LED Bulbs working on A/C

LEDs with out a rectifier will still operate, just flicker with the AC cycles.

They run off my AC 18 volt just fine...with a resistor..forgot the value.

One of the things I've read (but not verified empirically) is the need to place a separate diode in series with the LED to stop the reverse biased current from hitting the diode.  Now, one might ask (as did I): an LED is already a diode, so why would you need a second one?  The reason that's given is that no diode is "absolute" - meaning, it would prevent current from flowing backwards (i.e., reverse biased) for any arbitrarily high voltage.  In reality all diodes will "break down" if presented with sufficient voltage pushing backwards against them - something called out in their specs as the "reverse voltage" .  Now, my understanding is that most LEDs have a low reverse voltage as compared to virtually any other (non-Zener!) diode, typically around 5V.  Virtually any use in AC-powered O scale will result in more than 5V hitting the LED reverse biased 60 times a second, leading the LED to repeatedly break down its diode capability.  And this, in turn, creates heat and wear that will quickly cause the LED to fail. Add a properly oriented, regular ol' diode in back of it (that has a comparatively higher reverse voltage) and it will stop the voltage from ever reaching the LED. Or so the supposition goes.

Some people (as discussed earlier in this thread) seem to use LEDs with "raw" AC with no problem, though.  So, my question is: is the secondary diode really necessary?

Here are some thoughts from the EE StackExchange on this topic for consideration - there they posit reverse breakdowns of common LEDs are typically higher than 5V.

LED's will enjoy a much shorter life if you routinely exceed their reverse voltage specification.  For some time I used to simply wire LED's in model train applications with the resistor to limit current, and I was perplexed as to why I would have seemingly random failures of a component that I felt should outlast me.  Since I've been using a diode (or wiring two LED's back to back for AC), I have stopped having random failures.

This is just an opinion of one, but I believe the diode is necessary if you want the LED's to enjoy a long life.  If you have two LED's and are using AC current, wiring them back to back allows each LED to protect the other from excess reverse voltage.

As far as flicker, 99% of the population will not detect a 60hz flicker of an LED running on AC.  Where you will notice it is when you take a video of a model train with LED lighting that's driven from 60hz.

If an LED is rated for 12 volts, it is more than just an LED. That is, there are other components in the assembly such as diode(s) and resistor(s). I refer to these as LED Lamps, not LEDs. There are even LED strings that have active logic components that allow controlling the RGB of each lamp. Some of these strings operate from 5VDC and some from 12VDC.

My point is, to be aware if you are working with an LED lamp or an LED component.

Most LED replacements for 12v landscape lighting do have a bridge rectifier and any other necessary components contained in the "bulb".  All I have tested in recent years do, even the ultra cheap ones direct from China.  You can easily test with a 12v battery, or in most cases a 9v battery.  If the led lights when hooked up both ways - you have more than just leds on a chip, they took care of everything for you.  

gunrunnerjohn posted:

This is just an opinion of one, but I believe the diode is necessary if you want the LED's to enjoy a long life.  If you have two LED's and are using AC current, wiring them back to back allows each LED to protect the other from excess reverse voltage.

As far as flicker, 99% of the population will not detect a 60hz flicker of an LED running on AC.  Where you will notice it is when you take a video of a model train with LED lighting that's driven from 60hz.

When you say "back to back", I'm assuming you mean in anti-parallel, correct?  I've got a bunch of LED work coming up soon, so I'm going to take this as an opportunity to put to use some of those 1N400x parts I've got floating around to good use.

I know I'm part of that 99% - especially if the LEDs are frosted, I've never been able to see any flicker.  If someone actually could, I would think that TVs (well, the old analog ones...) and movies would be problematic as well...

Yes, if you wire LED's in parallel with reverse polarity, they protect each other from excess reverse voltage.  Obviously, you still need current limiting.  For a single LED, you can simply wire the diode in series.

I see many examples where the diode is wired in parallel with the LED with reverse polarity, that configuration makes little sense to me for most situations.  While the diode protects the LED, you're dissipating twice the power in the current limiting resistor.  I suppose if you were worried about the imbalance on one half of the AC waveform it would make sense, but why not just wire half the LED's with reverse polarity to power them on AC, problem solved with half the power dissipation.

There is a difference in old TV rep rates and LED flicker.  The phosphors used in picture tubes had persistence of illumination, so the image actually remained for the rep rate and really filled in the image.  The LED turns off immediately, hence their use to transmit high speed data.  If they had persistence like picture tubes, they'd be useless for data transmission.

The 12V LED "bulbs" that I plugged into my landscape lights that are fed with 12VAC are two pin and can be inserted either way. The lamps do not flicker and have been fine for 3 months now. My neighbor has had his burning for 5 years and they are still doing fine. They must contain an internal resistor but they are made for automotive use 12VDC and unlikely to have a rectifier. This all must go contrary to what is taught in theory but they work.

Well, the peak reverse voltage from 12VAC RMS is around 16.8 volts.  That would clearly exceed the max reverse voltage of most LED's.  The resistors help to minimize the damaging effects, but it is harder on the LED's.  I used to connect LED lighting directly to track power with just a resistor.  After losing several LED's for unknown reasons, I added a blocking diode, haven't lost one since.  YMMV.

Even if the supply voltage is DC, it's a common practice to employ a bridge rectifier so that the end user doesn't have to be concerned with polarity.

I rarely use a bridge rectifier as I know what polarity I'm feeding them.  For TMCC lighting, frame ground is positive, the diode is to protect when it drops into conventional and receives AC track voltage..  If I'm adding LED's to PS/2 locomotives, the P5V is the positive side.

John

I was responding to the comments about landscape lighting; common practice in mass produced products where the end user doesn't know what he is doing. I wouldn't add any unnecessary components in one of my own projects either.