Convert 120 V AC to 3.3 V DC without a transformer

I finally made some progress with the AC to DC converter (without using a transformer.) I recently graduated from NAU and got a job, so I actually have a little more time on my hands to work on stuff like this (even with a 3-month old baby.) If you will remember from my previous post on this topic, the challenges facing this project are that it should convert 120 V AC to 3.3 or 5 V DC, handle a relatively high output current (more than 15 mA, and the more the better), have a small profile (so no transformer or huge capacitor,) and draw very little current when no load is attached.

Preliminary Ideas
I knew my previous 2 ideas weren’t very promising. These poor design choices were:

  • Using voltage divider resistors, which requires that you know exactly what the load resistance will be,
  • Using Zener diodes to drop the voltage down, which uses up a ton of power and isn’t worth the effort in the end.

Also note that using the Zener diode method can create some really high current that can blow up some of your parts (the bridge rectifier, in my case.) What tended to happen was that if one of the Zener diodes was flawed then it would burn out and basically become a short – causing no voltage drop across it and burning out any parts that came after the diode.

I definitely needed a way to limit the voltage in a more indirect way (without using a lot of components that have to be in series with the load.) I had heard about switching power supplies and that they are fantastic because they use a transistor that turns on and off at a given frequency. By putting a capacitor on the output, the output voltage become the average of the on/off voltages. The output voltage can then be changed by adjusting the duty cycle of the switching. However, most switching power supplies that I am aware of are driven from a relatively low DC voltage (15 to 20 V) that is generated from a different power supply or from a transformer and capacitor. The problem with this solution is that I don’t want to use a transformer, and I don’t have an external DC power supply to drive my switching circuitry. My converter needs to directly change 120 V AC to 3.3 or 5 V DC.

Design Plan
Some early design choices would make the design easier to implement with fewer roadblocks. For example:

  • The circuit needs to to lower the voltage before tries to do anything with it, which can help to avoid high current problems.
  • The circuit needs to have as few parts in series with the load as possible, to avoid a chain reaction of part failures and unusual voltage division between the parts.
  • Capacitors will inevitable need to be used to stabilize the output voltage, but the smaller the physical size of the capacitor the better.

I came up with a few really good ideas to implement this, which I am sure will work and won’t have the flaws the other concepts had, that are listed above. However, I must stop here (temporarily) for the reason explained here:

I just graduated from Northern Arizona University, and am now working for General Dynamics C4S. Unfortunately, anything I create on my own time belongs to them if it is remotely related to any business that General Dynamics has their hands in. Since I am a hobbyist electrical engineer besides also having my degree in it, pretty much any hobbying I do on my own time could potentially belong to them. For this reason, this project is on hold for a while and I will work on other hobby projects that have already been done by other people before and that could not be considered GD’s intellectual property. I’ll probably even help my wife with her online digital scrapbooking…

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