x100 LEDs - Is a Resistor needed @ 3.0v ?

[spondootre]

Hi,

I am setting up a 100 LED lighting system, i'm going to use a computer PSU 3v rail to power them. I don't want to solder individual resistors to each LED so what size resistor would I need to run all 100 safely? i'm thinking about 0.5 Ohm, 10W wire wound? Or is this completely wrong? Or is there NO need for resistors when using this type of supply??

LEDs are 75mA MAX , 3.0v supply.

Malc

 

[Hero999]

I've never seen any LEDs with a forward voltage drop of 3V before. What colour are these LEDs?

If they're infrared, red, orange, amber, yellow, or yellowish green then they'll work but if they're blueish green, cyan, blue, violet, ultraviolet, white or magenta then you'll need a higher voltage.

Sorry it's hard luck you need a separate resistor for each LED. this is unless you're certain they were all manufactured from the same piece of silicon and only the manufacturer can assure that.

 

[Tarsil]

I used white LEDs at 3.6V without a resistor. Where 10mm/50mA LEDs ..made in China. Got a bit warm but that's it. If pulses used u can put them directly at 4V+ (below 5V thought).
About 5mm leds...hmm and 3V. I've seen green led's giving light at ~2.8V but was a very very dim light.
DO NOT use one resistor fo all the led's. Ev use one for 5 pcs or so ....all thought.....

 

[RODALCO]

Are you sure that it is 75 mA continuous or peak ?
Most LED's are rated for 20 mA dc continuous and may be pulsed at a higher current as long there is enough cooling time in between for the LED junction.
Say ratio 1 to 4 ( 75 mA - for 1 mSec then 0 mA for next 3 mSec's ) as example.
Each LED requires it's own resistor to allowe for correct voltage shareing.
If not then the LED with the lowest threashold voltage will fail first then the next and so on till only a few are left.
What colour are youre 3 Volt LED's? It's an odd working voltage.

 

[spondootre]

Thanks for the replies guys, I am sorry i've not been very clear or accurate!! The LEDs are slow colour change ones from here :

**broken link removed**

and have a forward voltage of <=3.6v, I am using an ATX PSU 3.3v rail to power them, so would I still need resistors as the supply is less than the max LED voltage? I have already wired it up without resistors and it runs OK, but is this just false security? Do I need to add resistors or even 1 resistor to be safe??

 

[Hero999]

I don't know, I'll need to see a datasheet to be sure. These sorts of LEDs normally have a built in current limiter so no resistors are required, but you'll need a higher voltage than 3.6V, 4V is normally the minimum.

 

[spondootre]

Running as they are they are bright enough @ 3.3v so I won't need to up the voltage, Just to add to the confusion I am also using another 100 5mm white LEDs which have a rating of 3.0-3.6v and 20mA on another supply. I've been using the online resistor calculators for LEDs and they all say the resistor I need is 0 Ohm!! So I am wondering if my origonal thought was correct that if they are running at or below there rating voltage they wouldn't need to have a current limiting resistor as they will only draw their max current of 75mA / 20 mA

 

[Nigel Goodwin]

It's your funeral! - but LED's require current limiting - and you're not giving them that, if you do this make sure you build it so it's VERY easy to replace blown LED's, as it's likely to be a frequent operation!.

Why not do the job properly?, use a higher voltage supply and current limiting resistors, with LED's in series per resistor.

 

[Hero999]

Have you actually measured the current with a multimeter?

The problem with running LEDs of a constant voltage source without any resistor is their voltage can vary qute a bit. One LED might be 3.3V while another 3.7V, from a 3.3V supply one might draw 100mA while another might only draw 10mA, even if they're both the same type.

Your LEDs aren't normal LEDs, they have a built in IC thant changes the colou, this IC might have current control built in or it might not. You really need to look at the datasheet.

 

[spondootre]

OK, so @ 3.3v what size resistor would I need to be safe? Or would I be better switching to the 5v rail? What resistor should I use with that? I don't want to be taking risks with it, I want to install it and forget about it!! not maintain it every 5 minutes, they are all already wired up to the PSU without resistors on the 3.3v rail but they aren't installed in location yet so adding resistors is not that big a deal at the moment.

So should I stick with 3.3 + resistors or jump to 5v with resistors?

 

[Nigel Goodwin]

As it's in a PC I would suggest the 12V rail, connect the LED's in three's in series, with a resistor for each three.

 

[spondootre]

That's a big alteration now, I can add resistors to each line easily but not re-wire the LEDs in series now so I would need to be adding a resistor to each line as they are curently wired. The other problem is that the ATX PSU gives a max of 10A @ 12V and x100 LEDs that run at 75mA is 7.5A, whereas the 5V rail claims to have a max of 25A. So should I go with the 5V rail and a suitable resistor value? given the wiring and PSU problems? Also could you advise what would be a suitable resistor ratiing for the colour change and white LEDs, 75mA and 20mA @ 5V

Thanks for your help!!

 

[spondootre]

Again I think i've misled what's happenning!! It's not actually going into a PC it is going into a bathroom ceiling, to be turned on and off via a capacitive switch hidden behind tiling, the power supplies are just from a PC.

 

[Hero999]

You're not listening, these don't behave like ordinary LEDs!

You should email the supplier, asking them if you can power them directly from a constant voltage source or wheter you need a current limiting resistor or not.

 

[spondootre]

Thanks Hero999,

There are a lot of conflicting views!! I have alreasy emailed both of the suppliers of the LEDs after your 1st post, the UK supplier (white leds) has said they will not need a resistor to be running withing their working voltage parameters and I await the response from the Hong Kong supplier whom is usually prompt to reply to questions.

Thanks again

 

[audioguru]

Their max continuous current rating is 30mA, not 75mA.
So operate them at about 25mA max because you have them enclosed and they won't have open air circulation for cooling.

Their voltage is not 3.0V nor 3.6V but is anything in between. Some won't light with a supply of 3.3V and others will burn out.
So use a higher supply voltage with a current-limiting resistor for each one, like the instructions say to do.

 

[user1453]

couldn't he wire all the leds in series, use a 300v supply and a resistor?

 

[audioguru]

couldn't he wire all the leds in series, use a 300v supply and a resistor?

Then if some need 3.6V like in their spec then none will light.
Maybe use 400V or 500V and a resistor.

 

[fingaz]

hi,

well, everyone seems to have given advice about the current limiting resistor. It really depends on whether you've got them in series or parallel. you could go here

http://www.metku.net/index.html?path=mods/ledcalc/index_eng

for an explanation of how to calculate resistor requirements for any given led's. . .

then you don't have to keep asking. hope this helps. . .

 

[Nigel Goodwin]

The calculations are trivial to do - but you shouldn't put LED's in parallel, it's SERIOUSLY bad practice and gives poor reliability.