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small power supply

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AtomSoft

Well-Known Member
Hi all, i was thinking about those little cellular phone chargers and how they use those tiny transformers to supply 5v @ 500ma.

Where can i find these kind of transformers? I was thinking about just buying a bulk amount of chargers and taking the boards for my project but i would love to find out how and where to get the parts.

I live in US so its a normal 120v-125v from wall so i need something to step down that. So any links to possible solutions would be cool!

Remember my goals:

1. Small size/form
2. 5-12v output would be great.
 

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They aren't mains transformers, they are high frequency switch-mode transformers as part of a SMPSU.

As every single circuit uses a custom transformer you can't usually buy just the transformers, and the assoociated electronics to make them wiork would make it far more expensive than buying complete PSU's.
 
So you are telling me its not worth buying the parts separate and building my own?

Can you recommend a way to drop 125v to 5VDC @ 1A or 500mA.
 
Im going to try and make a transformer less supply.

Is it ok to use larger sized resistors?
Like if a design calls for a 1W resistor. Can i use a 2W or larger?

Im reading :
Transformerless Power Supplies: Resistive and Capacitive - AN954

**broken link removed**
 
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Atom,
**broken link removed**
and down load app. note RDR158 and design RDK160
PowerInt has many designs for low power switchers, usually includes the transformer disign.
 
thanks ill check it in a minute. im getting into the groove of calculating these Transformer less supplies: (from equation 4:)

Image 1: Top of Equation: (Square (2 * VRMS)) - Vz
1-png.29909

Image 2: Bottom of Equation: 2 ((1/2*PI*f*C1) + R1)
2-png.29910

Image 3: Top / Bottom:
3-png.29911


Im just happy i was able to get the same answer they got lol which means i can follow this PDF lol
 

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I bought some name-brand, 5VDC-2A tiny and lightweight switching power supplies at a surplus store. They were made for dial-up modems (remember them?). I paid less than $2.00 for each one and they had thousands of them.

I also have some similar ones that were cell phone chargers.
 
The big problem with transformeless supplies is there's no current limiting inherently built in like with a transformer, if a component fails or it takes damage from a fall or what not bad things happen.
 
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