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Topic: Multiple leds on one arduino pin? (Read 11243 times) previous topic - next topic

dc42

#15
Nov 20, 2011, 11:25 pm Last Edit: Nov 21, 2011, 09:52 am by dc42 Reason: 1
Looks like you have a maximum current consumption of 10 x 20mA per colour per board, total 200mA (which is just fine for a BC337 to switch). Times 3 for the colours and you have 600mA total per board. Multiply that by 14 and you get 8.4A. So yes, you will need a high current power supply. One possibility is a standard ATX computer supply.

It would be easier to use higher voltage and lower current. If the LEDs drop 3.2v each, then instead of 10 led/resistor combinations in parallel, you could have 2 sets of (5 leds in series + one resistor). This would drop around 16v, so you would need (say) a 20v supply to drive it, and drop 4v in the series resistor. But it would consume 40mA per colour per board instead of 200mA. That brings the total power requirement to 1.68A at 20v, which is more manageable than 5v 8.4A.

Another option is 5 parallel groups of 2 leds in series, which would need about 9v @ 4.2A. If your boards had 12 leds instead of 10, you would have even more options, i.e. 12x1, 6x2, 3x4, 4x3.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dutchboy

Thanks dc42, I think a big revision is order, but its good to learn. I do have a spare ATX  power supply but it is a little unsightly, so maybe a trip to Ebay is order to source a possible supply.

Can't thank you enough for your help.

Grumpy_Mike

The transistors just switch. The power supply gives them something to switch.

dutchboy

#18
Nov 21, 2011, 12:48 pm Last Edit: Nov 23, 2011, 08:16 am by dutchboy Reason: 1
Hey Grumpy_Mike, yeah felt a little stupid after asking and not thinking about it properly lol. Thanks.

Edit: So been thinking about what dc42 said and rummaging through my box of hoarded power supplies I have found an old laptop power supply rated at 20V 3.25A DC does anybody think this might be ok?

Edit 2: Final circuit being added below! (hopefully!!!!lol)

Grumpy_Mike

Add up the volts drop, I think there are too many LEDs. Anything with a current limiting resistor under 50R is pushing it for constant current like behaviour.

dutchboy

Ahh ok so green and blue look ok but red need a rethink?

Grumpy_Mike

No other way round, green and blue have bigger forward volts drops so you reach 20V with less LEDs than the red.

dutchboy

Awww man thought I had it!

Ok so the LEDs in my circuit:

Blue Vf = 3.1V:
So 3.1 x 5 = 15.5v

Green Vf = 3.07V:
So 3.07 x 5 = 15.35V

What am I missing?


Grumpy_Mike

Quote
What am I missing?

The fact that on that schematic you showed 10 LEDs in series not 5. This changes the maths.  :)

dutchboy

#24
Nov 22, 2011, 05:27 pm Last Edit: Nov 22, 2011, 05:59 pm by dutchboy Reason: 1
Oh I thought I had done the red as 10 in series and the blue and green as 2 x 5 series each, I will check!

Edit: I've check my circuit and the red led's have 10 in series and the blue and green have 2 x 5 led series in parallel each.

dc42

The green and the blues look ok, but the voltage drop across 10 red LEDs is too close to 20v for the series resistor to regulate the current well enough. Go for two chains of 5 red leds too.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dutchboy

Thanks dc42 so should be good with this final schem!

Grumpy_Mike


dutchboy

Okey dokey, off to produce a few pcb's. Can't thank everyone enough for your patience and help.

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