Using External DC Power Supplies & Questions On Setup/Sizing of Peripherial Item

Hi Guys,

So still trying to wrap my head around using external power sources to power peripherial items beyond the arduino's 5v limitation. First thing I want to understand is calculating power requirements based on the current spec sheet:

Spec Sheet:
Computer Fan Blower: 12v DC - .30amp

I understand that a 12v .5a wall wort could power the blower no problem. But what if instead I had a 6v power supply that cranked out 1 amp. Since the consumption rate of the blower is .4 Watts, and both the 12v and 6v power supplies exceed that, could I just as easily use a 6v power supply as a 12v?

Now, let's say the project I have in mind requires a peak of 32v with the following peripherial items:
1 x 12v .35a blowers
1 x 5v .41a TFT display
1 x Thermometer board 5v .050a
1 x 5v LED light strip 12v 2a

A friend of mine just gave me some massive, old school power supplies that are 12v, but can run up to 15 amps. Is this where a transistor would help me? I've read they can act as an amplifier or a switch. In this case, I 'think' I'd want to amplify the 12v signal to be capable of handling up to 32v, correct? Since the power supply has 12v and up to 15a of current, it should be able to provide up to 32v at around 5a due to ohms law. Maybe I'm totally wrong about all this stuff, so please feel free to correct my noobish notions on power consumptions.

You are wrong, but in a way you are right :slight_smile:

You are right that 12v 0.5A and 6V 1.0A power supplies do produce the same Wattage.

You are wrong regarding running a 12v fan from a 6 volt source.

Most electronic devices are made to run at a specific voltage, and at that voltage will require a certain current, from those two factors you can calculate its wattage.

Things like some fans and led will allow you to reduce the voltage to slow or dim their output and the current will similarly fall, and so the wattage.

However things like ics and micros must run within specific limits, typically 5v or 3v3 and these voltages must be kept within tight tolerances.

So for your project I would built it like this.

I would use 2 x 12vdc 3A+ supplies. *

One to power just the led strip lights and the second one for all the other devices.

The second one you first power the 12v blower and then feed the Arduinos power socket .

Then the 12v should be fed into a lm7805 5v regulator to create the smooth 5v supply for the TFT screen; you could also use this voltage to supply the Arduinos +5v input.

The thermometer board , being a sensitive device, should be powered from the +5v line of the Arduino so the fan and leds do not affect its reading.

The Ardunio needs a Relay /Transistor/Mosfet on its outputs to handle the fan and leds 12v power.

  • Why 3A when at most your leds only use 2A, if you use a 2A supply it will be running at maximum and therefor get quiet hot which shortens its life.
    Like driving a car, you would not drive it everywhere at maximum revs all the time and expect it to last very long.

Thanks Ricky! Lot of good advice there, so I gave you a +1 to your karma :slight_smile:

Let me ask you this. Instead of using multiple wall warts to power the different items, what if I used some sort of step down device that they could plug directly into a standard 110/120v? Do you think that would make things easier?

You could get a 5A 12vdc Wall wart, that would simplify things.

The alternative is to build the power supply into your project using a transformer etc, but given your knowledge of electrics I would avoid that for now; a plug in, sealed wall wart much safer !

Whenever you supply power to a device, eg your fan or led strip. fit a fuse, so if anything goes wrong you have more protection.