Which external power supply for Uno/LED project?

Hi,

I'm using an Uno to drive those nifty 3W "star" LEDs and need to select an external power supply for the final product, as I'll not be powering things via USB. Supportive data then my question:

  • Uno overview sheet says Uno has 500 mA fuse to limit current via USB to that value.
  • Data sheet for these high-power LEDs, which draw from the 5V supply, says maximum forward current is 700 mA. My measurements show a range of 658 mA up to 936 mA.
  • Only one of the 3W LEDs is on at a given time.

How can the Uno be powering itself, the project's LCD and these current suckers and not be blowing the fuse when I am using USB as the source?
What current capacity does my to-be-purchased 12 Vdc external wall wart have to have in light of this? I was planning on 1 amp, but now I'm scratching my head. More current means higher cost, and I want this thing to be saleable some day.

I'm an electrical engineer and this "don't make sense" to me!
Art

How can the Uno be powering itself, the project's LCD and these current suckers and not be blowing the fuse when I am using USB as the source?

Basically you can't unless you have a different source for the LEDs. How are you driving the LEDs?

What current capacity does my to-be-purchased 12 Vdc external wall wart have to have in light of this?

Well at 935mA I would go for at least 1.5A to give you a bit of head room. It is always wise never to run a power supply close to its maximum rating.

Hi, if the leds run on 5V and the Arduino runs on 5V, why would you buy a 12V power supply?

Paul

PaulRB:
Hi, if the leds run on 5V and the Arduino runs on 5V, why would you buy a 12V power supply?

Paul

Because the Uno overview sheet says: "External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack." and "The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts."

See also, at http://arduino.cc/en/Main/ArduinoBoardUno, under the "Summary" heading: "Input Voltage (recommended) 7-12V."

Grumpy_Mike:
Basically you can’t unless you have a different source for the LEDs. How are you driving the LEDs?
Well at 935mA I would go for at least 1.5A to give you a bit of head room. It is always wise never to run a power supply close to its maximum rating.

I am using the method described by “customgeek” Jeremy in his video at Controlling high power (or a high number of) LED's with an Arduino - YouTube. Anode of LED to +5Vdc, cathode to collector of TIP122 transistor which is serving as a high power switch. The Uno turns the Q/switch on and off via logic level applied to base. It works well. The +5Vdc is that available on the Uno header which, in my present situation, is all coming down the pike from my MacBook Pro via USB.

If your leds ran at 12V it would make sense to buy a 12V supply and also feed that into the Arduino's jack socket so that its on-board regulator can make the very small current at 5V it needs to operate.

But your leds run at (less than) 5V. If you use a 12V supply, more than half its output power will be wasted as heat in the led series resistors. So it makes more sense to use a 5V supply. As long as it is stabilised or switch-mode, you can power the Arduino directly from it by connecting it to the Arduino's 5V connector, bypassing the jack socket and built-in regulator.

Paul

I watched the video and hope that he has only misspoken when he claims that he opens up the full 12V to make the small LEDs brighter, because that is of course total nonsense!

LEDs are constant current devices. You need a little more than the specified froward voltage Vf ,which is between 2.5-3.5V for most single die LEDs. So 5V is enough for your circuit. Much more than that, as someone already stated above will only result in energy that is dissipated elsewhere e.g. in the current limiting resistors.

Using simple resistors for limiting current is fine for small LEDs, e.g. the little white ones in the video, but for LEDs such as the high power LEDs you are using I'd suggest a cheap constant current driver.

Thanks for the input, but what about what I quoted earlier, from the Uno's spec sheet? "The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable." How can you thus say that I should feed the Uno with +5Vdc via the power supply input?
I am building a prototype which will operate off an external commonly-available power supply (i.e. wall wart) and I am still baffled, in light of the spec sheet's notice, that folks suggest I go with an external 5Vdc supply. Please explain, as inquiring minds want to know!

Art

If you want to feed an arduino from a 5V regulated power supply you connect it to the 5V pin and the ground pin, not the power jack.