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Basic Electronics » Power Rectifier

December 03, 2010
by Hexorg
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Hey everyone, i'm sourcing for a transformer that can bring US's household 110VAC downto something I could rectify into 5VDC.

I'm planning to use LM7805, MOSFET based voltage regulator similar to the one that comes with the nerdkit. It's rated for 1A of current, however I doubt my device will use much more then maybe 100 mA (it'll drive microcontroller, RF transmitter, and a relay coil). I'm planning to use 10:1 turns ratio transformer, to bring 110 Vrms to 7.77 Vp. So If i'll have max of 0.1 A on the 7.77 side, i'll have max of 0.01 A of the 110 side, right? So as long as transformer is rated for 1.1 W (1.1 VA) on the primary coil, it wont get destroyed, i hope.

I found a transformer on mouser.com that is "Transformers 2.3 VA 9V@256mA" it has primary voltage rating of 115V instead of turns ratio. And I just wanted to make sure it's the right transformer before I buy it.

December 03, 2010
by BobaMosfet
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In the USA, voltage is (technically) rated at 120VAC. In truth it's more like 126VAC, but the RMS voltage from this is about 115-120VAC (typically). Some utility companies (based on business model and market) will provide more or less voltage inversely proportional to current (which can be okay or not, depending on what you have connected).

If you're reading 110VAC on your house outlet, I would make sure you don't have a ground fault somewhere, causing voltage to pull down on the primary. Check the voltage between positive and ground (common), and then check the voltage between positive and neutral and ground and neutral.

WARNING: Remember to make sure your meter is set for AC.

Typical is:

  • Ground <-> Positive: 118 VAC
  • Neutral <-> Positive: 118 VAC
  • Ground <-> Neutral: < 1 VAC

And FYI, your meter typically provides RMS Voltage automatically, as a matter of how it's made (when using AC mode because it must rectify in order to get a steady reading).

Now, to address something else in your post. Your transformer current rating is NOT a rating of how much current it is going to push through your circuit (unless there is a short), it is a rating of how much current is AVAILABLE (only if needed).

The typical nerdkit uses about 100mA, so your mouser.com transformer will probably do what you need. I found the transformer you were talking about and then found the datasheet for it, as mouser's link was broken.

  • Primary: 115VAC
  • Secondary: 9VAC @ 256mA
  • No Load Voltage: 13.8VAC

It is a straightforward transformer with no center taps or other things. No rectifier either. You will need to use a bridge rectifier and the appropriately sized capacitor (for ripple) to convert this to DC.

Or, you can simply find an affordable wall-wort that does all this for you.

BM

December 03, 2010
by Hexorg
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BM, Yea I was planning on building the diode bridge + capacitor + LM7805 for the rectifier. The reason for that is I'm trying to keep the project's size as small as possible. There's been a little misunderstanding though - I knew that rated current is how much is available, I didn't know however if that was the maximum current on the primary coil or secondary, but it seems like it means secondary, so thank you. :)

Hexorg

December 04, 2010
by BobaMosfet
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Hexorg-

Understood. You can use a diode bridge, or a full-wave rectifier (1 part).

Yes, 256mA is the maximum output current available from the secondary winding.

For safety, please put fuse inline with your AC circuit. Rate it appropriately, based on what you're doing, so that it blows quickly if that value exceeded. That way, if you become the load, the fuse will blow (hopefully) before you do. As little as 50mA for more than 200ms across your heart can kill you. Make sure your fuse blows faster than 200ms.

Be careful, and good luck. BM

December 14, 2010
by danuke
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Is this because you want to build something? I was googling around this morning and found some listings for powered breadboard that were not all that expensive. They had a 5 volt supply and - and + variable 0 to 15 V power supplies. I think that I will get one after Christmas seeing as how I just spend a small fortune on the book, THE ART OF ELECTRONICS.

December 14, 2010
by Hexorg
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danuke, yes, and no. I do want to build something, but I want the end-product to have the ability to be plugged in the wall for power.

December 15, 2010
by Hexorg
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Hm... I'm starting to wonder if there is a better power supply for my project then a rectifier...

The device is going to be very close to the power line, so it did make sense to tap into that power directly. But from what I see, most transformers that are capable of supplying needed current are either too expensive, or too large. So I started to wonder what other ways can I use? changing batteries will be hard, since the device will be in a hard to reach area.

What if I run a coil around a power line? Could I get 12V peak like that?

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