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Support Forum » DC adapter (wall wart) questions

November 29, 2009
by Haprog
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I just got my USB NerdKit + LED Array Kit on monday and I'm very happy with it. I already got initialload, tempsensor, led_blink and dip_arithmetic projects working in a day (even trying out small code modifications).

I'm very excited to get started with the LED Array project, but I noticed that I can't ofcourse use the US-style wall DC adapter here in Finland. So now I'm looking for an alternative adapter for the USB NerdKit for use in Finland, but I came up with some questions due to lack of knowledge.

Quoting the USB NerdKits Guide:

you may need extra "bypass capacitance", as described in step 3b, because these power supplies can be more noisy

How do I know if I need extra "bypass capacitance" and how much?

Is there a way to know if a DC adapter is isolated via a transformer or not, by looking at one?

I noticed some DC adapters have specified VA (volt-ampere?).
If specified, does this value matter when choosing an adapter for use with USB Nerdkit?
If so, what does it actually mean?

I found three unused AC/DC adapters in the suggested 7-30V range:

  • "7.5V 1A"
  • "12V 1200mA (equipment up to 15W)"
  • "12V 5.0A"

Can I use one of these?
In what range should the adapters output current (A) be?
I noticed the NerdKits US-style adapter output is 9V 300mA.
How does the amount of current actually affect?

November 29, 2009
by Rick_S
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If I were to choose between the three you have listed there, I'd go with the 7.5v 1A adapter. While the 1A rating is beyond what you need to power the kit, it won't cause damage AS LONG AS YOU RUN IT THROUGH THE REGULATOR PROVIDED IN THE KIT.

Other adapters to look for are old Cell Phone chargers. I've see several that are 5v and regulated.

As to the need for "bypass capacitors" that will depend on the adapter. Additional capacitors at the point of connection to the breadboard will not hurt anything and will definitely help if the supply doesn't take care of that itself.

As to the Volt/Amp rating... I think (and someone can correct me if I'm wrong) that is the power rating for the adapter similar to how many watts it can draw from your ac outlet max.

I've made a couple of posts recently regarding the use of ac adapters. They may or may not be helpful but here are the links.

Link 1

Link 2

I hope that helps and, Welcome to the forum!


November 29, 2009
by Haprog
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Thanks for your answer.

I have several 5V adapters. So I could use one of these without the voltage regulator if the adapter is regulated? How do I know if an adapter is regulated or not?

I have:

  • 5V 890mA (Nokia charger AC-8E)
  • 5V 1.5A (Creative SoundBlaster Audigy 2 NX USB sound card power adapter)
  • 5V 1.6A (some old Archos charger)
  • 5V 2A (TomTom Mobile 5 GPS charger)

I don't yet fully understand many of the things I'm dealing with here.

November 29, 2009
by Rick_S
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If you have a volt-meter (multimeter), you could check the voltage. If it reads 5v within +/- a couple of millivolts, chances are it's regulated. Often modern cell phone chargers use switching power supplies to convert the voltage rather than the old transformer/bridge configuration. These supplies tend to be much cleaner power wise and tend to be regulated well. They will also be much lighter in weight and often smaller in size than the transformer type with equivalent amperage rating.

The main thing you want to be sure of is that the voltage going to your mcu is somewhere between 4.5v and 5.5v DC, clean power if you plan to use it by itself. Amperage rating isn't as much of an issue as long as it's enough to supply what you want to drive. The micro-controller itself draws little power and even if you had it sourcing all its outputs at a max of .02A each that would only be around 1/2 amp draw. So any of the adapters listed above would supply adequate current.

If you aren't sure and can't check the adapter, try running it through the regulator provided in the kit. It will prevent over-voltage from reaching the mcu. If you get erratic results when hooked up that way, place a capacitor across the input of the regulator to ground and another from the output of the regulator to ground. This cleans up the power and will often eliminate erratic behavior. If the adapter is regulated, you may not get power through the regulator because it needs at least a volt or so above its output to do it's job.

If you don't have one, a multimeter is a must in this hobby.


November 29, 2009
by Haprog
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I think I have enough information for now.
I don't have a multimeter yet, but I'm going to get one.

November 29, 2009
by Haprog
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For now I ended up using the 7.5V 1A adapter with the voltage regulator since it was the only one for which I happened to find a suitable jack atm, and it seems to work fine (atleast tempsensor runs at it should) :)

November 29, 2009
by Rick_S
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I figured that one would work out for you. Glad to hear you're up and running!


December 27, 2009
by BobaMosfet
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The 7.5V 1A adapter is the lowest rate you can go (should go). The 5V (LM7805) regulator requires the voltage source to be approximately 2 to 2.5 voltes above the 5V regulated output, or it won't be able to sustain 5V output.

Absolutely get a multimeter, and make sure it has a diode tester.

Power adapters have a built in transformer, and a diode bridge to convert AC to DC. If they have a regulator, other than just a capacitor, the voltage generated will be relatively clean and smooth. If they don't have a regulator, it will be a bit choppy. The only way to actually see which it is, is with an oscilloscope.

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