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A 20mA, GaN, blue LED, 5mm (forward Voltage 4 volts max, reverse voltage 5 volts) in series with 47K 2W resistor worked fine as a continuity checker using power from a standard 120V AC electrical outlet. (No reverse polarity diode in parallel was used, that was going to be the next step). After several months of general testing and usage the LED would not light. When that same LED was tested on a pair of 1.5V AA Duracells in series (3V) the LED glowed as bright as in the AC circuit. How is this possible? If reverse current/voltage breakdown was happening shouldn't LED have failed completely? I want to have 4 of these led in series, run from AC. Would 47K 2W resistor be acceptable and is the reverse polarity diode actually required in this case? (Setting up 2 of the 4 LEDs in the reverse polarity?)

Schematic

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  • \$\begingroup\$ "shouldn't LED have failed completely?" - Didn't it? You said the LED does not light now, right? LEDs don't have a very high tolerance for reverse bias, so I'm surprised it ran as long as it did. You should include a reverse polarity protection diode. Regarding the 3V test, was that with the 47k resistor also? \$\endgroup\$ – AngeloQ Mar 31 '17 at 1:37
  • \$\begingroup\$ The LED works fine with the duracells (no resistor) thus not a complete failure. That is what I find perplexing. \$\endgroup\$ – Joe Vokt Mar 31 '17 at 1:45
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    \$\begingroup\$ Maybe it got mad at you ... \$\endgroup\$ – Trevor_G Mar 31 '17 at 1:50
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    \$\begingroup\$ @Trevor: Yes, any abused LED has the right to be mad at it's owner. It definitely lost ability to operate at a lower current (2.6 mA). Does this diagram represent what you were talking about? Any recommended changes to the resistor or diode?4 LED Diagram \$\endgroup\$ – Joe Vokt Apr 1 '17 at 3:48
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    \$\begingroup\$ Still needs to be a 200V+ diode, I'd go with the good old 1N4004. \$\endgroup\$ – Trevor_G Apr 1 '17 at 14:35
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If you are going to use four LEDs. why not make it a bridge.

The following would be my alternative implementations. I'd put the LEDS in reverse parallel as indicated. That way they each turn on half cycle, and protect the other while they do.

Note the bridge is basically two stages of the TWO LED version. It is debateable whether the H-Bar is needed. However, do not be tempted to put the resistor there.

schematic

simulate this circuit – Schematic created using CircuitLab

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@Trevor:

An alternative answer you hinted at but reformulated here:

In the four LED rectifier you posted, when current is coming in the positive phase (top down) current will flow trough D3 and D4. It is not possible for current to flow through the bridge because D2 is in reverse polarity. Similarly, when current is flowing in reverse phase (bottom up) current will flow through D2 then D1, but current cannot flow through the bridge because D3 is in reverse polarity. The bridge is essentially a "dead zone" so it is justified to remove it in this particular case. See diagram.

Rectif-nobridge

Here is a live link that is editable. Rectif-nobridge-live

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  • \$\begingroup\$ @Trevor: I noticed you were using CircuitLab. I am curious if you have used Schemit. It is unlimited no-fee usage with a no connection limit. Free customer support. If you look at my live version it even stores the vendor number can give me a bill of materials spreadsheet for tallying costs. \$\endgroup\$ – Joe Vokt Apr 2 '17 at 20:36
  • \$\begingroup\$ @Trevor:I was interested in any comparison between CircuitLab and Schemit you could provide. Also, if you can comment on choice of circuit components that would be a plus. (visible in the Schemit link in the answer.) You might have been contacted by Mike of CircuitLab, he is now trying to write an electronic simulation textbook using CircuitLab technology. \$\endgroup\$ – Joe Vokt Apr 3 '17 at 7:56

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