Multiple SMD LEDs running off 12v, little help please.

[gaskell]

Hi everyone, this is my first post.

The idea to what I am doing is replacing the lights in an old LCD screen with LEDs. The reason for this is getting the bulbs for the screen is an expensive task IF you can even find them and running different bulbs requires an inverter circuit added so I am converting to LED's and running straight for the PC power supply using a spare molex socket.

The LEDs I have ordered are tiny surface mount ones. I was always good at soldering when I did my electronics course altho it was so many years ago a lot of what I learned has been forgotten.
The LED specs are:
0603 SMD LED
3.0V - 3.2V
I(mA) 20

Hopefully this helps.
I am thinking of hooking these into a number of 3 LEDs, 1 resistor circuits. Each circuit is series with 4 or 5 of these circuits running parallel together for the bottom and the same for the top, all running off the single molex.
I hope this makes sense.
I was told to run 3LEDs + a 120ohm resistor instead of running 4LEDs
Does this sound right? If so there are so many different SMD 120 ohm resistors I'm not sure what wattage rating is right.

Are there any issues with running this many parallel circuits together from a single molex connector from the PC PSU?


The area available for this is about 3-4mm wide by 300mm long (oldschool 4:3 screen). I was thinking of getting the copper CB sheets that you use to etch your own circuit boards with and use a craft knife to make 1 board 300mm long by 3mm wide with 2 copper lines, one along top and one along bottom. Basicly it would be LED, LED, LED, resistor. With alternating breaks between the LEDs. The positive wire is connected to the top copper line and then after the resistor the negative wire, then resistor, LED, LED, LED, positive wire....
I hope this is easy to follow.

In a summary I'm after the resistor value and making sure this is fine to run this many parallel circuits from one 12v molex.

Big Thankyou to everyone reading this wall of text and helping me out.

[admin]

The LEDs need about 20mA current and 3.1V drops off each LED. So if you use a 12V supply you cannot have 4 in series as that would drop more voltage than the 12V supply. So 3 LEDs in series is correct for each branch. The resistor value would be:

R= (12V - (No. of LEDs in series x Voltage drop of each LED))/LED Current

So in your case, if we limit the current to 18mA (20mA would be max. rating):

R = (12- (3.1x3))/0.018 = 150ohm.

Check what is the max. current rating of the 12V output of your SMPS, then number of parallel branches you can have (assuming this 12V isn't powering something else) would be:

No. branches = Current rating of 12V supply in amps/0.018

Eg, if your 12V supply is rated for 500mA or 0.5A, then

No. of branches = 0.5/0.018 = 27

The above driving method would have poor efficiency since the resistor drops of voltage and wastes power. A more professional approach to driving LEDs is to use a switching constant current supply. Nowadays there are many LED driver chips from companies like National semiconductor, Texas instruments and Fairchild Semiconductors.

Hope this helps.

[gaskell]

Excellent, Thankyou so much.
Now I guess I'm going to have a lot of reading and research to try and work out the whole LED driver thing. This does have to be cheap tho.
If I was to run the circuit with the resistor it would waste power, but with the enegy efficiency of LEDs compared to bulbs would this be more wasteful?

As I see it, the above is doable but not a good solution. If the drivers work out to be too costly this may be an option but I will look into driver solutions and see what I can find.
Unfortunately I am in NZ and electronic components are not as cheap as other countries. The LEDs for example I ended up ordering from china for about 10% of the cost of buying here in NZ. Now I have to wait around 3 weeks for them to get here.

[admin]

Hi Gaskell,
For a homebrew project the resistor in series should be more than sufficient! I wouldn't recommend using a switchmode driver for a hobby project.

The wastage would be very small about 0.05W per branch, which is negligible.

[gaskell]

ok cool. So now to work out how many branches will provide good light.
Now comes the fun of playing around with it.

[admin]

Keep up posted how it goes!

[gaskell]

little update.
The parts take ages to arrive but I guess thats one thing you have to put up with when ordering from overseas. The price difference is ridiculous.

I bought a 300mmX300mm sheet of blank circuit board copper/fibreglass (I dont know what its called but I call it custom PCB). That was the easy task. cutting it was the next issue. I found 1 place where I live that could do it but unfortunetly not as accurate as I would like. I now have about 20 strips of 300mmX4mm which are tapered so one end is about 3.9mm and the other is about 4.2mm so I have to file each one down to fit into the metal reflector strip.
Currently I am cutting the mid strip out with a ruler and craft knife, long slow process. When done I will end up with 2 copper strips with a gap in the middle (kinda like railway lines).
My LEDs have arrived but I'm still waiting for the resistors. I'm thinking of 30LEDs per strip (1 strip for top and 1 strip for bottom). The sheet of PCB is actually about 310mm so 5mm gap at each end then 1 LED every 10mm which leaves room for adding resistors later.
Once I get a bit further on I'll upload a couple pics of my progress. I'm hoping that the 60LEDs will light up the LCD enough.

[gaskell]

I am hoping that the single strip of 30 will light the screen as due to wire thickness I can only fit one strip.
Heres a cple pics of the finished strip. I resized them down and put them all together into 1 pic.
Next I'll put this into the screen and see what it looks like.