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Support Forum » wal wart

September 27, 2009
by Jesuz
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I read the section in the nerdkit guide on using a wal wart. It tells you to check a few things but does not clearly tell how. I have a few laying around, but I am scared to just hook them up and try them in fear of damaging my computer. How can I check my wal warts to make sure they are ok to use?

Also can anyone point me in the direction of a write up on how to switch to battery when the wal wart is not hooked up?

Thanks in advance.

September 27, 2009
by rusirius
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I don't recall exactly what it mentioned to check, but my guess would be voltage, polarity, and possibly current... All three of these should be clearly marked on the walwart itself. You need to make sure the voltage is within the limits of the voltage regulator... (I think it can handle up to 18v? Though you may want to add a heat-sink at higher voltages.. Remember that excess power is given off as heat...) And obviously you'll need at least 6v or so to keep a nice consistant 5v.

If you decide to use a walwart that IS 5v and not use a regulator, keep in mind that a LOT of walwart supplies are NOT regulated.. Meaning the more/less current you draw, the less/more voltage they supply... Some of these will output WAY more than 5v unloaded. And your MCU generally isn't going to be much of a load.

As for current, really it just comes down to making sure it can supply enough current for whatever the needs of your project... Most walwarts will be fine for your purposes I'm sure...

As for polarity, again it should be clearly marked on the walwart rather it is "center positive" or "center negative". (Indicated by a little drawing) Center positive means the INSIDE of the connector is + and the outside is ground... That's the most common arrangement.

If you choose not to use a "jack" for it, then you can just lop off the wires... If you're unsure about the polarity you can use a toner to test the other end, or use a multimeter with the walwart plugged in... Generally speaking one of the wires will have a "stripe" and that stripped wire is usually the positive side.

If you really want to make it robust, you can use 4 diodes in a network so it will allow the polarity to be plugged in EITHER way and still work just fine... The diodes just "route" it correctly. However keep in mind that you will always be running through 2 of the 4 diodes, each diode giving a .6 voltage drop, which means you'll need to size your supply at least 1.2v larger than you originally intended.

Last but not least, as for switching to battery when not plugged in... If you want to do this "automatically" there are "jacks" you can get for the power adaptor that will do this mechanically. When the adaptor is plugged in it "disconnects" the battery terminal, and reconnects it when the adaptor is unplugged.

September 27, 2009
by Jesuz
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Thanks for the reply.

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