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Help me step down AC voltage without a transformer? Answered

Hi again.
Well, the title explains everything. The goal is to make a 65W power supply the simplest way. I have capacitors and diodes, and I'll desolder other components if necessary.
Just, if someone can give me instructions and detailing a little bit.
May be you can post some schematics or useful links.
I've already created the circuit schematic but I would like to see yours first. I will post mine later but it doesn't include the values of the components and maybe it contains some errors. It was enough hard for me to draw it. I'm no professional afterall!


you can make a step down without a transformer. I will upload a diagram for you in my main website; emmih.com in 2 days.


2 years ago

How Safe do want the supply ?


2 years ago

Try to check the following link


Sorry, but for a beginner like me I can't really understand why it's pretty hard to draw some amps this way.
I'm no dumb, I just love to learn basis of all things.
What are safety issues?
What are the risks I'm taking when I turn on my so ever called "Power Supply"?
Is it gonna be fine if I run my project, some meters away from it.
Dudes, you're already scaring me with those stories!
I make a step forward, and you're pulling me just backward!

Risks: Electrocution with possible death, and/or fire. Destruction of any equipment connected to the device powered by the supply.

READ Iceng's comments. They are crystal clear, if they aren't to you, go away and work out what has been said. By the time you have, you'll understand.

I have to wonder if you understand current, voltage and power properly to even begin this. To get further still, you need to understand AC electrical theory.

A little learning is a dangerous thing;
drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain,
and deep ones largely sobers us again.

Thanks again for spending your time trying to help me.
I've changed my mind.
I mean my plan. LOL!
Sorry, but I've just figured out that one LM138 or an LM338 would fit much better because they can handle more current much easier.

I'll be detailing the project:
-I'll use a PP capacitor (1.5µ/400v AC) at the input.
-A voltage divider based on electrolytic caps at the output of the bridge rectifier (not sure).
-Maybe then a Zener diode if necessary.
-A voltage regulator rated 5 amps (LM138 or LM338).
-And just before the regulator, a CPU FAN will be used to cool down the components.
-I wont mount all this junk on a PCB for some reasons (actually blowing my house doesn't count).
-Praying to GOD day & night.

Here's a first version of the deadly circuit.
Please help me to determine all components values.
I'll be making it anyway, and you will have no more fun if I die trying! ^_^

65w Power supply.PNGFancap.jpg

And at what input voltage?
What kind of out voltage - DC, AC?
What form of AC? Is rectangle okay or has it to be a sinus?
What has to be regulated - voltage or current?
How good has the regulation to be?

It's AC-DC sir. AC input is a sine wave at 50-60Hz under 220-240v.
I would like to use a voltage regulator I don't know about current regulation. DC output 19.5 v so current will be 3.34 amps. The more current the better I think.

Look up the theory of switched mode power supplies these go some way to doing what you want in this question

My recommendation (not DIY but pragmatic): Check ebay or similar for laptop power supplies. Some of them are around 19V and the current should fit as well. 

A 65W switching PSU is not a project for a beginner, especially if you only have salvaged parts of old PCBs. You'll need HF transformers, handle more than 400V, need feedback optocouplers, a foldback circuit etc. etc. etc.  Not a trivial task.

Yeah, even pros avoid PSU design, if they can !

It is CRITICAL to note what Iceng says. The metalwork of your laptop WOULD BE AT MAINS LIVE POTENTIAL.

Yeah, but it just got re-listed:

Although, my point was that it may be easier to buy this power supply than to build it.  Similarly rated laptop power-bricks are out there. 

That is unless buying it is against the rules somehow. I mean from reading the original question it might be that OP really, really, wants to build a power supply, like "from scratch", and just buying one, well, that would be insufficiently from-scratchy.

Dumb question:  Would it be cheating to just buy a power supply with the specs you want?

This one looks nice:

That link should be pointing towards an eBay listing for a black brick-shaped power converter that takes mains power,at 110-240 VAC,  47-63Hz, and outputs regulated voltage at 19.5 VDC, at a maximum current draw of 6.7A,  I mean according to the little numbers printed on it.