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I’m using a 5v 20a power supply run through a barrel connector into an Arduino Uno to control an LED strip. When only the barrel is connected, the green light on the Uno comes on but the yellow light lights dimly for a few milliseconds then goes dark. The light strip doesn’t light at all. When a USB is also connected, the board lights up fully (green and yellow LED) and the LED strip runs through its routine.

I’m making this as Christmas lights on a tree outside so I want it to run on the stand alone power supply. Why doesn’t the barrel work on 5v?

measuring voltage from barrel, board with just barrel, board with barrel and USB

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    because it is not a 5 V barrel jack. the specs says it is for 7 to 12 V
    – Juraj
    Nov 22, 2020 at 17:25

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Voltage Drop

Well, if you put 5V into your barrel jack, your UNO's onboard 5V regulator is going to drop that down to 3.5V or so. You must have more than 5V into the regulator to get 5V out of it. This is referred to as the regulator's "drop out" voltage. The specs for an UNO say to power it from at least 7V for that reason.

As an experiment you can power it this way without your lights attached and measure the "5V" header to GND to see this happening.

Power Dissipation

Linear regulators like that found on the UNO are not efficient. They waste energy as heat in proportion to the current through it and voltage dropped by it.

If you provide it with 7V at the regulator and your project draws 0.5 amps from the regulator, you will try to generate 0.5A * (7V - 5V), that is 1 Watt of heat. Your regulator can probably dissipate half that with how it's mounted on the UNO and normal ambient air temperature.

Note this situation gets worse if you put 12V (for example) into the barrel jack, as you'd be dropping 7V across the regulator.

Many linear regulators have thermal overload protection built into them. They'll stop producing as much voltage at the output as they heat up to try to survive the event. And that's likely what you're seeing if you're trying to power your christmas lights from the regulator.

If you want to see this yourself might try very briefly tapping the regulator with your finger; it may be blazing hot. You might just dampen your finger and hold it near the regulator's surface to feel the heat coming off of it.

You will more than likely need to route power around the Arduino board to your lights to prevent this.

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  • Thanks! That makes sense. Nov 22, 2020 at 17:18
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    @JohnnyRollerfeet If you have 5V already, you can wire it to the 5V rail (to skip the regulator). But be avare of possible backpowering of USB hub
    – KIIV
    Nov 22, 2020 at 19:14
  • @JohnnyRollerfeet, you may also want to consider getting a USB breakout or take the B end off a USB cable and use that to power the board. That will allow you to keep the using the polyfuse that the UNO has connected to its USB jack. And it will effectively plug the USB jack while you're powering it with 5V more-or-less directly.
    – timemage
    Nov 22, 2020 at 19:36

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