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Tuesday, April 23, 2013

7. AC Adapters

I would just like to say a word or two about AC adapters.  Although I mentioned above that this work was safe, you’re now talking about mains voltages, and you don’t mess with that unless you really know what you’re doing, so parents – keep an eye on the little children playing with electronics to make sure they haven’t graduated on to the mains type voltages. 

Here is a picture of a label on an AC adapter:

Important things to look out for are: 

  • the input voltage – the 120 V AC labelled above would not be suitable for use in the UK’s 230 V (±10 per cent) AC mains supply. 
  • the output voltage – 6V DC may need to be further regulated to suit your project.
  • the output current – 500 mA would be more than adequate for most simple projects, but for the Raspberry Pi, there is plenty of advice on the internet which recommends a maximum available current of 1000 mA (one amp).
  • polarity indication – it is very important to know which of the outer or inner parts of the plug are positive or ground.  Please note – the polarity may not be indicated at all.
The popular plug in use with such adapters is the 2.1 mm plug:  This is so-called because of the inner diameter of the plug.  The one illustrated is indicated to have positive voltage on the centre connector, while the outer connector is ground.  Breadboard-friendly versions of the socket for this plug are readily available, and extremely useful.  See the Adafruit version on the right:

2.1mm x 5.5mm female DC plug connector with center positive connection
Breadboard-friendly 2.1 mm DC barrel jack from Adafruit (this one has been mounted on a breadboard)

For Adafruit's very useful article on transformer-based AC/DC converters, go here:

Remember that, especially with old or cheap units, you can’t rely on the stated output voltage and available amperage.  Always measure with a DMM.

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