not in close proximity to mains cables and thus minimises interference.

Earthing points
Within our ducting we need to split the earth wires at various points. We've done this with 6-way spade connectors. All of the spade connections we do are crimped and soldered for lower resistance and improved reliability. In our current loft, the trunking and the earth/ground wires form a ring loop right around the roof space, with drop points down to various sensors and modules.

In out next home we might use a slightly larger trunking as we can envisage more Cat 5e cables running through it.

Earthing points within ducting
This shows how we split the earth wires fitted within our ducting. The IP network hanging below also run in this ducting.

Appliances

There are a huge number of devices and appliances that work well from 12V DC power supplies, typically designed for use in cars and caravans. All of our phones and basically any device that charges via a PC USB port, portable games consoles, battery chargers, routers, etc. You can also get Freeview recorders, televisions and larger appliances such as fridges that work from a 12V DC power source.

Lighting

Lighting is covered in another section.

Related Reading

Projects