# understanding leds on ac

Discussion in 'General Electronics Discussion' started by sharpen047, Nov 20, 2012.

1. ### sharpen047

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Nov 20, 2012
Hey guys ive managed to get a working lamp using this instructable.
I was wondering if anyone knew how the values were found.id also like to know how to make one with say red or amber leds but would rather calculate it myself.
Any help would be great.
current leds are 3.3 vf and the reds are 1.9.
instrucables dot com /id/LED-TUBE-LGHT-AC/

2. ### sharpen047

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Nov 20, 2012
transformers and induction

Hey guys im trying to learn how to make transformers and induction coils for an led driver.
I am trying to convert 12v to 34v using induction but dont know much about it except that i need magnet wire and a core, Id like some help finding a way to calculate how many winds i need and what size core etc.
I know it needs to be ac to convert through a transformer or an induction coil and that they are current limited but id like to be able to make one myself because they are so expensive to buy and parts are cheap.

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Jan 21, 2010
4. ### sharpen047

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Nov 20, 2012
I appologize for not being clear.
I am trying to learn how to power a single 30 watt led with a forward voltage of 30-34 volts but cannot find a power supply for under 50\$ each.This is why i cannot simply rearrange the leds. With the LED costing about 15\$ its silly to spend that much on a power supply when i could just wire 500 leds up in together, but I am trying not to make a 12" by 12" panel full of leds.
I dont know how to make my own transformer and am looking for a site or page or something that will tell me how to calculate how many turns with which core etc. I do know the basics and have found a dc to ac converter (from pc power supply to transformer to rectifier to LED) to convert from 12v to 30-34.

So long story short i am looking for directions on how to calculate wire gauge turns and how large i need to make a transformer for whatever voltage i want from 120v.
A way to make 12v into 40 ish volts.
How to calculate capacitor and resistor values for leds on mains following that schematic on the instructable but instead of white, id like red (1.9v rather than 3.3 forward voltage.
thanks again!

EDIT:
Id like to make this:
http://www.ebay.com/itm/250861934069?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
to power this:
http://www.ebay.com/itm/30W-High-Po.../230884297775?pt=Lamps_US&hash=item35c1c6cc2f
and to make a 120v to 40v transformer. with 1.5 A or more current

Last edited: Nov 20, 2012
5. ### CocaCola

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Apr 7, 2012
Last edited: Nov 20, 2012
6. ### sharpen047

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Nov 20, 2012
Its more about learning how to do it so i can calculate whatever voltage i need for future projects.

7. ### CocaCola

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Apr 7, 2012
LEDs are current driven devices not voltage, and they should also have DC current not AC... This is an important fact you need to understand when you are driving LEDs...

If your LEDs are 30W and you pump in 40V into them you will need to limit the current to 750mA or you will face potential thermal overrun... See *Steve's* Sticky on LEDs...

Transformer windings are a simple ratio... If you put 120V in and want 40V out that is 120/40 or a 3:1 ratio... But, in this case you want DC and when you rectify full bridge rectify AC you get out ~1.414 the volts you put in... So in this case if the end goal is 40VDC then you want to have about 28VAC coming out... Thus a ratio of 120/28 or a 30:7 ratio... This will net you about 28VAC out once rectified to DC will be about 40VDC... But, you also have to factor in that the electric company is generally stingy, thus they rarely give you a full 120V at the outlet, in many cases (depending on location) it's closer to 110V... When using a transformer if your input varies so does the output as well, output is fully dependent upon the input... This is why most wall warts have additional regulation built in to hold the output voltage steady...

The current the transformer can deliver is determined by the number of windings in each coil of that ratio (and wire size)... So a 3:1 transformer with 300 and 100 windings will produce more current then one made with 30 and 10 windings...

Last edited: Nov 20, 2012
8. ### sharpen047

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Nov 20, 2012
Even without LEDs, some audio amplifier ics use 24v and I can't power that at 12v.
I am asking about how to calculate the current of the LEDs in that schematic so I can find out how to limit current to what I want rather than not knowing. The author of that schematic for the ac LEDs said at 120v they are driven about 20 ma at 75% duty cycle because of the 60 hz(stingy power company here is at 58 hz). I want to know how he came up with a 1k resistor and a .33 uf cap rather than a .8 uf cap an why he came up with a 1k ohm resistor rather than an 800 or 1200.
If I know those two things I can find out how to power red LEDs with a lower forward voltage.
I know LEDs are current driven and that they properly dim using pwm rather than voltage by use of resistors.
I also know that 30:10 is a 3 to 1 ratio and am wondering when a ferrite core is used rather than a toroidal core. Mainly how to calculate it.
I am new to electronics but please bear with me and my bad expliations.
I have designed and built a cnc for circuit boards and have made led dimmers, iphone chargers an audio amp and an led tachometer. But I've only copied free schematics online. I would like to be able to design a circuit for what I need and would like to continue with a dc dc boost converter from 12-??v. So if I need a 26v power supply I can make one and change the inductor for the same board as the 36v for example.
I've tried finding guides for making smps power supplies but I basically get "it's too dangerous you should start somewhere else"
I am we'll aware of the dangers of electricity and protect my ac led strip with fuses and isolators etc.

9. ### BobK

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Jan 5, 2010
Add up all the forward voltages of the LEDs.
Subtract this from 170 (the peak voltage of the 120V AC line).
That is how much voltage the capacitor has to drop.

The voltage drop of the capacitor is:

V = I Xc

Where Xc = 1 / (2 * pi * f * C)

Solving for C we get:

C = I / (2 * pi * f * V)

For example 15 3.3V 20 ma LEDs (like in the instructable)

V = 170 - 15 * 3.3 = 120.5

C = 0.020 / (2 * 3.1416 * 60 * 120.5) = .44 uF

The resistor in the instructables circuit is unnecessary, but it would drop 1000 * 0.02 = 20V and the voltage should be adjusted for that. I suspect it is there basically as a fuse.

This will set the peak current at 20ma. You would probably want to set the peak a little higher (maybe 30ma) becuase the LEDs can tolerate a little more current for short periods and each LED in well under half the time in this circuit,

Bob