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Fairly simple LED circuit help please

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denno

Member
I'm looking to put lights in a shadow box/frame, and I thought it would be fairly straight forward, but as with most things, I think I have underestimated what I need to actually do..

I have purchased white and red of these LED's off eBay:
https://www.ebay.com/itm/10-pcs-PLC...ltDomain_0&hash=item1c13a3dd9c#ht_6026wt_1181

As you can see with those LED's, they're 3-chip, and I initially thought about connecting one side of a resistor wire across the 3 terminals, but now I'm thinking that I'm supposed to put a resistor to each chip of the LED? I just want them all to light up at once. Can I get away with soldering the resistor across the 3 positive terminals? Basically connecting them all to the one point, if that makes sense..

Also, I am going to be powering 14 LED's at a time, and I want to run it off batteries... Would it be ok for me to just connect each LED in parallel with the power supply? I was thinking of using 6V (4 x AA batteries), as I don't want the batteries to take up much space.

Sorry for writing so much, but I like to make my question clear, so hopefully someone will take the time to read through it..

Thanks in advance
Denno
 

audioguru

Well-Known Member
Most Helpful Member
The contacts on the Chinese LEDs are very close together and are underneath because their case is surface-mount. The sketch does not show the functions of the 6 contacts.

4 x AA alkaline battery cells make 6V when new and drop to about 4V when they are almost dead. The White LED forward voltage is from 3.2V to maybe 3.6V so it will be difficult to calculate a current-limiting resistor and the LEDs will be fairly dim when the battery voltage drops. The red LED forward voltage is from 2.8V to maybe 3.2V so it will also be difficult to calculate the resistor.

The sales sheet does not show if the 3-chips in each LED product are separate or are already connected in parallel, they are probably separate and each chip probably has two contacts on the case.
They do not say if each chip needs its own current-limiting resistor or if it is safe to parallel all 3-chips on one product then use a single resistor for each product.

There are many unknowns when you buy Chinese products from E-Bay.
 

ikalogic

Member
Each led should have its resistors.
yes it's okay if you connect all resistors to the same point.
your device - running on AAs - wont last for long. certainely not if they are alkaline. try Ni-MH rechargable.
 

KJ6EAD

Active Member
Do you know the pinout? There's scant data from the seller.
View attachment 63566
They have this drawing with pin numbers but no indication of which pins go to which die or any polarities. Do you know how to find out?
 
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denno

Member
Okay, so now I'm thinking I should have just bought the single chip one to avoid this confusion :/.. I was just going to use trial and error to figure out the contacts and which is the positive/negative sides.

I was afraid that the batteries wouldn't last that long.. I do have an AC to DC 12V power supply, so maybe I'll switch to using that as the power supply instead of batteries..

Do you guys have any other suggestions of how to connect it all up with this many LED's? Using the 12V power supply I'll obviously connect them in groups of 2-3 LED's in series

Thank you for the replies, they're very much appreciated

Denno
 

panic mode

Well-Known Member
which would raise operating voltage...
 

KJ6EAD

Active Member
...to 10.2V per device (90Ω @ 20mA) for the white LEDs and 9V per device (150Ω @ 20mA) for the red LEDs, which is quite manageable using the 12V supply.
 

denno

Member
@KJ6EAD, did you just finish the sentence started by panic mode? It looks alot like you did lol..

Anyway, thankyou for the confirmation.

How do people power their huge LED arrays? For example, the white LED suit that a snowboarder recently wore while shredding down the slopes and being recorded.. I'm 110% certain there isn't an extension chord trailing him to a AC to DC converter :/..

Thanks heaps
Denno
 

KJ6EAD

Active Member
@KJ6EAD, did you just finish the sentence started by panic mode? It looks alot like you did lol..
I saw the ellipsis and couldn't help myself.

How do people power their huge LED arrays?
They use several of these strategies: They use good LEDs from Cree or Nichia, high density bateries with spares available to change between shots, boost converters and microcontrolled multiplexed matrices. You or I might want 10 or 100 LEDs to work in an art exhibit for a few hours but many commercial effects only need to run for 5 or 10 minutes on a battery while recording.
 
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denno

Member
How would you or I create such a thing that wouldn't drain the batteries so quickly? Or is it just a matter of using more batteries in parallel so as to increase the effective current capacity of the power supply..

Instead of explaining it all yourself, if you know of any examples of people blogs where they've talked about their power management and stuff? That would be greatly helpful.

Thanks
Denno
 

audioguru

Well-Known Member
Most Helpful Member
How would you or I create such a thing that wouldn't drain the batteries so quickly? Or is it just a matter of using more batteries in parallel so as to increase the effective current capacity of the power supply.

Instead of explaining it all yourself, if you know of any examples of people blogs where they've talked about their power management and stuff? That would be greatly helpful.
It takes a lot of power to make a lot of light. If the light is dim then the battery will last longer or a smaller battery can be used.
For a lot of bright lights to be turned on for a long time then use a larger and heavier battery.
Some batteries are cheap, large and heavy (lead-acid). Other batteries are expensive, small and lightweight (Li-Po).
 

colin55

Well-Known Member
Supply voltage . . . . Resistor value
5V . . . . . . . . . . . . 91 Ohm
9V . . . . . . . . . . . . . . . 330 Ohm
12V . . . . . . . . . . . . . . . 470 Ohm
13,8V (Car) . . . . . . . . . 560 Ohm

View attachment 63612

A very messy device, when you can get the same effect with a LED-chip having just 2 leads and requires 3.4v @20mA.

To get white light out of this 3-chip device, you need three resistors of 100R to illuminate the three colours and this will require 60mA for a 5v supply.
 
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audioguru

Well-Known Member
Most Helpful Member
To get white light out of this 3-chip device, you need three resistors of 100R to illuminate the three colours and this will require 60mA for a 5v supply.
The LED has 3-chips with the same colour. If it was an RGB multi-colour LED then the currents must be adjusted to produce white.
 

denno

Member
Ah ok. That's what I feared, that I would need 3 resistors for each of the chips in the LED.. Really wishing I just bought single chip LED's now :/..

Thankyou all for your replies, I really appreciate them.. I think I'm just going to go ahead and use 1 of the chips from each LED, which should still provide enough light for what I'm doing.

I'm still very interested in any other large scale LED projects that run off batteries and how they do so, if anyone know's of any?

Thanks again,
Denno
 

panic mode

Well-Known Member
it was already mentioned that you can connect the three elements of the device either :

individually (each with own series resistor, for example when operating from low voltage such as 5V) or

in series (when operating from larger voltage source such as 12V or so). in this case you can use one series resistor for all three elements.

to calculate series resistor you can use

R=(Vin-n*Vd)/Id

where
Vin = supply voltage (5, 9, 12V or whatever)
Vd = forward voltage of single element
n = number of elements in branch
Id = forward current (in Amps), such as 0.020A (=20mA)

grouping more elements in strings of LEDs allows reducing amount of power wasted by resistors (and reduces number of components).

If for example you have 24V or higher at your disposal, you can connect more elements in series (from 2, 3 etc LED "chips")
It's the same principle.

they don't have to be in same string either. for example you can connect one of elements with one resistor, and remaining two elements with another resistor (if the available voltage permits two but not three LEDs in series for example). you can also connect elements of two chips in series (element 1 from both chips in one string, element 2 from both chips into second string, etc.).

It all depends on what you have there (voltage of power source) and a bit of creativity.
 
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KJ6EAD

Active Member
One reason for putting 3 die of the same color with separate leads in a single package is to allow simple brightness control in three steps with only a switch.

For some other low power LED projects take a look at the Adafruit and Sparkfun sites. To run a strip of LEDs for illumination efficiently from a low voltage battery supply look for LED boost converter drivers such as the BoostPuck.
 
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