External 5V power question

I picked up a couple of 5V regulated 3A power supplies today to use for projects.

The voltage from the power supply measures 5.4V with or without a load. Can I safely hook this power supply to the 5V and Gnd pins on my Arduino even though the output is a little bit high?

Thank you.

Can you give us a link to the power supplies you bought?

5.4V is a bit high for a regulated PSU

What kind of meter did you use to measure the voltage?

The PSUs could be off spec or your meter could be miscalibrated, your meter's battery may be gone or any combination thereof

For comparison my regulated 5V breadboard PSU supplies 4.97V

If my calculations are correct 5.4V is 8% over spec and that is a bit much

At 5.4V some 5V components may overheat, fail or behave erratically, each component has a tolerance range, this is usually 1-2% sometimes higher or lower

If you have an arduino Diecimila with an onboard regulator you can use an external 6-12V unregulated power supply instead and plug it into the external PWR jack and set the EXT PWR jumper accordingly

:slight_smile:

The maximum operating voltage of the chips used on the Arduino is 6 volts so I would think you should be ok with 5.4 volts. But if you want to play safe, you could connect a diode (like an 1N4001) in series with the positive rail to drop the voltage down to a little under 5 volts.

But as gnu_linux says, it may be your voltmeter that is off. If you do have an arduino Diecimila, check your meter by measuring the output of the onboard regulator, they are usually within a percent or two of 5v.

Thank you.

The power supplies are Compaq wall warts. Here is a link from Jameco. http://tinyurl.com/6ovr8b

I measured the voltage with a cheap RadioShack multimeter which could be miscalibrated. If it is accurate my concern was whether or not my system could tolerate a supply that was that almost 10% over spec and it sounds like it could cause problems.

I have an 9V unregulated supply that I could use to power my Arduino but I'm building an LED project and estimate that I may need about 2A total. My hope was that I could use the regulated 5V supply for everything. I may have to rethink that idea.

See if you can borrow or buy a better meter or do as mem suggested and measure the voltage on the arduino's voltage regulator

The large heatsink pad on the regulator is Vout ( pin 4 ) +5VDC

The center pin ( pin2 ) is cut off and is also Vout

You should get almost exactly 5V on pin 4, most of the time it should be between 4.95 and 5.05VDC

If you get 5.4V at the voltage regulator then your meter is most likely off

:slight_smile:

I checked the output of the regulator and got 5.2V which means my meter had problems.

The meter is old so I replaced the battery. Now I'm getting 4.95V off of the voltage regulator and 5.19V from my regulated power supply. Not great but better than 5.4V.

Thanks for the help.

Cool :slight_smile:

4.95V is great :slight_smile:

5.19V is still a little high though

I guess the lesson here is, always use fresh batteries :slight_smile:

You will find that a lot of wall wart type power supplies which are rated for high current will be over by a few hundred millivolts to compensate for cable loss. With a 3 amp load, even .2 ohms in the wire will have your 5 volt supply down to 4.4 volts at the connector end of the cable.

You can expect most wall wart supplies that are rated over one amp to be on the high side of their rated voltage output.

Does anyone else find it ironic that Arduino fails the spell check in this Forum :wink:

KG2

Throw a diode in there and enjoy your new 5V power supplies. That's what I did and it worked well. It was nice to have lots of 5V power available for bright LEDs without worrying about the temperature of a voltage regulator.

I just grabbed some larger diodes out of my junk box and tried a few until I found one that dropped the output below 5V but not too far. Then I soldered it in with the power plug I was using.

Does anyone else find it ironic that Arduino fails the spell check in this Forum

Sorry to be Grumpy but ironic is not the correct word here.

It is "perverse" and "just one of those things" and "wouldn't you just know" but ironic means conveying the opposite meaning to the words used. :wink:

ironic means conveying the opposite meaning to the words used

Isn't also like:

  • rain on your wedding day
  • a free ride when you've already paid
  • good advice, that you just can't take

:slight_smile:

According to Alanis Morissette ((( shudder )))

According to Alanis Morissette

That song is notorious for not having any examples of irony in it, in fact you could say it was in it's self ironic as the examples sung about were not actually examples of irony.

Unfortunately there is no international arrest warrant for crimes against the English language. :stuck_out_tongue:

Anyway I forgot to point out that the spell checker is inside your web browser and not on the board.

Back to the topic...:slight_smile:

I see a bunch of power supplies at Digikey, but I have NO idea what will work.

For example, will this work?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=T311-P5P-ND

Is there a smaller one that will do the job? I don't want to blow up my Arduino.

Keep in mind that how to hook-up an external 5vdc supply to an Arduino board is not an easy slam dunk. There is all ready a 5vdc regulator on board and if you feed an external 5vdc to the normal external power in then that built in regulator will not have enough input voltage to remain in regulation. Also it is probably not a good idea to just hook up an external 5vdc power to the board's 5vdc buss as there may be problems with the on board regulator having voltage on it's output pin but not it's input pin and I'm not sure that is good for the on board regulator, anyone know for sure?

The best method would be to get a USB cable and cut off the computer end connector and sort out the two 5vdc power wires and hook that up to your external 5vdc power supply.

Lefty

You can take +5VDC from a computer power supply which is regulated

Just make sure the power supply is in good shape and the voltage is within tolerance with a good DMM ( digital multimeter with fresh batteries )

The unit at digikey is regulated and costs almost $40 which is how much you would pay for a good 300-400W computer PSU

Since the digikey unit is regulated you should be able to tie it directly to the +5VDC pin but as I said before verify the voltage is ok with a good DMM ( digital multimeter with fresh batteries )

:slight_smile:

Hi,

I was reading up on this for a project and noticed that the "Atmel" can take up to 6V... but the "FTDI" chip is only rated for 5.25V.

So I would be careful powering the 5V pin on the Arduino with anything more than 5.25V.

I would suggest using the Vin pin instead of the 5V pin.