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I'm repairing the circuit for a motion-activated LED light. The PCB uses a capacitive dropper circuit as the power supply. Photos of the PCB and the corresponding reverse-engineered power schematic are shown below.

PCB

Schematic

In this case, the two glass diodes in the bridge rectifier have failed short (D4, D5). Before I replace them, I'm wondering what's the significance of using two different diodes for the bridge rectifier? I would have thought that for such a simple circuit, any efficiencies saved by a potentially lower diode forward voltage drop wouldn't be worth it.

Edit Here's a closeup of the glass diode markings. The band appears to be green, which, according to some of the links posted in the comments, would imply a Schottky diode. Update after finding some more information online regarding this specific circuit, I can confirm that the glass diodes are indeed zeners.

MELF Diodes

Update I'm happy to report that after replacing the two glass diodes with 24V 500mW MINIMELF diodes, as well as transistor T2 (BC81-40), the light now works fine.

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  • \$\begingroup\$ This system is galvanically connected to the AC mains, it seems. Assuming an intelligent designer and not some arbitrary reason, I'd have to offer the suggestion that there may be a difference in the way these diodes may fail that led the designer towards these differences. But now I feel ignorant and also curious, too. Perhaps someone here will be able to provide the missing piece and we'll both learn something new! Good question. +1. \$\endgroup\$
    – jonk
    Commented Aug 22, 2020 at 19:46
  • \$\begingroup\$ Only the neutral is galvanically connected, the high side is dielectrically coupled. \$\endgroup\$
    – D.A.S.
    Commented Aug 22, 2020 at 19:58
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    \$\begingroup\$ The diodes marked M7 are plain rectifier diodes. \$\endgroup\$
    – JRE
    Commented Aug 22, 2020 at 20:20
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    \$\begingroup\$ The MELF diodes have a blue band. That says they are Zener diodes. \$\endgroup\$
    – JRE
    Commented Aug 22, 2020 at 20:37
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    \$\begingroup\$ @Tony, or only the live is galvanically connected ... \$\endgroup\$
    – Transistor
    Commented Aug 22, 2020 at 21:01

3 Answers 3

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The two glass diodes are zener diodes.

They serve two purposes

  • as one pair of a bridge rectifier

  • as a 24V bridge input voltage clamp.

A couple of 1N4007s (the marking 'M7' is clearly visible on one of them) make the other pair.

enter image description here

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  • \$\begingroup\$ In your circuit, you've placed the glass diodes at the bottom, connected to 0V, but on the PCB, the glass diodes are actually the top half of the rectifier, connected to the 24V out. \$\endgroup\$
    – Amr Bekhit
    Commented Aug 23, 2020 at 10:04
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    \$\begingroup\$ That would make no difference to the functioning, which would be the same in either case. \$\endgroup\$
    – vu2nan
    Commented Aug 23, 2020 at 10:11
  • \$\begingroup\$ Ah yes, you're right. In any case, I think the assumption that the glass diodes are zeners may not be correct - the band is actually green, which may indicate a schottky diode according to the links posted by some other users. \$\endgroup\$
    – Amr Bekhit
    Commented Aug 23, 2020 at 12:01
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    \$\begingroup\$ But, I suppose, the probability of those being zeners is more. \$\endgroup\$
    – vu2nan
    Commented Aug 23, 2020 at 18:11
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    \$\begingroup\$ Thank you, Amr Bekhit. \$\endgroup\$
    – vu2nan
    Commented Aug 24, 2020 at 11:04
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The larger diodes in the schematic are 24V Zener Diodes. They clamp the incoming line voltage to 24 V pulses.

The small diodes in the schematic take those pulses and feed them to the bulk reservoir capacitor..

This is similar to a standard capacitive dropper except that it is full-wave.

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    \$\begingroup\$ The big ones are marked "M7" \$\endgroup\$
    – Oskar Skog
    Commented Aug 22, 2020 at 20:25
  • \$\begingroup\$ M7 is not a Zener ! -1 \$\endgroup\$
    – D.A.S.
    Commented Aug 23, 2020 at 6:53
  • \$\begingroup\$ @Tony Stewart: the large diodes in the schematic diagram appear to be Zener Diodes. I see now that they are marked D7 & D8. At no time did I say they were marked with anything. \$\endgroup\$
    – Dwayne Reid
    Commented Aug 23, 2020 at 16:24
  • \$\begingroup\$ Appearances can be deceiving \$\endgroup\$
    – D.A.S.
    Commented Aug 23, 2020 at 16:25
  • \$\begingroup\$ Although a quad zener bridge is what I would have done with a ceramic power R and abig cap. \$\endgroup\$
    – D.A.S.
    Commented Aug 23, 2020 at 16:27
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Assuming they used the cheapest diode 1N4148 300mA < $0.01 in vol. There is no doubt a line transient would blow the diode junction. The resistor has a larger junction but might also blow open from 9A

The 1N4148 is rated for impulse current rate of 4A single shot 1us. Abs Max. A Resistor might have a 0.5 s time constant

A cheap 1N4007 1kV diode can handle 30A 1.2kV half-sine 1 shot but then the SMD resistors will blow. Install it with bent leads carefully with 2 tweezers folded to fit on SMD pads or any other 1A SMD silicon diode.

Check all components in the front end for component values open or short by some sort of test. [![enter image description here][1]][1] https://store.comet.bg/download-file.php?id=13687 The large M7's are similar to 1N4007 1kV silicon diodes rated for 1A, or 30A impulse

All 4 diodes should have been M7's and 100R . A line filter would reduce the impulse voltage but stretch out the duration. [1]: https://i.sstatic.net/WWweG.png

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  • \$\begingroup\$ There needs to be something like a Zener Diode in the circuit to set the output voltage. The large diodes in the schematic appear to Zeners. \$\endgroup\$
    – Dwayne Reid
    Commented Aug 23, 2020 at 16:28
  • \$\begingroup\$ It doesnt have to be efficient,just safe from impulse surges \$\endgroup\$
    – D.A.S.
    Commented Aug 23, 2020 at 16:28
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    \$\begingroup\$ Why is this down-voted with no comments or wrong assumptions? \$\endgroup\$
    – D.A.S.
    Commented Aug 29, 2020 at 17:06

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