I have a circuit running an ESP-12 at 3.3v DC that I would like to power with a doorbell transformer that will deliver between 8 and 24v AC. I was wondering what would be the best way to do that.
I'm currently powering the circuit with a 5V DC adapter and a HT7333 LDO (datasheet) to convert it to 3.3v.
I was thinking that I could use a bridge rectifier like the MB6S (datasheet) to turn AC into DC, and then use an LDO to convert to 3.3V.
The HT7333 has a maximum input voltage of 12v, so I will be in trouble when I use a 24v bell transformer, but there are LDO's that should fit the bill and which have the same footprint, like these.
I was wondering if there's anything else I would need to do to make this work? or if there are better solutions than the way I'm going?
Best Answer
I would not use a linear voltage regulator to convert from 24V AC to 3.3V DC for anything other than a milliamp of current. Your EPS-12 draws up to 320mA of current.
If you were to rectify your AC voltage, you would get a peak DC of 34V, which aside from being outside the input voltage range of many standard linear regulators, also means that you are dropping 31V across your regulator. At 320mA, that means you are dumping ~10W of power in your regulator, a waste of power which will cause massive heating problems.
Instead, I would highly advise rectifying the output and then feeding it into a wide-input DC-DC converter to produce 4V to 5V DC output. You can then use an LDO to clean up the output and bring it down to the required 3.3V (much like you are doing currently with your 5V supply).
There are many converters that will do the job, and if you are not comfortable designing the circuit from scratch, there are many cheap converter modules that can be acquired through well known internet stores. If you do want to built your own, there are tools such as TI's Webench (no affiliation) that will do a lot of the design work for you.