1. I would like to use a microcontroller to drive a TRIAC (Z0103MA0,412). The TRIAC will be switching a 120 volt a/c resistive load. I would like to derive a 3 volt or 5 volt CD power supply directly from the a/c line. So, I would be very grateful if someone could give me a schematic (with explanation & calculation for this).

  2. How would I connect the microcontroller to the TRIAC?

  • \$\begingroup\$ Why would you want such a noisy non-isolated solution? \$\endgroup\$ Commented Apr 1, 2019 at 0:47
  • 10
    \$\begingroup\$ If you have to ask, it's too dangerous for you. \$\endgroup\$ Commented Apr 1, 2019 at 1:10
  • \$\begingroup\$ I am going with this solution because of the small (1.75" wide X 1" length X 0.75" high) space that I am working with. If I can locate a 10 watt 120 volt transformer (off the shelf) to fit in this space, I would go with that. \$\endgroup\$
    – user136789
    Commented Apr 1, 2019 at 23:32
  • \$\begingroup\$ what type of cap are C2 & C3 ? Are they electrolytic caps? \$\endgroup\$
    – user136789
    Commented Apr 1, 2019 at 23:42

1 Answer 1


Something like this, used in home-power-line metering


simulate this circuit – Schematic created using CircuitLab

The 16v zener provides a clamped charging voltage for the 220uF capacitor. There will be substantial ripple on the 220uF. For 10mA I_Load, refreshed 1/50 seconds, given dV/dT = I/C, we have dV = (0.01amp / 0.00022Farad) * 0.05sec = 0.0005/0.00022 = 2volts ripple. So the linear regulator has some work to do, at 10mA load.

  • \$\begingroup\$ Wouldn't this be extremely sensitive to load current? \$\endgroup\$
    – SRR
    Commented Apr 1, 2019 at 7:33
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    \$\begingroup\$ Note to anyone who uses this: All points in this circuit must be treated as though they are at line voltage. Nothing powered from this circuit may have any electrical connection to other circuits. This circuit must be enclosed at all times when in operation. \$\endgroup\$
    – JRE
    Commented Apr 1, 2019 at 11:17

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