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I'm building a simple temp PID controller for turning an SSR on and off (yes, I know I can buy them fairly cheap, but its a learning project) and I need DC to power my RBBB arduino and mains AC for the SSR. I'd like to do the AC to DC transformation in my project and I'm really clear on how to do it. Ideally, I have a small box that I'd like the project to fit in and the transformers of the circuits I keep seeing are way too big. Obviously you can do it very small - the Blackberry travel charger* is the size of a large gumball and does 5v @ 750mA.

Is there different technology used in such a small device or am I simply not sourcing a small transformer correctly?

*Worst case, these are like 5 dollars and I'll crack one open. It should be a sufficient current for me.

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rfusca
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    Switchers will be smaller, but for arduino-class currents the final generation of linear cell phone chargers used some pretty small transformers and four diodes in a bridge. And usually omitted the filter caps - I though that was just the dollar store clone, but I checked the original and it was the same. – Chris Stratton Oct 23 '11 at 05:36

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The size difference is due to different approaches.

A large transformer implies AC operation at the mains frequency, stepping the mains voltage down without an intermediate conversion stage.

A small transformer implies a high-frequency switching regulator, where the AC is first rectified and smoothed to DC before being chopped into high-frequency AC and then stepped down and rectified to DC again.

As frequency goes up, transformer size goes down for a given power level, as you need fewer turns to do the job. Don't try using the travel charger transformer at mains frequency - it won't work so well.

endolith
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Adam Lawrence
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  • "where the AC is first rectified and smoothed to DC before being chopped into high-frequency AC and stepped down." - Its rectified to DC and the chopped back into AC?.... You've lost me somewhere. The travel charger does plug into mains power... – rfusca Oct 23 '11 at 04:57
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    @rfusca The question you linked to uses something called a linear power supply. Madmanuruman was talking about a different type of power supplies called [switch-mode power supply](http://en.wikipedia.org/wiki/Switched-mode_power_supply). It does exactly what he said. First rectify the "high voltage" mains AC into high voltage DC, then it turns the DC back into AC and steps it down and rectifies it again to low voltage regulated DC which will be used by the device. Such power supplies are in fact connected to mains power and are used in all sorts of chargers. – AndrejaKo Oct 23 '11 at 07:42
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    It took a bit more research, but I understand now. You rectify AC to DC so that you chop it to high frequency AC. With high frequency AC, you can use a smaller transformer. After the transformer, you're left with high frequency, lower voltage AC which then gets re-retified to DC. Roughly correct? This is pretty much what you said, but I didn't get the, 'then re-retified to DC' part. – rfusca Oct 23 '11 at 21:46
  • I wasn't really suggesting ripping out their transformer and using it at mains frequency, just ripping out the whole circuit possibly. – rfusca Oct 23 '11 at 21:47
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    This makes sense to me *why* a switching supply like that can be smaller. Thanks. – rfusca Oct 23 '11 at 21:48
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What power output do you require?
What are the dimensions of your "small" box?
What maximum transformer dimensions would you find acceptable?

See diagram below - this is an offline universal supply rated at about 2 Watts - transformer is about 16mm square and could be smaller.
Is that small enough?.

In the absence of dimensions you can work out the size from the components lead spacing. I estimate the transformer to be about 0.66 x 0.66 inch square. And it could certainly be made smaller.

This is the LNK58c series offline switcher aimed at "zero power" standy and low parts count. Datasheet here

enter image description here

Based on an IC package pin-pin spacing of 1.27mm = 0.050 inch that makes the transformer about 0.7" / 16mm square.

enter image description here

Russell McMahon
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