Multi-Function Travel Adapter
In August 2013 we went on a family holiday to Santorini. Having got home and whilst unpacking, we realised that we had taken a huge number of power adapters, cables, travel plugs, etc. The objective of this project is to simply replace more than 80% of them with a single device that can charge numerous types of devices and also charge three devices at the same time.
Our requirements are:
- The main power unit must work with a (separate) 110-240V ac mains power supply that provides a regulated 12V dc and also work with a 12V dc car cigarette lighter socket.
- The 12V dc power supply have a figure-8 power input socket, to enable a range of mains power cables to be used. It must also work with 110V to 240V ac mains power.
- One USB ports suitable to work with Android devices (rated at 3A). This requires the pins to be wired correctly.
- Two USB ports suitable to work with Apple devices (rated at 3A). This requires the pins to be configured and wired correctly, with a suitable network of resistors.
- All ports can be used to charge other devices such as our Contour+ action camera.
- A 9V power output rated at 1A for our Philips active travel speakers.
- Switched LED (warm white) night light built in.
- The whole unit must be compact and light-weight.
- The unit must not get warm when in use under maximum load conditions for long periods of time.
We want this device to also work in vehicles, to also have to avoid taking in-car chargers along with us. Standardising on 12V also means that we can use a single 12V mains power adapter to power everything. In reality the voltage output from a car is often much higher than 12V (typically 13.8V to 14.3V), so we have chosen dc-dc converters that can handle these variations.
The other advantage is that this device will also work during a mains power cut and can be connected to our 12V dc Uninterruptable Power Supply (UPS).
Power input is via a standard 5.5/2.1mm panel mount power socket.
Mains Plugs & Cables
Our 12V dc power supply requires a 110V to 240V ac power source and we are using this combination of cable and UK mains plug adapter. This will work in most of Europe.
For the USA, we have a similar cable with a suitable plug to match the USA power sockets.
Ports & Cables
Most devices come with a standard micro-USB cable these days except our many Apple devices, which either use a 30-pin connector or the Lightning cable. Because these cables are 'fairly standard', we have decided to work with them and modify the USB ports on our charging device.
We are using panel mounted USB sockets.
For the regulated 5V dc power supplies, we are using compact and efficient dc-dc converters bought on eBay. We have used these things a lot and they are very good. They can come with micro-USB connectors, making them extremely useful for powering a Raspberry Pi. They provide an accurate and stable voltage and can supply up to 3A.
Charging Android Devices
Many Android powered devices like the Samsung Galaxy S4 and the the Nexus 7 have two modes of charging, 'USB mode' and 'AC mode'. In USB charging mode, your Android device charges slowly, only drawing a maximum of 500mA from the USB host device, even if that device is a USB charger that is capable of supplying much more power. The mains power charger that comes with Android devices that support fast charging on 'AC mode' are special in that they have a 200Ω resistance between the two (D+ and D-) data pins inside the USB socket. When you connect the device to the charger it detects the 'shorted' data pins and switches to AC fast charge mode.
Typically, this means drawing about 1.1A from the USB socket and even if it can supply more current, it will not be taken.
Interestingly, many Android device we have tested using an Apple iPhone charger or an Apple iPad charge have all charged in 'AC mode'. This includes a Google Nexus 7, Google Nexus 10 and a Samsung Galaxy S4.
Charging iOS / Apple Devices
Apple have gone with a proprietary approach of modifying the USB port to signal to Apple devices, how much current can be drawn from the USB socket. Many standard USB chargers will not work at all with a Apple device.
To enable high-speed charging of an Apple device, the USB port needs to have a network of four resistors added across the pins
Philips SBA290 Portable Speaker System
We have used a Philips SBA290 portable speaker for travel use for a number of years. It is ideally sized (180mm × 65mm × 35mm) and weighs just 210g without the batteries inside it. It can be powered using six AAA batteries but the sound is much better and has much less distortion if the supplied power adapter is used. This unit has a spatial enhancement switch that improves the stereo sound. It also has a dynamic bass boost switch that enables an decent amount of bass to come from such a tiny speaker system. In short, it is perfect for travelling.
The speaker has a short cable with a 3.5mm stereo plug for audio input. It also has a built-in stand and comes with a quality travel case.
The power adapter supplied is rated at 9V dc and 500mA. This seemed a bit low to us, so we put our meters on it to see how much power was really being used. The first thing we noticed was that that the supplied PSU is not regulated. It puts out about 12.3V under no load and as the volume is increased, this drops to about 9.5V but, also fluctuates a lot. The speaker never consumed more than 300mA.
We then put the device on a regulated 9V dc power supply and it sounded better, with less distortion at higher volumes. The speaker was quite happy with a regulated 12V dc power supply too. This means we don't need a 12V to 9V dc-dc converter.
There is the option to use a 12V - 9V dc-dc converter but it doesn't look like this is needed.
We bought a Maplin aluminium extruded case. It is is just 120mm × 72mm × 19mm and is nice and compact, whilst having a nice quality finish and profile.
This is the case we are using. It is very compact, so we have limited our design down to just three USB ports, in order to fit it all in the case. It comes with some nice rubber feet.
This is what it will look like with the speaker on top.
We tested the output of the OEM Samsung Nexus 10 charger and it measured 5.13V under no load. It is rated at 5V and 2A.
We tested the output of the OEM Asus Nexus 7 charger and it measured 5.19V under no load. It is rated at 5V and 2A.
Variable Charging Rates
The current taken from a USB port depends on the current battery state of a device. When we measured the current taken to charge both a Nexus 7 and a Samsung Galaxy S4 with ~80% battery, we only ever saw a maximum current of 260mA drawn regardless of what we used to charge the device.
Android Data Pins
We tried a number of resistances across the data pins from 220Ω to a wire short and it made no difference to the charging rates. In the end we stuck with a 47Ω resistor across the data pins when charging Android devices.
We did some simple tests to see how the USB lead used affects the charging rate of a Google Nexus 10 tablet with a battery at 15% charged. We used a current meter on the 12V dc side because putting a current meter in-line with the USB power on the 5V dc side causes a voltage drop across the meter itself due to the shunt within it. This way we can be sure the device sees exactly 5V dc and our measurements won't affect the charging of the device.
Our tests showed that a cheap Amazon 15cm micro-USB lead took 0.66A from the 12V supply (about 8W). Using the OEM Asus Nexus 7 micro-USB lead (95cm long), this dropped to 0.55A. Using the OEM Samsung Nexus 10 micro-USB lead (1.5m long) this dropped to 0.26A.
This is a really surprising result and one we are going to try and repeat with other Android device.
12V Supply Current
How much current is taken from the 12V power supply under full load? Full load would be charging a Google Nexus 10, an iPad and an iPhone 5 with the music system on full volume too.
We have all the parts required and are now testing the charging performance with a range of devices. We are now mounting all the components in a nice case.
This device doesn't just have to be used whilst travelling. We have a clutter of chargers and leads distributed around our home and this also provides a means to charge them from a single charger.
3rd October 2013
Have booked a skiing trip for January 2014 and this will be my first opportunity to test it abroad.