Hi guys...
I was just wondering if you could help me with a project i'm working on...

All I need to know is what resistors I should use and what rating power supply is needed.

here' what i need hooked up:


50x Blue LED

@

Forward Voltage : 3.2v – 3.8v

Forward Current : 20mA (Typical) 30mA (Max)


+


130x Red LED

@

Forward Voltage : 1.8v – 2.2v

Forward Current : 20mA (Typical) 30mA (Max)


+


1x Fan

12v

0.07a

I need to connect them in rows like this:

the thing that concerns me is the different voltages that the red and blue have...

Thanks in Advance....

 

For paralleling different voltage LED's, the best way is to connect a resistor in series with each LED.
So that all the LED's will be at it's max even though they have different Vf.
But the resistor calculation depends on the PSU that you planning to use.
Post ur power supply spec.

For the no. of led's u have posted gives a current rating of at least 5 Amps. (typical)
Voltage can be 3.3V or 5V or 12V .
So you choose what you can get ur hands on

 

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

You will need two different types of resistors for each color of LED.

You will need to decide on a power supply too, batteries generally don't last long. LEDs are more efficient than incandescent, but they still use power.

If the power supply is not regulated you will need to build extra circuitry. Regulated power supplies are pretty easy to get though.

 

BTW, I recommend a 24 VDC power supply. More voltage becomes hazardous. Something like this perhaps...

http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12940

A wall wart would be better though, especially if you don't have any experience building AC projects.

At 24V you will need around 600ma. At 12V you will need around 2A power supplies.

 

You can connect different colored LED's in series. The current thru the series string will be the same for all the LED's. On problem is that the intensity for the different colors may be different. Even thou the LED's are rated a a typical current of 20mA doesn't mean you have to power them with that much current. The current does affect the intensity, and it is alway not linear. For example going from 10mA to 20mA is not going to always double the intensity.

Looking at the picture you posted there are 15 by 15 rows. I would find a 24VDC @ 600mA wall wart and make 30 strings of LED's 15 strings of 7 LED's in series and 15 strings of 8 LED's in series. I would use a 470 ohm 1/4 watt resistor in series with each string.

 

As long as the voltage drops add up to the same for each LED string you are good to go, but this is an assumption at this time. We're still waiting to hear what kind of power supply the OP is planning on using.

 

Hey guys, thanks for all your input.

I want them to be at max brightness and the power supplies i have are:


9v - 1000mA 9VA

+

17v - 400mA - 6.8VA

+

12v - 12VA


Would one of those be suitable??

I dont want to buy another unless its really nesassery as I have a very tight budget and still need to get resistors.

Thanks

J_R

 

You missed a key comment, these power supplies will need to be regulated. If they don't have them as part of the wall wart (a typical situation) you will likely have to add it.

You can do this without regulation, but it means you have to be very careful, actually measuring the current these LEDs get.

In this case, VA is equal to Watts (with DC circuits this is the case).

The 12V 12VA may work (I don't think it will, but math will tell). VA is Volt Amps, you will get around 1 amp out of this unit, it is not enough. Reread post #4.

This power supply will vary it's voltage depending on the load. So until it is fully loaded you don't really know what it will put out. Worse, most of them have internal fuses, and it isn't meant to be replaceable. If it blows you will have to buy a new wall wart. If you want to go through the exercise using the 12V wall wart I will help, but I think it's life span will be short. It may work for a week, but it is going to blow eventually.

You have 50 blue LEDs, each drawing 0.02A and dropping 3.8V. Since watts equals voltage X amps (X quantity), this means the blue LEDs will draw 3.8 watts.

You also have 130 red LEDs, going through the same math using 2.2V it is 5.72 watts.

The fan is another .84 Watts. This adds up to a grand total of 10.36 watts. Since nothing is 100% efficient you will need double that, or 20.72 watts (or VA).

If this was a 12V power supply you would need 1.7A (pretty close to my original estimate of 2A). Increase the voltage as I suggested and the efficiencies go up.

The short of it is you need a different wall wart or power supply. If you can find one that is regulated that is a major bonus, also a higher voltage is good.

The good news is resistors are very inexpensive. My personal cost is 2¢ each, 3¢ in small quantities.

We can help you find parts, but we need to know what part of the world you are in. You'll note the old hands list their city, state, or country that shows up in the right hand upper corner (part of the profile settings).

 

Here is a hypothetical design using 12V.

It would draw around 0.92 amps. That 12V wall wart might work, I wouldn't do it, but it is up to you.

It would be worth making / finding a stable 12V source to tweak these resistor values. You can actually use the resistors themselves to measure current using Ohm's Law, I = V / R. Measure the voltage on the resistor, then divide by the resistance.

 

Hi Bill, Thanks for the swift reply.

i found this...

http://cgi.ebay.co.uk/REGULATED-12V-DC-2AMP-POWER-SUPPLY-SWITCH-MODE-CCTV-/120458731725

Price is ok and i can just cut the end off..


If you could tell me what resistors I need to get so I can have a row of 13 red and a row of 13 red and blue.
So I get 13 rows of 13 and i'll just leave the other 11 as spares

I really appreciate your help,

Thanks

J_R

 

The problem is we have an approximate Vf, you need to measure them a little more exactly. Turn 10 of each on (I assume you already have the LEDs), measure the voltage (I also assume you have a meter), and let me know what the values are. I could guess at it, but it would be better with real numbers. The specs given are kinda sloppy.

I'll think about the arrangement. How many blue in each row of 13? You could break it down, say 11 rows have 4, 2 rows have 3 blue leds.

 

Hi Bill,

I'm not quite sure what you mean 'turn 10 of each on'.

I have the leds and a multimeter, do you mean connect them to a power supply?

I found a led calculator and have created a circuit diagram based on that(see attached)
these are the first two rows...

What do you think?

 

You can't use a resistor value as low as 1 ohm to limit the current to 20mA. 1 ohm is a piece of wire.
If the supply voltage is slightly low or the voltage of the LEDs is slightly high then they will be very dim or will not light.
If the supply voltage is slightly high or the voltage of the LEDs is slightly high then they burn out.

You need at least 1/10th the power supply voltage across the current-limiting resistor so if the power supply is 12V then a resistor of 75 ohms limits the current to 20mA for three 3.5V LEDs in series.

 

Qty 6 at 2.2V (Vf of red) = 13.2V. You have exceeded the voltage of the power supply, an illegal condition.

If the red LEDs are 2.0V then Qty 6 = 12V, another illegal condition.

If the red LEDs are 1.8V then Qty 6 = 10.8V, you would need 60Ω

This is why you need to measure the LEDs, the variation you give is too great. We need good numbers.

Use a 1KΩ resistor and a 9V battery and measure the voltage on 10 of those suckers.

This is also why we don't use too many LEDs per chain, and why I recommended you getting more voltage on your power supply. 10 LEDs is a valid statistical sample, no two LEDs are quite the same (close, but not the same).

 

Wouldn't be easy if he just wire a 220 ohm in series with each and give 12V

 

Do the math, the fewer per chain, the more current it takes total. Efficiency is worth something.

OK, I drew a possible pattern. It has a 48 blue LEDs and 121 red LEDs, 169 total.

Is this what you want?

 

Bill, your pattern makes me dizzy when I scroll it.

 

Hey, Thanks for the layout that would do just fine...

I've been searcing for a 1kohm resistor for testing but cant find any, would 5x 200ohms be the same?

Also, I found 2 more power supplies during my search which are...

(Probably Unregulated)

12v 3.8A

+

24v 3VA

I would love to be able to use one of those if possible.

I'm Gonna test the voltage drop across the LEDs 2mro morning and will let you know.....

Thanks Guys

 

I love that you named this thread "Simple LED Project".

You know you jinxed yourself with that title.

But then again, Bill_Marsden is the man, he will do you right.

 

The 24VDC is much better. Is that what you're going to use, and is it regulated? I need to know to go from here.

Yes, the 5X200Ω would be fine.