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wiring lots of LEDs together?

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algorithm:
Ok, so I am trying to use 98 5mm LED's to create 1 big light.
The LEDs are 3v 20mA, reverse current (uA) vr=5v max=10
My first question is what would be the best way to wire this up, pairs of 5 in series, then wiring those bundles in parallel? or...?

Another question, what would be the best way to do this? both inside and out?
Should I get a bunch of little blank boards for all those LEDs? or...?
Also, what would be the best way to make a case for this light? Would a pan work? I think I will need something to soak some heat up.

Thanks in advance for any advice.

Monittosan:
leaving the full lead lengths of the leds and having them sit standing off the pcb should allow for some cooling.
Else you could drive them at a lower current to increase life and reduce heat.

mariush:
My advice would be to wire them in such a way that you'll be able to use a common power supply.

For example, I would aim for a 12v or 24v power supply or a 18.5v laptop adapter.
As the forward voltage won't be exactly 3v for all leds, it's a good idea to make individual chains of leds that have the total voltage a bit lower than the power supply's voltage so you can properly limit the current going into the leds instead of blowing them up.

For a 12v power supply, 3 leds per chain would work fine... 3 x 3v = 9v. For 24v, I'd make chains of 7 leds .. 7x3 = 21v  ... this would also make it exactly 14 chains of leds.  With 18.5v , you'd make chains of 6 leds  6x3 = 18v.

You need to limit the current going into the leds, and the simplest way is to add a resistor in series with the leds. The value of the resistor you calculate with the simple formula :

[ Input Voltage - Led Forward Voltage (total)  ]  / Current.

For 12v , 3x3 leds you would have   (12v - 9v ) / 0.02 = 150 ohm   ... and the power dissipated in resistor is P = IxIxR  = 0.02x0.02x150 = 0.06w so a resistor rated for 0.125w will do.
For 24v , 7x3 leds you would have (24-21v) / 0.02 = 150 ohm .. same value.
For 18.5v , 6x3 leds you would have (18.5 - 18) / 0.02 =  25 ohm 

Once you figure out how many you want in series, with the resistor limiting the current each chain will use exactly 20mA at that input voltage, so you can multiply with the number of chains to get the current amount. For example, for 24v power supply, you'd have 98/7 = 14 chains so that means you need a power supply capable of  20mA x 14  = 280mA.

algorithm:

--- Quote from: mariush on August 19, 2014, 11:39:48 am ---My advice would be to wire them in such a way that you'll be able to use a common power supply.

For example, I would aim for a 12v or 24v power supply or a 18.5v laptop adapter.
As the forward voltage won't be exactly 3v for all leds, it's a good idea to make individual chains of leds that have the total voltage a bit lower than the power supply's voltage so you can properly limit the current going into the leds instead of blowing them up.

For a 12v power supply, 3 leds per chain would work fine... 3 x 3v = 9v. For 24v, I'd make chains of 7 leds .. 7x3 = 21v  ... this would also make it exactly 14 chains of leds.  With 18.5v , you'd make chains of 6 leds  6x3 = 18v.

You need to limit the current going into the leds, and the simplest way is to add a resistor in series with the leds. The value of the resistor you calculate with the simple formula :

[ Input Voltage - Led Forward Voltage (total)  ]  / Current.

For 12v , 3x3 leds you would have   (12v - 9v ) / 0.02 = 150 ohm   ... and the power dissipated in resistor is P = IxIxR  = 0.02x0.02x150 = 0.06w so a resistor rated for 0.125w will do.
For 24v , 7x3 leds you would have (24-21v) / 0.02 = 150 ohm .. same value.
For 18.5v , 6x3 leds you would have (18.5 - 18) / 0.02 =  25 ohm 

Once you figure out how many you want in series, with the resistor limiting the current each chain will use exactly 20mA at that input voltage, so you can multiply with the number of chains to get the current amount. For example, for 24v power supply, you'd have 98/7 = 14 chains so that means you need a power supply capable of  20mA x 14  = 280mA.

--- End quote ---

Beautiful, thank you very much.
I suppose that just about solves it. The only thing left now is to decide what to mount the strip board onto.
Thanks again

christopher iles:
Hello,

I made some Christmas lights with 100 leds, All I did was to solder a 1k 1/4 watt resistor to one leg of each led and then connected them all up in parallel just like you would bulbs.

I powered the lot from an old 12v dc power supply and they worked (and still do) fine.

They don't even get warm at all.

Hope this is of use.

Regards,

Chris.

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