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Topic: Question about unregulated ("wall wart") DC power supply... (Read 1 time) previous topic - next topic

moses1592

I want to use an unregulated ac to dc converter to power an Arduino board. I know Arduino can take a supply up to 12V on Vin. My power supply says that its a 9V 300mah supply, but when I put it on a voltmeter it reads almost 15 volts. I thought I remembered reading that the unregulated supplies will typically read higher-than-stated until they are applied to an electrical load close to what they are rated for. Is this true? Furthermore, is it safe to power my UNO board with this power supply? I'm pretty sure the power supply is not faulty. I have several of this exact same power supply and they all read around 15 volts (with no load applied to them).

Thanks!

-Brian


krupski

I want to use an unregulated ac to dc converter to power an Arduino board. I know Arduino can take a supply up to 12V on Vin. My power supply says that its a 9V 300mah supply, but when I put it on a voltmeter it reads almost 15 volts. I thought I remembered reading that the unregulated supplies will typically read higher-than-stated until they are applied to an electrical load close to what they are rated for. Is this true? Furthermore, is it safe to power my UNO board with this power supply? I'm pretty sure the power supply is not faulty. I have several of this exact same power supply and they all read around 15 volts (with no load applied to them).

Thanks!

-Brian


Your Arduino board will likely not load the wall wart enough to drop the voltage much below the 15 you measured. It would work, but you are dropping an awful lot of energy into the on-board voltage regulator. It (the Arduino) will probably get very warm to quite hot just running by itself.

I suggest either building a little 7805 based post-regulator (a 7805 on a small heatsink) to provide clean 5 volts to the Arduino. Or get a regulated wall wart.  Or better yet, get a small bench top power supply. In the end, if you get serious playing with Arduino boards, you'll need more versatile power anyway.

Good luck!
Gentlemen may prefer Blondes, but Real Men prefer Redheads!


raschemmel

That's because the wall wart is a VERY old fashioned LINEAR transformer and NOT  a modern switching supply. Throw it in the trash and buy a switching supply !

Boardburner2

Litter lout.
You could at least unwind all that copper wire and use to make a kitten zapper.
 :)

krupski

That's because the wall wart is a VERY old fashioned LINEAR transformer and NOT  a modern switching supply. Throw it in the trash and buy a switching supply !
I've used "old fashioned" iron transformer wall warts in situations where I need quiet (i.e. no hash noise) power and absolute certainty of isolation from the mains.

Call me old fashioned if you like, but I still can't get myself to trust switchers. I know that switchers ARE isolated (high frequency transformer and opto-isolator), but I still don't like them.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

krupski

Litter lout.
You could at least unwind all that copper wire and use to make a kitten zapper.
 :)
That's not nice. I like cats. Those who don't like them simply don't understand them.

Sure, a dog will come running and slobbering when called.. a cat probably will not. Why? Maybe because he simply doesn't WANT to!

Cats have a mind of their own, and this ticks off a lot of humans who are used to "blindly obedient" dogs.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

raschemmel

Then you need to add a bleed resistor to bring the output voltage down to 9V with the arduino connected.
Experiment with different values and measure the current through the bleed resistor so you an calculate the proper wattage you need.

Boardburner2

For certain applications such as analogue radio and audio i would agree.
However i have a bucket full of analog wall warts saved which i rarely touch these days.

For digital stuff its much easier to get a switcher as it avoids the extra regulator.
Provided its not a chinese incendiary one of course.

OldSteve

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.

Boardburner2

Well mine used to be blind in one eye.
The cat was suspected.

OldSteve

Well mine used to be blind in one eye.
The cat was suspected.
That's the wrong type of "blindly obedient". :(
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


Boardburner2

I should have used a cast statement.  :smiley-zipper:

Another good use for old linear supplies is for powering motors , worth keeping a few.

OldSteve

I should have used a cast statement.  :smiley-zipper:
I had to think about this for a moment. :)
(Type-cast a 'blind' dog to a 'blindly odedient' type.)
I like it.
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.

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