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I have this circuit with a 1N4735A 6.2 V zener diode. I am trying to figure out why am I measuring 5.1 V across it. The circuit is a transformerless power supply with more or less 60 mA of maximum current. I've read that insufficient current through the diode may be the problem but 56 mA should be enough according to the datasheet. Right now I am using a simulator but some time ago I had the same problem with a 12 V zener in an implemented circuit. I hope I am not making a silly mistake. Thank you!

EDIT: There is 11.2 V across C2, Zener's current is 55 mA and LED's current is 3.5 mA. I don't think the load is draining too much current. The original design uses a C1 = 470 nF. I made it bigger but the result is pretty much the same, lower voltage across the zener diode than expected enter image description here

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    \$\begingroup\$ What is the voltage at C2+? \$\endgroup\$
    – Kevin White
    Commented Mar 27, 2020 at 16:13
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    \$\begingroup\$ measure the Zener current. Include it in question. \$\endgroup\$
    – Sadat Rafi
    Commented Mar 27, 2020 at 16:25
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    \$\begingroup\$ I've done another simulation and it appears to regulate properly. In fact, the 1 μF capacitor seems excessive unless you need significant current out of this 6.2V rail; it still works just fine with a 330 nF cap. \$\endgroup\$
    – Hearth
    Commented Mar 27, 2020 at 17:17
  • \$\begingroup\$ There is 11.2 volts across C2, Zener's current is 55 mA and LED's current is 3.5 mA. I don't think the load is draining too much current. The original design uses a C1=470 nF. I made it bigger but the result is pretty much the same, lower voltage across the zener diode than expected \$\endgroup\$
    – Gabi M
    Commented Mar 27, 2020 at 23:07
  • \$\begingroup\$ Are you sure that it is not a 5.1V zener diode? \$\endgroup\$
    – Kevin White
    Commented Mar 28, 2020 at 18:45

2 Answers 2

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I suspect you're not getting enough voltage across the bridge. It's limited by a 100 ohm resistor and a 1uF cap, and then your diode drops kick in as well. This is clearly intentional, since you don't want to zener regulate from 300V to 6.2V, but it's not particularly reliable since the current draw on the DC side will affect the voltage directly. That (and personal safety) is why most such designs use cheap transformers or even complete wall warts.

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    \$\begingroup\$ You're not going to get a cheap mains-frequency transformer; even small low-power ones are going to be several dollars each. That's the reason this type of power supply is used: it avoids the complexity of switching and the cost of a mains-frequency transformer. \$\endgroup\$
    – Hearth
    Commented Mar 27, 2020 at 20:35
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Ok, after changing every parameter in the circuit y decided to put another zener. I replaced the 1N4735 6.2v with a 1N4736 6.8 v... and it works just fine! It must be a problem with that especific component in the simmulator. Thank you for your time and for helping me with this silly problem

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