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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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Nigel Goodwin

Super Moderator
Most Helpful Member
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.
 

AtomSoft

Well-Known Member
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.
 

AtomSoft

Well-Known Member
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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AtomSoft

Well-Known Member
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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audioguru

Well-Known Member
Most Helpful Member
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.
 

Sceadwian

Banned
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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