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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?

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  • \$\begingroup\$ Your use of the term "LDO" seems inappropriate. For a start it's a regulator and that is the term to use (or LDO regulator) except you don't need to have a low-drop-out voltage regulator given your AC voltages are sufficiently high. \$\endgroup\$ – Andy aka May 28 '18 at 8:40
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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.

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  • \$\begingroup\$ According to the datasheet the average current of ESP12 is 80mA and max is 170 mA, am I missing something? \$\endgroup\$ – Swagatam Majumdar May 28 '18 at 9:41
  • \$\begingroup\$ @SwagatamMajumdar Even at 170mA, that it still over 5W, so the same statement about excessive heating applies. Plus it would still burn 10x more power in the regulator than the device itself, which is a massive waste. \$\endgroup\$ – Tom Carpenter May 28 '18 at 9:52
  • \$\begingroup\$ I agree with you, in terms of efficiency this idea may not be recommended, however using a device like LM317HV would allow the user to get the thing working quickly, and permanently, since such devices have great protection features. Some heat could be expected which could be tackled with a heatsink mounted externally on the cabinet. Moreover the wi-fi wouldn't consume 170mA throughout its operating period. \$\endgroup\$ – Swagatam Majumdar May 28 '18 at 10:13
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According to me the best method would be to first convert the transformer DC output to 5V using a 7805, and then further drop the 5V output to 3.3V through a a few series 1N4007 diodes or through an adjustable emitter follower BJT regulator.

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    \$\begingroup\$ Converting 34VDC to 5V DC using an linear regulator is not a good idea. Using a 7805 is a terrible idea as the Vin(max) of a 7805 is typically 25V. \$\endgroup\$ – Tom Carpenter May 28 '18 at 8:56
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    \$\begingroup\$ Using two diodes to regulate for 5V to 3.3V if you want any kind of stable output voltage is also not good design practice. \$\endgroup\$ – Tom Carpenter May 28 '18 at 8:57
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    \$\begingroup\$ Bad idea, bad recommendation. \$\endgroup\$ – Andy aka May 28 '18 at 9:11
  • \$\begingroup\$ Sorry I completely missed that the rectified output would be over 35V, in that case an LM317HV based simple regulator would do the work sincerely. Here's the detailed overview of the IC LM317HV homemade-circuits.com/0-60v-lm317-variable-power-supply The idea here is to provide a simpler yet reasonably effective solution to the user. \$\endgroup\$ – Swagatam Majumdar May 28 '18 at 9:22

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