Quick external power supply question

Hi there

I recently fried an old hub by using the wrong power supply, so now I'm scared ;) (I think I just took a center negative instead of center positive supply, voltage was correct)

So now my question, is it safe to use the following power supply with the arduino duemilanove? Voltage is within the limits, amperage should be ok and it is center positive. But I'd like to get some confirmation :) I'm not allowed to post a link now, so please forgive me for double posting. See next post for image.

(click to enlarge)

How do you know that it is center - positive?

I don't see anywhere on there the letters "DC". Now, from my experience, you don't see AC - AC adapters nearly as often as AC - DC adapters, but you can never be too careful.

Other than those two things, it looks OK to me..

next to the last line of the text states center +

the



symbol = DC, ac looks more like a ~

6VA =

Transformer capacity is rated in Volt-Amps (VA) which is generally the same as wattage (Watts)

It should be fine for most light duty single supply projects, 12 volts going into a 7805 voltage regulator will get you about half* of what that supply can provide in amperage before getting obnoxiously hot.

(*under stock conditions, with a fat heat sink you could draw + 0.250 amps more than the supply can provide, resulting in excess heat and possible fire risk at the wall wart)

its a balance of numbers

Should work fine. Also the Arduino external power input first passes through a polarity protection diode. So in reality plugging in a reversed polarity DC module or even a AC module will not harm the Arduino. It may not function, but no damage should result. Only if you plug in a power source with too high a voltage can damage result.

Lefty

next to the last line of the text states center +

the



symbol = DC, ac looks more like a ~

Obviously I'm not hip with these symbols :P. Center positive I probably should have figured out but ah well ::). Thanks for clarifying :).

Also the Arduino external power input first passes through a polarity protection diode.

So that's what that little diode's for? Cool :).

Thanks for your answers. It didn't fry indeed ;) The voltage regulator gets warm pretty quickly even if I'm just blinking a LED, but I guess I'd burn my fingers on it before it would take damage. So in terms of heat it would be better to have a lower voltage and higher amperage?

Like this: 5V, 2.5A V a bit low, A over the top...

I also have a regulated one: 1.5-12V, 500mA max, 9W If I understood correctly you told me that Arduino will draw up to 750mA, so this power supply wouldn't give enough power...

This one even has the possibility to reverse polarity: I'm not sure which setting gives which direction, but since you told me the arduino can handle inversed polarity I think I should be fine.

It is best to have about 7v to power the arduino through the regulator. The supply also should have at least 500mA for general arduino projects. The arduino onboard regulator can only provide 400mA including powering the atmega chip and FTDI so high drain things have to be powered seperately (not just through the 5v pin) You can power the arduino through the 5v and GND pins but you need a [u]good[/u] power supply to provide a regulated 5v supply and I don't think there is any protection if you power it like that.

Mowcius

Ok, thanks. So they should all be fine (even the 5V, 2.5A), and best is to use the variable one at 6V (since I can't set it to 7v), right?

the 7805 regulator requires at least 7 volts to operate, 9v on your adjustable one would be fine, 6v probably wont work ... I dont know about yours, but the one I have 6 volts is more like 7.5 check it with a multimeter ...

the 5v one can be used if you hook it up to the 5 volt pins on the arduino, but you bypass the regulator and all protection, if you hook it up backwards it will fry the board, likewise if it is not a regulated supply over voltage could cook the board too, its not recommended

even the 5V, 2.5A

As Osgeld said, the regulator needs about 7v to function properly (provide 5v to the arduino) and you want to be really careful about sticking 5v into the arduino pins because it has no protection.

If you have a multimeter then I would recommend testing any power supply you want to use to check what the actual voltage is (sometimes they can be 1-2v more!)

Mowcius

If you have a multimeter then I would recommend testing any power supply you want to use to check what the actual voltage is (sometimes they can be 1-2v more!)

I've always been a bit confused about that. Are unregulated power supplies variable voltage depending on the load? Because I tested a 12V DC Wall wart I found, and it was about 16.5 Volts without a load! But when I hooked up an old 12 Volt fan, it went down to about 12 volts.

Because I tested a 12V DC Wall wart I found, and it was about 16.5 Volts without a load! But when I hooked up an old 12 Volt fan, it went down to about 12 volts.

That's why it's called an unregulated supply ;)

This is normal behaviour for that simple kind of DC supply. The way a regulator works is to have excessive input DC voltage avalible and throttle it down to the desired DC voltage by constantly testing it's output voltage relative to a fixed internal reference voltage and adjusting it's series resistance to raise or lower it's output voltage.

A unregulated DC supply just has a fixed internal impedance (resistance) and just obeys ohms law depending on the load resistance.

Lefty

That's why it's called an unregulated supply ;)

This is normal behaviour for that simple kind of DC supply. The way a regulator works is to have excessive input DC voltage avalible and throttle it down to the desired DC voltage by constantly testing it's output voltage relative to a fixed internal reference voltage and adjusting it's series resistance to raise or lower it's output voltage.

A unregulated DC supply just has a fixed internal impedance (resistance) and just obeys ohms law depending on the load resistance.

Lefty

That's what I thought. Thanks :).

Thank you all for your answers... I measured 19V on the 12V supply, still in the tolerance but quite astonishing... I'm using the adjustable at 9V now, works very well :)

Thank you all for your answers... I measured 19V on the 12V supply, still in the tolerance but quite astonishing... I'm using the adjustable at 9V now, works very wel

Probably a good call. 19 V would be pushing it. I'm pretty sure those limits are absolute maximums, meaning the Arduino will become damaged if you go above them. Best to stay (well) within that range. :)