I thought LED's run on DC .

Hi Jerry,

Yes, LEDs work on DC so when you power them with A/C they only use ~1/2 the wave, they actually flicker on & off 60 times per second.  if you want true steady state, you'll need to rectify the A/C.  For most of my train lighting uses, I go the easy way and just use a resistor large enough to protect them.

Mike

Mike thanks for reply.

Truthfully, I'd seriously consider the diode as well.  While they're currently working, you are subjecting the LED's to reverse voltages in excess of their maximum ratings.  Most LED's have a 5V to 7V maximum reverse voltage rating, if you have 14 VAC on them, the peak reverse voltage they're exposed to is over 19 volts.  That far exceeds their rated maximums and will likely result in much shorter life for the LED.

John I know about reverse voltages, but does that even include when a resistor is in circuit?  By the way I was playing around with your curent reg chip with led light strips. Have done about 12 William cars, the light strips throw off a nice light.

Jerry

It is just about good practice. On AC the LED is doing double duty as it is also serving as a rectifier blocking the reverse polarity that is not used. This may or may not shorten the life of the LED,it depends on the design. As mentioned to achieve maximum intensity a bridge rectifier will use both halves of the sine wave and an added capacitor will achieve steady current. In fact if a capacitor is used even with the single diode maximum intensity will also be reached as the half wave will charge to peak assuming the capacitance is sufficiently large. Using the bridge is the most efficient method. No matter how it is run the LED itself is a DC device.

Being that a diode costs about 2 cents,it is not a bad idea to include it.

Dale H

The peak voltage will happen in the absence of current flow, which should not happen in a reverse voltage situation.  Since the current is limited, it won't kill the LED outright, but I have had several fail after months of use before I decided to include the diode in my lighting designs.  None have died since I did that.  Since I wire them like they're going to be in there forever, it's a PITA when one croaks, so I don't want that happening.

I never spend much time worrying about maximum efficiency, the plain diode and resistor are fine for my projects.  Typically, I'm running headlights at around 10-12ma, so I'm not losing enough power to worry about it.

Glad the light strips are working out, I just got a couple more spools of them for a bunch of cars waiting for conversion.   It's amazing how little current it takes to really properly light a coach.

Thanks for the replys, I was going to put a bridge rect in circuit for the headlights, but will use a diode in stead. As once headlights ar in dont want to change them out.

 PS the led is still going strong on 14-15 v ac

The problem with acronyms!

(Jerry, the following obviously does not apply to you, but it is related to the subject your raised.)

A few days ago I bought a new Samsung LED LCD Smart TV UN60ES7100 from Amazon mainly because the local stores did not have this model in stock yet. During my conversations with a couple of store clerks about these TV’s regarding the role the LED’s play, I discovered that they did not know what the LED acronym meant, or LCD either, for that matter!

An LED (Light Emitting Diode) inherently, as its name implies, is a diode in itself and makes its own direct current if alternating current is fed to it.

BTW, I fully concur with the advice given regarding the merit of rectifying the AC for use to power LED’s.

Alex

Alex, see my info on the maximum reverse voltage specification of LED's to understand why the protection diode is necessary.

While there can be no doubt that adding the diode is good advice, I would just like to point out that on several K-Line TMCC engines I have, the marker LED's are wired in series and powered directly from 18 VAC track voltage with a 1000 ohm resistor in series, period.

I have kept this wiring scheme intact on other K-Line engines that I have converted to PS-2.

Seems to work fine; have not lost one yet.

Rod

I've seen that as well Rod, I can only go from what I know from experience and reading the specifications for the parts.   I have some locomotives that are also wired that way, and I've yet to get around to sticking a diode in, though I fully intend to do that at some point.

John,

I know it well and never doubted it, which is why I concurred with what you and others had said.

I was simply attempting to explain why there may be some people who don't know that an LED is a diode, even if its intended function is to emit light and not to rectify alternating current.

Alex

True enough Alex.   You'd think the name would give it away.

Diode makes sense, would you happen to have a schematic of the circuit using a diode, resistor, LED combination.

Just put the silicon diode in the same orientation as the LED, and add the resistor, and all is good.

Another way to wire LEDs if you have 2 (ie: markers in a tender) is to wire them in "inverse parallel".  When wired this way, each LED protects the other as one is always conducting and limits the reverse voltages.

When 2 LEDs are wired in inverse parallel, you don't need the series diode, but do need the series resistor.

Absolutely correct Jon, I mentioned that in a previous thread.  It's a great way to save a component or two, since you also need a single resistor and no diodes.   Of course, that only works if you need an even number of LED's, and you're running them on true AC. 

Which is how a lot of the manufactures do it, so it may be worth your effort to look at that K-line pair again.  G