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Hello everyone!

Why it is not recommended to use the 5v pin to power up arduino uno? Is there a problem with the regulator? It is not the same as when I connect only the usb?
If i use a zener as voltage regulator to enable the pin?

Thanks.
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There is no problem with the Voltage Regulator.
On the Arduino Uno board, the voltage supply is flexible. Either +5V on the +5V pin, or an adaptor to the voltage regulator, or USB power.

The onboard voltage regulator could get hot. And the USB power is often less then 5.00V and only 500mA. If you need more current from the 5V for your project, and external +5V power supply is needed.

A zener is not a voltage regulator.
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There is no problem with the Voltage Regulator.
On the Arduino Uno board, the voltage supply is flexible. Either +5V on the +5V pin, or an adaptor to the voltage regulator, or USB power.

The onboard voltage regulator could get hot. And the USB power is often less then 5.00V and only 500mA. If you need more current from the 5V for your project, and external +5V power supply is needed.

A zener is not a voltage regulator.

he is referring the the current warning the arduino product page for the current Uno and Mega boards:

Quote
5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.

Vs the older definition the arduino folks used in their older boards:

Quote
5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
I'm aware of no official reason given for their change of recommendation on the use of the 5V pin.

Lefty
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Thanks Krodal.
That's very good news! I was worried because I red this:

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5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
http://arduino.cc/en/Main/arduinoBoardUno


Zener is not a regulator but you can make a voltage regulator with a Zener and a resistor. I am thinking of making one in order to power up the UNO from the 5v pin.
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retrolefty, so the text has been changed.

I thought it was only with the Mega a problem but not with the Uno.
The warning can be read in different ways. Perhaps they ment to say that without using the voltage regulator the board is 'unprotected' against higher voltages. And there are many cheap adapters that supply more than 5V.

Tilemachos, I don't know what your plan is with the zener, but it doesn't sound good.
If you need more power, use a good power supply with a good +5V. The Uno board is designed to be able to get power via the +5V pin (but only if it is +5V and not higher).
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Quote
Vs the older definition the arduino folks used in their older boards:

Quote
5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
I'm aware of no official reason given for their change of recommendation on the use of the 5V pin.

Lefty

Thanks Lefty! I didn't know that. That's really odd!
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Tilemachos, I don't know what your plan is with the zener, but it doesn't sound good.
If you need more power, use a good power supply with a good +5V. The Uno board is designed to be able to get power via the +5V pin (but only if it is +5V and not higher).

Well, I don't need more power, just power up Arduino from 4xAAA rechargeable batteries, 4x1,2V=4,8V, which voltage can't enable regulator.
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retrolefty, so the text has been changed.

I thought it was only with the Mega a problem but not with the Uno.
The warning can be read in different ways. Perhaps they ment to say that without using the voltage regulator the board is 'unprotected' against higher voltages. And there are many cheap adapters that supply more than 5V.

No their recommendation on the older boards was it was OK to use a regulated 5vdc power supply to the 5V pin, the newer recommendation is to not try and power the board via the 5V pin. There must have been a reason for them to update the pin 5V recommendation, we just don't know the reason why.
Lefty


Tilemachos, I don't know what your plan is with the zener, but it doesn't sound good.
If you need more power, use a good power supply with a good +5V. The Uno board is designed to be able to get power via the +5V pin (but only if it is +5V and not higher).
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The reason was discussed a while ago. The 1117 5V regulator is susceptibe to damage when reverse driven. The earlier posts discussed bricked boards after putting 5V into the board via the header pin. A member of the team discussed it with us.
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The 1117 5V regulator is susceptibe to damage when reverse driven. The earlier posts discussed bricked boards after putting 5V into the board via the header pin. A member of the team discussed it with us.

Thanks CrossRoads.
Is there a solution about this? Sure, I can regulate the external voltage, but how can I protect the regulator?

When I plug in only the usb, there is no reverse current in the regulator?
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I just notice that Arduino Leonardo has no problem with a regulated voltage at 5v pin. Am I right?
http://arduino.cc/en/Main/arduinoBoardLeonardo

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5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
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The 1117 datasheet suggests a diode from 5V to Vin, anode on 5V and cathode to Vin, as protection.

The Leonardo uses the same NCP1117 5v regulator with no protection.

See page 10 of the data sheet.

I think the Uno statement about 5V is applicable:

5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.

* NCP1117-D.PDF (222.5 KB - downloaded 11 times.)
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The 1117 5V regulator is susceptibe to damage when reverse driven. The earlier posts discussed bricked boards after putting 5V into the board via the header pin. A member of the team discussed it with us.

Thanks CrossRoads.
Is there a solution about this? Sure, I can regulate the external voltage, but how can I protect the regulator?

When I plug in only the usb, there is no reverse current in the regulator?

That is the somewhat puzzling part. From the perspective of the on-board 5 volt regulator I see no electrical difference between powering the board via an external regulated +5vdc voltage source and when powering the board via USB power only, both place a Vcc voltage on the output of the on-board regulator. So if it's risky to power the board via the 5V pin why is not as risky to power it from USB again from the perspective of possible damage to the on-board 5 volt regulator? Not the first time I've posted that question, but yet to hear a what the difference is for the on-board regulator.

Lefty
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Thank you CrossRoads, for making this clear.
retroleft, well said. But the USB +5V is only 500mA and it passes a mosfet before it applies voltage to the output of the voltage regulator. Perhaps that makes it a little less harmful.

What is the best workaround ?  A diode over the voltage regulator ?
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Thank you CrossRoads, for making this clear.
retroleft, well said. But the USB +5V is only 500mA and it passes a mosfet before it applies voltage to the output of the voltage regulator. Perhaps that makes it a little less harmful.

Possibly, but if the thermo-fuse was being activated that would normally cause the board to reset and/or the PC to complain about excess USB current draw. But then again maybe, but if so then that is a very weak engineering solution if there is a real design error in the way the on-board regulator interacts with the rest of the board.

What is the best workaround ?  A diode over the voltage regulator ?

Well crossRoads posted the 'wrap around diode' across the regulator that the regulator's datasheet recommends. But then how does one test it's effectiveness, by powering the board via external regulated +5vdc in spite of the arduino warning?

 The basic problem is that the arduino folks have not explicitly stated the technical reason for their warning of not powering via the 5V pin, so speculation and best guesses is what we are left with.

Lefty

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