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Topic: DC power for USB Arduino (Read 15 times) previous topic - next topic

Ian Jones

I have a couple of new power related questions:

1. When powering the board with USB, what voltage and mA come out of the pins? what about the power pins, the 5V and 9V are these usable?

2. What are the specs when powering from a DC adaptor? eg. 9V 500mA?

3. I have read somewhere that powering devices, sensor / actuators from the board is not the best way to work because microchips like the ATMEGA8 don't like power fluctuations.  Is this true?

I'm probably asking pretty silly questions, but my electrical knowledge is very limited atm so any help would be appreciated.

Daniel

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1. When powering the board with USB, what voltage and ma come out of the pins? what about the power pins, the 5V and 9V are these usable?
The USB board and the serial board use a 78M05 regulator for the 5V supply, which gives potentially (sorry for the bad joke ;) ) 500ma of current at 5V. It will run hot at that current, so it's good to use less current if possible. the Arduino uses perhaps 50-100ma, so it would be safe to draw 200ma from the 5V pin, making total consumption about 300ma. See question #3 for why.  

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2. What are the specs when powering from a DC adaptor? eg. 9V 500mA?
yes that will do the trick.

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3. I have read somewhere that powering devices, sensor / actuators from the board is not the best way to work because microchips like the ATMEGA8 don't like power fluctuations.  Is this true?
Microprocessors like the Atmel and the PIC do not like power fluctuations. If you draw a lot of current quickly  from your regulated supply, you run the risk of a voltage fulctuation that will disrupt the delicate task of computing going on inside the chip.  The answer is to use lots of filter capacitors to satisfy anything that uses current in 'spikes', to limit by design the current drawn from the microcontroller's power supply, and to never never never never use inductive loads like motors and relays that take their current from the regulated 5V supply.  For those, you want a separate supply, preferablt diurectly from the main DC source.  

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I'm probably asking pretty silly questions, but my electrical knowledge is very limited so any help would be appreciated.
Not at all! We're all here to learn and share...  

Ian Jones

I bought a DC power adaptor today, from your advice.

I got a 12V, 0.7A Max with what I believe is the correct diameter connector cos it seemed to fit just fine... but the arduino doesn't do anything.  The power LED comes on, Pin 13 LED (only for a 20ms or so) which I have hooked up also comes on and the TX, RX LEDs also turn on.

The program doesn't run though.  What could be wrong?  I'm pretty sure the power supply is center positive, but I can switch it and test it the other way around if it is safe?

The board works fine, I tested again after with USB power.

Massimo Banzi

Hello Ian

quick question:
there is a jumper between the power plug and the usb plug (called sv1). Is that jumper connected between pin 1 and 2??
this tells the board where to get the power supply from. between 1 and 2 is the external dc plug while between 2 and 3 is the USB plug.

If your power adapter has a selector for multiple voltages make sure it is set to 9v or more..

let me know

massimo

Ian Jones

Yep I changed the jumper for the DC power closer to the DC input jack, but to no avail.  It's also 12 volts already.

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