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Schematic circuit for 25 leds Answered

I  want to use 10   3m Blue  3.3Vf  20If  and 15   3mm Green 2.2Vf  30If LEDS  spaced over about 24" or 600 mm  using a 12 V AC 500ma  power supply  They can be wired in 2 arrays ( blue set  and green set    Could you kind people put me on the right track to design a safe working circuit for this project  I would like a on/off switch and say 1/2 watt  fuse or what ever size fuse is required  
thank you for your help



Best Answer 8 years ago

If you can get one, I would recommend running them on a 12VDC transformer, as it will simplify things. You will run the green and blue leds in separate series strands, parallel to one another, each with its own current limiting resistor. http://led.linear1.org/led.wiz will do the calculations.
Blue:  enter the numbers:  12, 3.3, 20, 10. -- it wil suggest a couple options.
Green, again, enter the numbers 12, 2.2, 30, 15...whatever this spits out, build that and run it as its own circuit in parallel to the blues.

Hi I,m a retired engineer Electronics is out of my line and this is my first post so bear with me Please explain the difference to me and which 12 V unit I should use 12VDC 300mA 12VOLT AC/500mA I buy these transformers just reading the output (12V) thank you for calculator web site I would like to mount the resistors, fuse, toggle switch etc in a box What type of connector or junction box ( a simple type quick release ) would be great to connect the 2array wire looms too an engineer trying to write about electronic stuff is hopeless Make some sense out of this please (part names would be great) Thank you for your time Arob2665

Willard's reply shows the circuit diagram you need (based on the output of the wizard website), Hooking it up would be as simple as soldering the strings of leds with their respective resistors -- then attaching those in parallel...likely with just small gauge wire and solder. The components are a bit fine to use much bigger to connect them.

Thanks fellers for your Tutorials you have been a great help Take care arob2665

Here is the circuit that frollard is explaining.  It will only work using 12VDC not AC


Hi Willard2.0
I would like to control the brightness of each array of leads Like a lounge light dimmer switch So what parts would I need to add to this circuit to achieve this Show me how to connect things up between on/ off switch and the array's Thank you very much You look pretty smart under Graduation cap Willard
Have a great day

I'm not sure how you would do that. I know the basics of LED wiring. I don't know how to dim them, other than lowering the voltage.

I would post your current circuit in a new question and ask how to dim each array individually.

Thanks for the compliment on the grad cap.

thank you Willard2.0 I will do just that in the next few days
have a good one

To dim the LEDs, you want to pulse the power to them. Do this with some transistors switching to ground on each string, a 1k resistor on the base of the transistor, and some kind of variable duty cycle square wave supply. You could try find an example using a 555 timer. Look for a 555 PWM controller. You will need one of these per string if you want to alter the brightness of each string, and you will need to manually control the potentiometers.

You might also try using a microcontroller such as the arduino to perform the PWM switching.

Hi Wilard2.0 Thank you for a great diagram easy for a old codger to understand have a great day arob

It would work with AC if you put the greens and blues in reverse polarity, but would be just bad form...They would act as their own bridge rectifier. Trouble is AC is the average RMS voltage. At 12VAC you'd get peaks of about 17 or 18 volts, and your leds would die a quick and painful death.

Well, all you do is check that the maximum LED current is exceeded at Vpk Its BETTER run on AC than DC, from the point of view of the transformer.

Agreed. Also, I hate the visible flicker of leds when viewed in motion running on AC :S

*and you are WAY too nice :D I like to make people explore, think, and learn. If you feed it to them they will never understand why you asked them to do it a specific way.

Yes, I know I'm too nice. I agree with what you're saying, but sometimes it's easier to understand a picture than verbal diarrhea (no offense), especially when it comes to combining series and parallel circuits where doing it wrong can fry LED's.