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Imperf3kt
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How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 12:52 am

I hate my camera, it takes very poor quality images despite being 12MP
Either the light is too dark or the flash is too bright.

So, I have decided to use my Pi0 and some scrap melamine particleboard to create a sort of box where I can control the lighting conditions.

I settled on using five 3535 SMD LEDs
Which I'll buy from here: https://au.rs-online.com/mobile/p/visible-leds/8140866/

I intend to recycle a disused 12v, 1.5A SMPS to power the LEDs and the Pi (with a buck converter between)

I used an online calculator to determine the best way to place the LEDs and it has given me the following result:
Image
http://led.linear1.org/led.wiz

I am a little concerned with the warning it gave me. I don't fully understand the data so don't know what the implications are.
Will it just get excessively hot? I have heaps of aluminium to construct heatpipes and such.
Mostly though, I am concerned about efficiency and the LEDs stated forward voltage. The datasheet says 5.8v minimum, 6.1v typical, 6.8v maximum.


My experience with LEDs is limited to some wires and a battery that equals the total forward voltage, so I don't understand much about resistors and diodes in general.

Can anybody explain this message and of course, how should I control this from the Pi?
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klricks
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 2:54 am

Each resistor dissipates just over 1 Watt so the next available standard resistor wattage size is 2W.
Even 1W resistors are fairly large in physical size. 1W might be OK depending on usage or duty cycle, ambient conditions etc..... but I would go with 2W.

You will need some form of cooling depending on conditions.

To control this with the RPi you would need to use an add on relay board or some form of transistor / FET.
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KLL
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 3:13 am

how about (2*) 2 LED in serial ( and a very small resistor only )
that would be much more efficient.

http://docs-asia.electrocomponents.com/ ... 3f0b1b.pdf page 13

danjperron
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 3:17 am

Why not adding a DC/DC regulator to set the voltage to 7..7.5V and adjust the resistors to the correct current. This way the resistor won't dissipate so much and you will have reduce the total consumption.

http://www.dx.com/p/diy-dc-dc-adjustabl ... ioB3kria00

B.T.W There is an enable pin that you could used to control the regulator but you will need to solder a wire and add a transistor for GPIO control.

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Imperf3kt
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 9:03 am

I have considered different voltages. Just couldn't find a nice value, but a little after I posted this I found either 7.4v or 14.8v looked good. I could get that with 18650s, but I would prefer to use the 12v powerpack.

Thanks for the replies, now I've got somewhere to start googling. I'll look into all the suggestions, thanks!
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wh7qq
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 10:18 pm

Sorry this posting is a bit late...my ISP crapped out just as I was writting it last evening. What you don't know is how consistent (or not) the 3535smd is from part to part, or even if you would get LEDs off the same die. Trying to drive all 5 off the same power bus is going to be a problem for this reason.: The LED with the lowest Vforward is probably going to hog the current and might even fry itself in the process. You will need to have a constant current driver for each LED. Search on google for "constant current LED driver". They are available in discrete packages from various manufacturers.

You don't tell us what you are using the RPi for in this case. If you are doing some kind of PWM to control the LEDs, it is a whole additional can of worms because LED's do not have persistence and syncing them to the actual shutter opening will be difficult. LEDs are on and off much faster than we can perceive so we can't see their on and off but your camera's image sensor can.

I would not try to power the RPi from the same supply as the LED's because there may not be enough current left over after the LEDs, especially if you have a USB bus with anything drawing serious current. Even small wires will drop too much voltage just coming from the supply. RPi power bricks are really cheap.
Sent by WH7QQ from an RPi3/Raspbian/pixel desktop.

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davidcoton
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Re: How to efficiently power 5x 3535SMD LED?

Fri Dec 08, 2017 11:02 pm

wh7qq wrote:
Fri Dec 08, 2017 10:18 pm
Sorry this posting is a bit late...my ISP crapped out just as I was writting it last evening. What you don't know is how consistent (or not) the 3535smd is from part to part, or even if you would get LEDs off the same die. Trying to drive all 5 off the same power bus is going to be a problem for this reason.: The LED with the lowest Vforward is probably going to hog the current and might even fry itself in the process. You will need to have a constant current driver for each LED. Search on google for "constant current LED driver". They are available in discrete packages from various manufacturers.
As long as there is sufficient excess voltage, the series resistor (for each LED or LED string) effective forms a constant current regulator. Strictly, the calculation of its value should take into account the Vf max and min for the LED string, and this may reveal a problem if the excess voltage is low. In this case a constant current driver can be useful.
wh7qq wrote:
Fri Dec 08, 2017 10:18 pm
You don't tell us what you are using the RPi for in this case. If you are doing some kind of PWM to control the LEDs, it is a whole additional can of worms because LED's do not have persistence and syncing them to the actual shutter opening will be difficult. LEDs are on and off much faster than we can perceive so we can't see their on and off but your camera's image sensor can.
Correct. You will also want to consider the effects of different colour temperatures of white LED.
wh7qq wrote:
Fri Dec 08, 2017 10:18 pm
I would not try to power the RPi from the same supply as the LED's because there may not be enough current left over after the LEDs, especially if you have a USB bus with anything drawing serious current. Even small wires will drop too much voltage just coming from the supply. RPi power bricks are really cheap.
Yes of course the PSU must be able to supply enough current for both LEDs and Pi, but there is unlikely to be a problem with wire voltage drop if the Pi's supply is separately regulated (eg UBEC) and the wires from the UBEC to Pi are short and thick. This applies to any Pi PSU.
"If it ain't broke, fix it until it is."
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Imperf3kt
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Re: How to efficiently power 5x 3535SMD LED?

Sun Dec 10, 2017 9:56 pm

Ah right, sorry, forgot to mention the Pi's role.

I was hoping to use it to control a PWM signal to adjust the brightness. I have other SMD3535 LEDs and they're very bright. Too bright for photography, especially in a confined, reflective area xD

More research is required because as it sits right now, my knowledge is limited to powering a single LED (of the 3v variety) from a single power source that matches the fv of the LED.
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drgeoff
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Re: How to efficiently power 5x 3535SMD LED?

Sun Dec 10, 2017 10:51 pm

For efficiency and less power dissipated in the resistors, change from all LEDs (and their resistors) in parallel to a arrangement with some degree of serialism. Exact configuration will depend on the forward voltage of the LEDs and the supply voltage you want to use.

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