Based on this article: https://hackaday.com/2017/04/04/the-shocking-truth-about-transformerless-power-supplies/

I have some rf wireless plug outless which use transformerless power supply.

I would like to build in an ESP32 (3.3v) in it. I don't know currently what output the transformerless powersupply in these produce but I seen some 24V relays so I have to check.

My questions are:

1, Regardless that it is not safe to connect an ESP32 through the mains as the article explains, is the output of a TPS fine grain enough to power an ESP?

2, In the article it states: "excess current is shunted to ground when the load draws less than the 25 milliamps", an ESP32 power consumption is between 100-250 mA (using wifi) why does it matter how much my load drains? For example let's assume that this circuit I wan't to modify would output 3.3V to the board, would I still need to modify it if my load is different?

3, Is there some online calculator for TPSes where I can put in my load value and desired voltages and it will calculate what cap and resistor(s) will I need?


  • 4
    \$\begingroup\$ It's not safe for a beginner so if you have to ask, you're a beginner so don't use it. \$\endgroup\$
    – Andy aka
    Commented Feb 25, 2019 at 9:39
  • 1
    \$\begingroup\$ Just use a USB phone "charger" and a 3.3V LDO regulator. Stay safe. \$\endgroup\$
    – Wesley Lee
    Commented Feb 25, 2019 at 9:58
  • \$\begingroup\$ I agree. A phone charger costs just $1. Add an AMS1117-3.3 or a TL431 and 2N2222. A lot less headache and portable, besides being safer. Alternately, one can use a 6V step down transformer. In some parts of the world these may cost just 50 cents. \$\endgroup\$
    – Indraneel
    Commented Feb 25, 2019 at 11:05
  • 1
    \$\begingroup\$ ESPs can use as much as 200ma and as little as 5ma, making it hard to design an efficient capacitive dropper that can handle the full range of currents needed. \$\endgroup\$
    – dandavis
    Commented Feb 25, 2019 at 18:22
  • \$\begingroup\$ Google "HLK-PM03". You can get like 5 pcs for $10 and be done with it. Note: ESP32 can draw up to 500mA(!) when transmitting. \$\endgroup\$
    – rustyx
    Commented Nov 11, 2019 at 13:20

1 Answer 1


Here is an online calculator for TPS in various configurations.

Play with the values and you will see that for 250mA load you will need a non polarised capacitor with a value over 4.7uf at 400V. At a quick search the capacitor I found is 25/25/35mm in volume and costs 3$.

The TPS will have a low efficiency and your project very hard to debug.

Please follow Wesley's and Indraneel's advice and use a phone USB charger or something similar, some simple chargers can be tricked to output 3,3V It will be not only cheaper but also smaller , safer and will heat less.


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