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# Arduino maximum current draw? Answered

What would the maximum current draw through an Arduino be? (ATMEGA328) - I'm planning on running 16 LEDs from it and I need to know if I should be powering them separately somehow.I have the appropriate resistors (100ohm and 180ohm LED dependent).

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## 9 Replies

1. A 2N3904 NPN transistor has a worst case saturated ON voltage drop of 0.3 Volts
2. A single RED LED has a low forward voltage drop of 1.8 Volts
3. Your power supply is 5 VDC
4. Calculate voltage across the Resistor = 5 - 0.3 - 1.8 = 2.9 Volts
5. You want to run 40 ma through the LED
6. Ohms Law states R = V / I = 2.9 / .04 = 72.5 ohms
7. You cannot buy that value so use a 75 ohm 5% resistor

Two RED LEDs in series case.
1. Calculate voltage across the Resistor = 5 - 0.3 - 1.8- 1.8  = 1.1 Volts
2. You want to run 40 ma through the LEDs
3. Ohms Law states R = V / I = 1.1 / .04 = 27.5 ohms
4. You cannot buy that value so use a 27.4 ohm 1% resistor

A

jonrb (author)2013-05-30

I'm afraid I don't really understand what you mean (I understand the diagram, but not the values).

As I stated above, the LEDs came with resistors for use under certain voltages (They're high power LEDs, I'd send you a link but the site is down at the moment). Should I not be using those ones that they came with?

I'm not fussed about using PNP over FETs. If you think FETs would be better, I'm following your advice!

jonrb (author)2013-05-29

The resistors came with the LEDs as follows:
4 Green, 4 Blue and 4 White: 100Ω for 5V and 470Ω for 12V
4 Red: 180Ω for 5V and 510Ω for 12V

They'll be done in groups together, with the resistor in series with each LED in the parallel circuit. Is that right?

steveastrouk (author)2013-05-29

Yes, THAT is OK, if you have resistors. I think Iceng was concerned you weren't using INDIVIDUAL resistor/Led combinations.

jonrb (author)2013-05-29

Actually, regarding that: Would it be better to have the LEDs in series with 1 resistor or 4 in parallel with their own resistors? Will one configuration use less power than the other, but maintain the same brightness? (Given that all 4 LEDs will be the same colour and power specification)

steveastrouk (author)2013-05-29

You don't have enough volts to put 4 in series, though if you HAD that would be most efficient, and could save power.

Are you running on 5V ? You MIGHT get two in series, with another parallel two in series.

jonrb (author)2013-05-28

I've got the appropriate resistors for powering the LEDs (Planning on doing it in parallel - is this wise?) with 5V and an LM7805 which does up to 1.5A so that should be fine.

Transistor wise, I have enough 2N3906s and enough BC558s - I'm guessing they'd be OK running 4 LEDs each? Is there one I should be choosing over the other?

steveastrouk (author)2013-05-28

The ABSOLUTE MAXIMUM package current of 200mA, and a ABSOLUTE MAXIMUM pin current of 40mA per pin is stated on the datasheet/

10mA per pin for 16 pins is the only safe point, unless you can guarantee that not all LEDs are ever driven at the same time

Powering them separately would require transistors

jonrb (author)2013-05-28

Cool, thanks!

I'm assuming there won't be any problem running them from the same power source as the Arduino (It can provide around 1.5A)?

If I'm running them off a separate power source, should I just run the pin's 'ground' combined with the LEDs' 'ground' (After going through the transistor) into the power source? (Like in this picture: http://bit.ly/15ep1ZB ) - I like to think that the arduino's GND just goes into the power supply's GND anyway, but I guess it's good to check.

Does it matter what kind (PNP and NPN) transistor I use, particularly? Will the models matter?

(Any switching will be done in millis)