Powering a mega through VIN

Hi, this is something I really should be able to work out myself, but I'm really tired and its got me scratching my head.

I've built a 12+v DC step-down 'power-board' to pull out a 5V supply via a LM7805 and a 3.3v line off a LM3940. It features all the caps and diodes and an inductor as well to clean it all up. Designed to run off a motorbike supply.

I run the out of the 7805 via the 3940 and on to the VIN pin.

My understanding was that if there was no external supply connected to the DC jack, then it would switch to taking power raw off of the VIN pin.

The mega powers up fine, but its only putting out 4.09V via the 5V rail.. im assuming this is because theres a series diode or something on the VIN?

Also - whats the best way to get a split 5V + 3.3V rail sorted ? Similar to what I'm doing or is there some magical reg that outputs both ?

If I read your post correctly, it sounds like you are powering the Arduino by applying +5vdc from your custom PS module to the Arduino Vin pin?

If that is the case, you are not meeting specification for external power input reccomendation of 9-12vdc. The only difference between the Arduino external jack's positive terminal and the Vin pin is that there is a series connected diode for reverse voltage protection. Both points feed the input of the on-board +5vdc regulator chip which requires about 7.5v minimum input voltage to meet it's voltage regulation drop out specification.

A better solution could be to add an additional regulator, a 7809, to your external power supply module and feed it's +9vdc output into either the external power jack or the Vin pin on the Arduino.

Lefty

Lefty, your point is valid, but to be precise, the Mega uses a low dropout regulator so only needs a volt or so more than 5 volts,

Lefty, your point is valid, but to be precise, the Mega uses a low dropout regulator so only needs a volt or so more than 5 volts,

Yes, I missed the Mega part. So 5vdc +1vdc (drop out value) + .7vdc (diode drop) gets us to what, 6.7vdc minimum input at the external power jack, or 6vdc if inputting via the Vin pin? A 7808 or 7809 would still be a good choice for the OP to feed the board.

I just get a little uncomfortable reading people powering their Arduino board through either the Vin pin or the +5vdc pin directly if there are purpose power connectors provided in the board design. To much room for error or mistakes.
Lefty

Yes, and its particularly important with a mega board where you can't just drop in another chip if you make a mistake and smoke it.

thanks guys - I’ll swap it to the 7809 i have here.

Totally agree re using the DC jack a safer option, but space is an issue with the mega lengthwise already… Also need a good solid 3.3V feed, so thought that using a ‘custom’ shield that powers via VIN was one of the few better options - saves trying to either have a seperate power ‘box’ or having stuff coming out and backinto the housing… Also, since the vibrations where these are housed along with just ‘vigourous riding’ means I thought a locking connector was a wiser choice.

If Adr1an has a regulated 5V supply "It features all the caps and diodes and an inductor as well to clean it all up", and he would like "a locking connector" for his power supply, [u]why[/u] can't he supply the Mega using the power conductors of the USB port?.

This is essentially what happens when the Arduino Mega is powered by an attached computer (via USB).

He'd have to strip a USB cable with type A connector, and join the power conductors to the output of his regulated supply.

why can't he supply the Mega using the power conductors of the USB port?.

That would be a perfecly acceptable way to power the board using a regulated +5vdc external power supply. Hacking a USB cable is not that difficult.

My issue is when users are tempted to power the board by sourcing voltage via either the Arduino Vin pin or the +5vdc pin connections. They are just not reliable connections and again just too much room for error.

Lefty

@lefty:

My issue is when users are tempted to power the board by sourcing voltage via either the Arduino Vin pin or the +5vdc pin connections. They are just not reliable connections and again just too much room for error.

I agree. It's just 'not the done thing'.

OK now you're making me nervous Lefty. I have a 5V switching power supply from another microcontroller project, and I am feeding that to my Mega via the "5V" pin.

My reasoning was that I have other things I need to power with 5V, and a 12V wall-wart. Better to feed the 1A switcher with the 12V and hang the Arduino off that, rather than demand that the Arduino on-board regulator power my other 5V stuff as well.

I'm not really clear on what risk I am taking here though. Is it just that I might do something foolish like switch the 5V and GND when sticking things in the Mega's power header?

OK now you're making me nervous Lefty.

Not my intent, but making the connection for the +5vdc and ground from the external supply via a hacked USB cable is a much more secure connection, with no chance of having +5v connected but ground fell off or visa versa. You also get the benefit of the built in 500ma thermal fuse when using the USB power connection that you don't get using the +5vdc pin.

Supplying power to the board using either the USB connector (regulated +5vdc) or the external power connector (>7.5vdc) is the proper method, anything else is just needless risk taking.

Lefty

Very thoughtful and well expressed explanation Lefty.

so can you give it 5.5v from the vin or is that too low.?

so can you give it 5.5v from the vin or is that too low.?

Too low, 7.5 to 12vdc is the normal range of voltage via the external power connector or the Vin pin.

Lefty

as discussed in earlier posts, 6 volts or more is ok for Vin on a Mega