# 3.3v Power Supply

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#### -Kitsune-

##### New Member
Hi all,

I'm a real noob at electronics and only really know the very basic principles.
I am looking for a good way to make a step down transformer to power high power 3W LED's, efficiency is key.

I have 2 input voltages I would like to use:
#1: 220v AC 50Hz
#2: 12v DC

The output voltage needs to be 3.3v at about 350ma, preferably regulated.

#1: I know there’s the classic step down transformer approach but I also know there is a way to do it using capacitors?

#2: I imagine there is a really simple way of doing this (voltage regulator), other than using a 4W 27Ω resistor, lol.

Any help will be much appreciated.

#### MikeMl

##### Well-Known Member
Just a nit! What you want is a power supply that delivers a fixed, regulated CURRENT of 350mA, regardless of what the voltage drop across the LED is! I.E, what you want is constant-current power supply; not a constant-voltage power supply.

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#### -Kitsune-

##### New Member
You see my limited knowledge of electronics makes this, well, complicated, especailly because I dont want to pi anyone off with stupid questions. But, I need to learn.

Yes, that is what I need. I also need to know how to increase/decrease the current supplied for when using different LED's.

#### MikeMl

##### Banned
Do a search (using google helps a lot) on the forums for "constant current source"

#### tcmtech

##### Banned
A basic constant current regulator with an adjustable output would work well.
Most of the common 3 terminal devices like the LM317 or its many cousins are able to be configured to do this and the circuits are very simple.
But being linear and not switching they are not the most efficient power wise when the LED to drive voltage difference gets bigger.

The capacitor method is very efficient when being ran off of an AC power source with good frequency and voltage stability (utility power) but it lacks isolation so a little more safety concerns are associated with constructing it.

#### -Kitsune-

##### New Member
Read this homework, and then come back for more discussion.
Thanks, will do..

The problem with just Googling for circuit diagrams is that in the end, sure I have the circuit, but I wont understand how it works and as soon as I want to change something, I'll be right back on the forum..

##### Banned
So learn how the circuit you find works... If you're not sure ask.
Keep in mind you can limit your google search to this website using the advanced search feature. The reason I suggested it is because I know for fact there is a constant current source using an opamp and a mosfet in the forums, cause I built it. It's extrmely easy to build and it's adjustable from very close to 0 amps all the way up to the max the mosfet and sense resistor can handle.

#### -Kitsune-

##### New Member
Thanks, I will search again, at least now I know what I'm looking for
I read the article Mike referred me to, that is basically what i'm looking for, what I dont quite understand about it is the linear and PWM control, I where does the extra power for the control voltage come from?
The only problem is most of these are 7-70v input, what do I do for the 220v input?

##### Banned
Use a step down transformer and a full bridge rectifier.

#### -Kitsune-

##### New Member
Use a step down transformer and a full bridge rectifier.
Does this not waste a lot of energy? I know from some of your previous posts that you hate wasting..

What about the capacitor method feeding into the current regulator?

##### Banned
Transformers are relatively efficient. You're also talking about 300ma here. The capacitor method also has saftey issues, you have to use special capacitors that are garunteed to fail open circuit rather than closed circuit, because if they fail your LED's are gonna go up in flames like a mini tiki torches, and anyone touching it is gonna be exposed to line voltage.

#### -Kitsune-

##### New Member
That could be fun, right?

Been doing some more reading and it seems no one seems to agree on a good way of doing this..