Go Down

Topic: Connecting 5V VCC to an Arduino Pro Mini (or to any Arduino) (Read 6717 times) previous topic - next topic

Wawa

Internal diagrams show no backfeed diode (but I remember having seen one many moons ago).
But the text talks about "the internal diode between the output and input pin".

Output capacitance is not an issue here. Input capacitance is.
We are putting 5volt on the output, while the input cap is still discharged.

"the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A"

The root of the problem stays the same.
The input cap on Uno and Mega boards is 47uF. Less on smaller boards.

I think we're safe to assume backfeeding (without extra capacitance connected to Vin) is not problem.
Extra capacitance in a supply, connected to the DC socket, is not a problem because of the reverse protection diode.
Leo..

OldSteve

Internal diagrams show no backfeed diode (but I remember having seen one many moons ago).
But the text talks about "the internal diode between the output and input pin".

Output capacitance is not an issue here. Input capacitance is.
We are putting 5volt on the output, while the input cap is still discharged.

"the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A"

The root of the problem stays the same.
The input cap on Uno and Mega boards is 47uF. Less on smaller boards.

I think we're safe to assume backfeeding (without extra capacitance connected to Vin) is not problem.
Extra capacitance in a supply, connected to the DC socket, is not a problem because of the reverse protection diode.
Leo..

Yes, if we're to believe the datasheet, backfeeding is fine, and the UNO isn't breaking the rules by supplying the USB 5V after the regulator.
The datasheet doesn't mention input capacitance in the 'backfeeding' equation, but it does recommend a minimum 10uF input capacitor.
Overall, the biggest issue appears to be making sure that 5V isn't connected to the output of the regulator while a voltage is applied to Vin.

I'm pleased that this topic was raised by the OP. It's a common question, and has now been completely cleared up for a few of us. I wasn't sure if the UNO, (and no doubt other Arduinos), was breaking the rules with the USB 5V connection on the downwind side of the regulator, but obviously it isn't.
And therefore my LM2940 connection is OK too. I'm happy. :)
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

Wawa

Overall, the biggest issue appears to be making sure that 5V isn't connected to the output of the regulator while a voltage is applied to Vin.
Don't get that part.

OldSteve

Don't get that part.
I just mean that it's never a good idea to directly parallel two regulators. That, (I assume), is why the UNO and probably other Arduinos uses the P-channel MOSFET to isolate the USB 5V from the 5V supplied by the regulator via Vin, so that when Vin is present, the USB 5V is switched out of circuit.

I keep referring to UNO because it's what I have, and I hadn't looked at other Arduino schematics, but I just checked the Mega schematic and it's the same in this regard.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

Wawa

It is not a problem to parallel two regulators.
The problem could be that only one does all the work.

The mosfets on the USB line prevent backfeeding into the computer.
But they only switch off when Vin is above 6.6volt (see opamp circuit).

When USB is just a diode drop higher than the onboard 5volt regulator, USB supply takes over through the reverse protection diodes inside the mosfets.
Leo..

OldSteve

It is not a problem to parallel two regulators.
The problem could be that only one does all the work.

The mosfets on the USB line prevent backfeeding into the computer.
But they only switch off when Vin is above 6.6volt (see opamp circuit).

When USB is just a diode drop higher than the onboard 5volt regulator, USB supply takes over through the reverse protection diodes inside the mosfets.
Leo..
I hadn't looked at the exact point of switching. (You've saved me the effort of working it out. :) )
And you're right, usually the problem of paralleling two regulators is uneven load sharing. I guess it really doesn't matter too much if Vin is applied at the same time , as long as the external 5V supply can handle sinking current as well as sourcing it, in case it's voltage is very slightly lower than that from Vin via the 1117.

I think I'd still prefer to ensure that Vin isn't connected at the same time. In my circuit, Vin is never connected, so it's not an issue. The 5V always comes from the LM2940. It's a better choice of regulator. It can supply more current, has a lower dropout voltage and has provision for a heatsink if It's input voltage is high and a fair current is being drawn. While a 1117 can supply 800mA, I'd hate to go anywhere near that with a 12V supply. I won't even run a servo directly from the Arduino board.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

raschemmel

Quote
Even without the above, the UNO does it all of the time when USB is connected and there is no voltage applied to Vin, and I don't see them dying all over the place from back-feeding the regulator.
Maybe I'm missing something here but I just looked at the UNO schematic and when the USB is connected , with nothing plugged into the external dc barreljack, the USB 5V is connected to the output of the onboard 5V regulator (which has no input). I don't see anything shorting Vin to GND I don't know what you mean by the UNO does it all the time. Can you explain (what you meant ?)
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

OldSteve

Maybe I'm missing something here but I just looked at the UNO schematic and when the USB is connected , with nothing plugged into the external dc barreljack, the USB 5V is connected to the output of the onboard 5V regulator (which has no input). I don't see anything shorting Vin to GND I don't know what you mean by the UNO does it all the time. Can you explain (what you meant ?)
I meant that the UNO connects a voltage to the output of the regulator all the time when USB is powered and that no damage is done. ie There's nothing wrong with this, despite the fact that the Arduino page I referred to earlier recommends against it.
I said nothing about the Vin pin being shorted to ground, and neither does the Arduino page.


Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

larryd

If you connect 5v to the 5 volt pin and then forget, then plug in a wallwart to the jack, might be what they are alluding to.

No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

raschemmel

Quote
I meant that the UNO connects a voltage to the output of the regulator all the time when USB is powered and that no damage is done.
Yes, I agree. That's what I was describing in the schematic. I guess it was someone else who was saying the regulator would self destruct if you shorted the input pin to GND and connected 5V to the output pin.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

OldSteve

Yes, I agree. That's what I was describing in the schematic. I guess it was someone else who was saying the regulator would self destruct if you shorted the input pin to GND and connected 5V to the output pin.
Must have been. The datasheet refers to this though, in relation to large output capacitances, so I guess there is a good reason not to short Vin to ground with 5V connected to the regulator's outout.
It says "With an extremely large output capacitor (≥1000 μF), and with input instantaneously shorted to ground, the regulator could be damaged."

I'm sure that the datasheet says "instantaneously" because if it's not instantaneous, the large capacitance will discharge slowly without problems, at a decent current. So if a continuous 5V is applied to the output, it follows that the regulator might well be destroyed.

While I have 5V connected directly to the regulator's output, I personally won't be shorting Vin to ground to see what happens. :D

Edit: it was Wawa, yesterday, who referred to an input short while 5V is applied to the regulator's output. In effect, he said what I just said, "Maybe someone (not me) could ground Vin, and put a current controlled 5volt supply on the output of the regulator.
See if it releases the magic smoke."
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

raschemmel

Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

OldSteve

I'll add that to my bucket list...
If I otherwise kill a board so that it's unusable, I'll try it too. I think I know what will happen.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

Go Up