LEDs - What do I have here?

A quick glance for "LED strip lighting" over on your favorite web search will provide a seemingly endless list of brands and vendors. Odd though that they are running off of AC voltage as most come without a rectifier and need DC voltage input. They come in all manner of lengths and colors and intensities. I have used cool white and warm white in upgrading passenger car lighting in the past.

Thanks XX101.....this is what I am wondering about.

I see where they are primarily DC....but I have had this plugged in for hours now on AC and still working fine.

Since there are 3leds in series, the reverse voltage from the AC is not as harmful but eventually they will suffer damage. So put a diode in series with the power.

Thanks Jack......"a diode in series with the power".

What diode would I use?....and in line with the AC power positive line?

Disclaimer I'm just another hobbyist not an electronics guru. That said I believe a 1N4001 diode placed on the common (u terminal or black wire) side of the AC supplied power should be plenty sufficient. Again these diodes can be easily sourced on the internet or if you have an electronic component supplier in you area they will likely have them. All the diode is going to do is prevent sending voltage backwards across the LED as LED's arent made to work with reversed polarity.

Thanks xx101

I placed a 1N4001 in line  -  silver stripe towards incoming negative.

Placed with silver stripe the other way resulted in no lights.

So now...see pic....the negative incoming goes into the diode (silver end) then onto the lights as originally pictured in first post.

Is this now complete?  The LEDS are now protected because of the diode allowing current to go only one way?

Appreciate the assistance

Dave

Yes, that’s right.

Thanks Jack

One more question.  I am only using one diode.  There are 3 strips it services.

Two strips have 3 lights and one strip has 5 lights.  Is one diode enough for this task?

Thanks Dave

As a test, I powered up a 10 foot strip of leds (about 180 leds) and applied 16 vac thru that diode. The string draws about 1 amp and the diode is good for 1 amp...it got mildly warm. So I'd say you're good for about 180 leds.

Well Thank You Chuck....appreciate the time you took and the testing results.  Very good to know.  I can button this job up now.

On these LED strips from China.....they all seem to be 12 V DC.

Should we use these with the diode and about 15 volts AC or are there strips of LEDS out there with higher voltages than 12 V DC that might better be used for our O Guage enviroment?

Dave

When you half wave rectify a sine wave, the equivalent dc value (the RMS value) of the result is Vpeak/2. So if you use say 16 VAC, the peak is 16 X 1.414 = 22.63 Vpeak.  And that is 22.63/2 or 11.31 Vrms. Which is equivalent to almost 12 Vdc. Minus the diode drop, about 11 Vdc Perfect for those strips. 

Thanks again Chuck

So what we did by adding the diode is "half wave rectify of a sine wave" ?

Dav

Soo Line posted:

Thanks again Chuck

So what we did by adding the diode is "half wave rectify of a sine wave" ?

Dav

Actually the LEDs are diodes and they half rectify the AC. But they, as diodes, can't take more than a 5 to 7 volts reverse, and that is why it's best to be safe and add the rectifier diode. Without the rectifier diode the LEDs could degrade and prematurely fail. Since there are three LEDs in series, it's not likely but still an inexpensive safety.

Soo Line posted:

So what we did by adding the diode is "half wave rectify of a sine wave" ?

Actually, the diode just removed the requirement for the LED to do the rectification.  As Chuck notes, they are not good rectifiers as they're too easily damaged by fairly low reverse voltage peaks.

Soo Line posted:

Thanks XX101.....this is what I am wondering about.

I see where they are primarily DC....but I have had this plugged in for hours now on AC and still working fine.

 

Depending upon how LEDs are wired, they can reliably run using AC.  I wasn't aware of that until I installed a solid state Dallee E unit in one of my post war engines.  The instructions that came with the unit included a bunch of additional circuit diagrams showing how the unit could also support different types of  lighting, including LEDs.  I was able to use one of those to wire up an operating directional backup light on the engine tender.  Circuit was super simple, just  requiring an LED, a resistor and a diode.  Here's a link to the Dallee instructions if you want details (I used the circuit labeled dwg #3 for the tender backup light, using their "substitute a diode for one of the LEDs if you only want a single light" option).  That backup light has been working fine for years now, with no signs of the LED weakening. 

https://www.dallee.com/PDFs/E_UNIT-400.pdf

Thanks John and Dan for your help.

Interesting....."removed the requirement for the LED to do rectification".

I found these and ordered them to tinker with.

They are AC DC converters     .5A 6W   LED Driver Transformer for $1 each.

AC in    12V DC out.

What might these accomplish in lieu of the simple diode approach?

Worth the dollar?

Thanks

Dave

That would be good for passenger lighting since the capacitor would supply current during the intermittent power losses as the car goes over switches, etc. However, you would probably have to select some resistance in series with the LED string to limit the current. The similar power supply that Henning's sells will do what that does as to no light blinking and provide as well an adjustable constant current source for the LED string. IE, you would have constant lighting at a brightness you can dial in for best passenger car lighting effect.

http://hennings-trains.shoplig...regulator-2-pcs.html

I can recommend the lighting regulator, I'm good friends with the designer.

Soo Line posted:

 

...I found these and ordered them to tinker with.

They are AC DC converters     .5A 6W   LED Driver Transformer for $1 each.

AC in    12V DC out.

What might these accomplish in lieu of the simple diode approach?

Worth the dollar?

That module is NOT in itself 12V DC out.  As Chuck says, you will need additional circuitry to limit the current flowing into your LED strips.  That is, with your 14V AC input, that module will put out about 18V DC which will overdrive the nominal 12V DC LED strips.

There are 2 things going on here.  1) convert AC to DC since LEDs operate on DC.  2) adjust or regulate the DC applied to the LEDs to within their operating range.

It's coincidence or just dumb-luck that using a single 1N4001 diode with the 14V AC Accessory output of a train transformer does not require step 2.  Chuck did the math earlier.  By using the 4-diode method (full-wave instead of half-wave) in conjunction with the capacitor (the big black component on the module), the math gets messier but bottom line is you now need step 2. 

Step 2 can be as simple as a 5 cent resistor which provides a fixed brightness; some math is required to choose an appropriate value based on how many LEDs you're driving.  Most guys will pony up $1 on eBay (free shipping from Asia) to get a DC-in, DC-out converter module which provides adjustability.  Two examples (photos hijacked from another thread):

You would need a meter to set the voltage to between, say, 10-12V depending on brightness you want.  For about $2 you can even get a DC-DC converter with a built-in LED digital voltmeter!  You can get DC-DC modules that have screw-terminal in-and-out rather than having to solder wires.  And so on.

Apparently you already purchased the AC-to-DC module, but for under $3 you can get a single module which performs both AC-to-DC conversion and output adjustability.  

Note that above module even has mounting holes; I noticed in your earlier photo that you neatly mount your components using screws.

Strange - your most recent post where you described what you purchased vanished.  It appears you bought something like this:

This module indeed puts out 12V DC.  But the incoming voltage must be 110V AC or 220V AC from the wall outlet.  It will not work with "only" 14V AC from a train transformer.  

Thank You for your replies Stan.

Yes I did post but could not seem to get the link to E bay to work so I deleted it.

Here is the Ebay # for the products I bought.  322366247650......I bought 6.

Unless I am wrong....I think input is AC (train AC) and output says 12 DC.

As well, I followed Chuck's advice using a single IN4001 diode and have had the lights on for over 10 hours now with no flickering or failure.

So this is coincidence ?  A layman here Stan in the fullness of the word !!     So I am trying to get all this straight.

Did I buy the right product for the job in mind?  Just to use on the light strips for lighting buildings.

I also ordered some of the more expensive bucks from China that you listed.

I have also used John's devices and they work perfectly as well.

Oh and yes....I did order a couple of the ones you listed above requiring 110 from the wall.  Just wanted to try a few things.

Just tinkering to learn more......

Appreciate the help....

Dave

is there a  link for that product(s)?

Search eBay for auction: 323231381316.

Select the indicated product in the drop-down.

This module will run on track power and you can adjust the voltage to whatever you like.  Note with 20 VAC, you can get up to 32 volts out of it.

Soo Line posted:

...

Yes I did post but could not seem to get the link to E bay to work so I deleted it.

Here is the Ebay # for the products I bought.  322366247650......I bought 6.

Unless I am wrong....I think input is AC (train AC) and output says 12 DC.

As well, I followed Chuck's advice using a single IN4001 diode and have had the lights on for over 10 hours now with no flickering or failure.

So this is coincidence ? 

Yes.  As mentioned earlier it is a quirk-of-fate, karma, stars-in-alignment, etc. that using a single diode with a 14V AC train transformer puts out a DC voltage suitable for driving 12V DC LED strips.  To be precise, using the single diode method, the LED strip actually does flicker.  That is, the LEDs are flashing on-and-off 60 times per second.  It's just that most folks over the age of 40 (?) can't "see" it...and the lighting looks smooth and steady.  But it can confound digital camera photos with fast aperture settings.  But that's another topic - so if it looks OK to you then so be it.

Here's the eBay listing you referenced.  Note that OGR moderators will delete direct links to an eBay listing.

As I described in a previous post, if fed 14V AC train transformer voltage, the 12V LED strip will be driven with too much voltage.  It will be seem too bright which should be a clue.  

The advantage of the module though is the 4-diode method of AC-to-DC conversion along with the capacitor provides the basis for a much smoother DC voltage source.  Combined with a separate method to reduce the operating voltage to the LED strip, you will get smooth, non-flickering light.  

Separately, once you have 12V DC you can use your 1N4001 diodes to create a string of voltages which can be used to good effect for building lighting.  Put one diode on the 12V output and you get about 11.3V DC.  Put a 2nd diode and you drop the voltage another 0.7V to 10.6V DC.  And so on.  The 12V LED strips light up between about 9V and 12V.  Creating multiple voltage taps allows you to have different LED strip sections at different brightness settings - very economical considering the cost of a diode is a penny or so. 

gunrunnerjohn posted:

....

This module will run on track power and you can adjust the voltage to whatever you like.  Note with 20 VAC, you can get up to 32 volts out of it.

The DC7-32V refers to the input range of the module.  The module accepts either 5-20V AC or 7-32V DC.

As a "step down" converter, the output voltage will be less than the input voltage.  This module is well suited for driving your DC 12V LED sections from 14V AC train transformer - and you get adjustable brightness and true no-flicker lighting. 

Soo Line posted:

...

Oh and yes....I did order a couple of the ones you listed above requiring 110 from the wall.  Just wanted to try a few things.

Just tinkering to learn more......

I trust it goes without saying that "tinkering" with 110 from the wall requires a level of care.

If the objective is 12V DC output powered by a 110V wall-outlet, I think you're better off just getting a 12V DC "wall-wart" which are about $2 shipped on eBay.  Then use a screw-terminal adapter so you don't have to splice the wall-wart coax-barrel plug.

stan2004 posted:

The DC7-32V refers to the input range of the module.  The module accepts either 5-20V AC or 7-32V DC.

As a "step down" converter, the output voltage will be less than the input voltage.  This module is well suited for driving your DC 12V LED sections from 14V AC train transformer - and you get adjustable brightness and true no-flicker lighting. 

Duh Stan, I didn't read that carefully enough!  

The good news is, the module still works great for what he was looking for.   I was simply keying on the 20VAC, I figured that was enough to do the trick for common model train use.

stan2004 posted:

Soo Line posted:

...

Yes I did post but could not seem to get the link to E bay to work so I deleted it.

Here is the Ebay # for the products I bought.  322366247650......I bought 6.

Unless I am wrong....I think input is AC (train AC) and output says 12 DC.

As well, I followed Chuck's advice using a single IN4001 diode and have had the lights on for over 10 hours now with no flickering or failure.

So this is coincidence ? 

Yes.  As mentioned earlier it is a quirk-of-fate, karma, stars-in-alignment, etc. that using a single diode with a 14V AC train transformer puts out a DC voltage suitable for driving 12V DC LED strips.  To be precise, using the single diode method, the LED strip actually does flicker.  That is, the LEDs are flashing on-and-off 60 times per second.  It's just that most folks over the age of 40 (?) can't "see" it...and the lighting looks smooth and steady.  But it can confound digital camera photos with fast aperture settings.  But that's another topic - so if it looks OK to you then so be it.

Here's the eBay listing you referenced.  Note that OGR moderators will delete direct links to an eBay listing.

 

As I described in a previous post, if fed 14V AC train transformer voltage, the 12V LED strip will be driven with too much voltage.  It will be seem too bright which should be a clue.  

The advantage of the module though is the 4-diode method of AC-to-DC conversion along with the capacitor provides the basis for a much smoother DC voltage source.  Combined with a separate method to reduce the operating voltage to the LED strip, you will get smooth, non-flickering light.  

Separately, once you have 12V DC you can use your 1N4001 diodes to create a string of voltages which can be used to good effect for building lighting.  Put one diode on the 12V output and you get about 11.3V DC.  Put a 2nd diode and you drop the voltage another 0.7V to 10.6V DC.  And so on.  The 12V LED strips light up between about 9V and 12V.  Creating multiple voltage taps allows you to have different LED strip sections at different brightness settings - very economical considering the cost of a diode is a penny or so. 

 

Okay .......more to ponder.......Thanks Stan

I'm over 40....a lot !     And I don't depend on karma or star alignment ect....

So.....I guess this little module will do a better trick and for $1 to the door.....

One more question (s) if I may .......

I have IN4001 up to IN4007.........can I use either for the dimming ?

And basically.....what is the difference ?

And lastly......in my first pic top of thread of a section of the LED strip.....what is that little device marked    R7 ?

Cool idea Ted..............nice looking!

Thank You members of OGR..........such a great resource.

Dave

Soo Line posted:

...

I have IN4001 up to IN4007.........can I use either for the dimming ?

And basically.....what is the difference ?

And lastly......in my first pic top of thread of a section of the LED strip.....what is that little device marked    R7 ?

Hmm.  Sounds like you have the Radio Shack 25-diode assortment of 1N4001 thru 1N4007 - was actually a good value at something like 10 cents a piece!  

Any of them will work for the matter at hand.  The difference is the so-called blocking voltage of the diode which ranges from 50V (1N4001) to 1000V (1N4007).  The idea is you are blocking 14V AC... so even the 1N4001 will work.  If you were working with 110V or 220V AC wall-outlet voltage, the 50V 1N4001 would not be suitable.

If you didn't already have the diodes, I'd recommend the 200V 1N4003 for O-gauge train applications to have lying around for this type of project.  These run about a penny a piece on eBay (free shipping from Asia).

To elaborate a bit on dimming see above.  The idea is once you have 12V DC a string of diodes (note silver band on the right side of each diode) creates multiple "taps" that you can use so the LED strips operate at different voltages to create brighter and dimmer strips.  I drew the diagram to show that multiple sections (a section being the 3 LEDs between the copper-island "cut" boundaries) can be hooked up to a specific tap and you can "skip" a tap if it does not provide a suitable brightness.  Again, the idea here is a typical O-gauge building layout that has absolute uniform brightness between all buildings is simply not as interesting (or prototypical) as having variations.  Of course this is just my opinion.

As for the R7, that is a resistor to limit the current.  Each section of 3 LEDs should have a resistor.  The idea here is LEDs are semiconductor devices which have a "curious" characteristic that once they turn "on" then a relatively small increase in voltage causes an exponential increase in current (which will eventually blow out the LED).  This in contrast to a traditional incandescent bulb where a small increase in voltage causes only a small increase in current.   Adding a resistor makes the LED behave more like an incandescent bulb where a small increase in voltage only causes a small increase in current.  I'm sure there are other ways to describe the phenomenon and perhaps too-much-information, but the answer to your question is, "it's a resistor."

Thanks again Stan.

You are very concise and thorough.  And the time you take to add the drawings and images makes understanding easy.

Possibly a teacher at some point.......... 

Dave