I'm using a 12V DC 2A power source to power a 12V LED strip, and controlling it using a WEMOS mini and a relay.

Yes, I know there is a power shield for WEMOS, but I tried to regulate the 12v input to power the WEMOS as well, which did not succeed.

First I tried with a 3.3V Zener diode via 3.3V pin on WEMOS- with no success.

Later I tried with a 5.1V Zener diode, and 5V pin on WEMOS - and again no succees.

  1. See schematic (please ignore diode's catalog number.) Circuit yields voltages as expected.
  2. Measuring current, using a multi-meter, shows a draw of only 10mA, when I guess a 200mA is needed at start.
  3. Since it is a 1W diode, at least P=VI should yield a nominal of 200mA when using a 5V diode.


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


If you need 200 mA at 3.3 volts, R1 must be no more than 43.5 Ohms. 1K would drop 200 volts when passing 200 mA.

Much better to use a switch mode voltage regulator (DC-DC converter). A linear regulator would dissipate 1.75 Watts, and require a heatsink.

  • \$\begingroup\$ Thank you. Made some research. Why does it called "switch-mode". It has nothing to do with switching, am I right ? \$\endgroup\$
    – guyd
    Apr 27, 2020 at 5:22
  • 1
    \$\begingroup\$ A switch mode regulator or power supply (SMPS) switches its output off and on rapidly such that the filtered average output is the desired voltage. An SMPS will have much better efficiency (and thus waste less power/generate less heat) than a linear regulator. \$\endgroup\$ Apr 27, 2020 at 5:28
  • \$\begingroup\$ OK. Saw an explaining diagram as well. I thought is it a On-Demand switch ON/OFF as in other IC's \$\endgroup\$
    – guyd
    Apr 27, 2020 at 5:31

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