I am recently working on mains socket outlet type LEDs lights. They all have very similar AC to DC conversion circuit.


simulate this circuit – Schematic created using CircuitLab

  1. C1 is a X-2 type capacitor and I know it has Independence $$X_c= \frac{1}{2\pi fC} \approx 9.6 k\Omega$$ Thus, the AC current will be limited by this cap at 230/9.6k=23mA. But what the R1 does in the circuit?

  2. R2 is a big through hole resistor, does it act like a fuse or something?

  3. D1 is a Zener diode. I guess it is here to regulator the voltage to its breakdown voltage, like 5V or 12V. Am I right?

  4. This circuit doesn't have a transformer like 230V to 12v before the rectifier, would it be safe in practice?

Thanks a lot!

  • \$\begingroup\$ Note that the circuit ground (symbol on DC side) should not be connected to mains earth. Both sides of the circuit must be assumed to be live. \$\endgroup\$ – Transistor Apr 22 '18 at 7:05
  • \$\begingroup\$ R2 is an inrush current limiting resistor. When you first connect the circuit to the mains voltage and the C1 capacitor is empty. R1 was added to quickly discharge the capacitor after you disconnect the circuit from the mains voltage os, you do not get an electric shock. And yes, the Zener diode "set-up" the output voltage. And try to google transformerless psu ww1.microchip.com/downloads/en/AppNotes/00954A.pdf \$\endgroup\$ – G36 Apr 22 '18 at 7:21

The circuit is safe so long as you can't touch it or any circuit connected to it, if there is inadequate protection between users and the circuit it is unsafe.

R1 bleeds the charge out of the capacitor when the circuit is unplugged this makes the plug pins safe to touch very soon after it is unplugged.


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