LED power supply question and FYI

[Anonymous]

Bob

I cut the string mentioned previously apart,no easy task and could not find any resistors.

I cut a second string apart one which I purchased in Canada. This is not UL listed as Canada has their own system. I know this because my daughter lives there and no one would fix an appliance she purchased in the US because it did not meet their codes. I wound up fixing it myself.

This second string has 70 lights warm white.. This thing is hard to cut apart and trace but my best guess is they use 2, 1n4007 diodes I found encased in special sockets. It looks like they half wave 35 using both halves of the sine wave (half with each diode). This would give pulsed half wave DC of about 3.5 volts per LED wired in series. Peak voltage on the LED would be about 4.7. The LEDs themselves are about 3mm. They do not give the voltage on a slip of paper enclosed for the model number but they do say each draws .04 amps.

I suspect my conjecture is true because when I lit them with the wall plug (before I cut them apart) they had a very distinctive flicker I would expect from using a half wave with no capacitor. I tested an individual LED on my circuit. I use an adjustable LM350 regulator circuit set at 3.2 volts. I install 4 Farads of capacitance on the regulated side, that way the LEDs stay lit for about 2 minutes during station stops in conventional. The mentioned LEDs seem to work good at this voltage though I dont know how long they will last. They are getting a flat 3.2 volts DC instead of a half wave pulsed 3.4 VDC. Without LED specs all I can do is guess.

Dale Hz

[jimhaleyscomet]

Thanks for the tips.  I will try next time I can work on this which will unfortunately not be for about a week!

Jim 

[lionelsoni]

Assuming that you're using a simple transformer, with a sine-wave output, the meter will read RMS volts.  "RMS" means "root-mean-square".  It is the square-root of the average of the square of the instantaneous voltage.  The peak voltage of a sine wave is 1.414 times the RMS voltage.  What a typical AC voltmeter does is measure the average of the absolute value of the instantaneous voltage.  For a sine wave, this is 90 percent of the RMS voltage; so the meter displays 1.11 times what it actually measures.  For any other waveform, all bets are off.

The current through those LED strings is probably very non-sinusoidal.  So, even if you had an AC-current setting on your meter, which is rare, it wouldn't help.  However, you could get an idea of what's going on by putting a single diode in series with your string.  (This slightly reduces the voltage, but not enough to worry about.)  This will cause half the LEDs not to light.  You can then measure the average DC current of the string, which will also be the average through each LED, assuming you have a DC-current scale on your meter.  This should be less than the 30-milliampere rating of your LEDs.

[jimhaleyscomet]

Sorry I was not more clear on what I am doing.  I am new to most of this and really do not know exactly what I am doing. 

I am running yellow LED's because I do not mind my interior lights glowing yellow.  The voltage I measured is the one I get when I set my digital multimeter to the AC setting (Radio Shack # 22-801 Digital Multimeter).  Is that RMS?  I have not measured the current yet.  I am not sure exactly how to do that with my meter (I need to find a manual somewhere).  

Thanks for the heads up on the reverse voltage failing shorted.  That means as one fails the others will tend to follow.

Jim H 

[lionelsoni]

Jim, the numbers in your description don't add up for me.

You say you have 2-volt LEDs, which I infer you are using for illumination, since there are so many of them per car.  Yet a 2-volt LED should be red or orange, not white.

You propose to use 9 of them at 18 volts RMS; but that is over 25 volts peak, quite a bit more than 9 times 2 volts.

You say that this does not exceed the current rating of 30 milliamperes.  Have you measured the current?

If an LED in a pair fails open, it stops protecting its companion from reverse voltage.  I don't know the failure mode of an LED specifically in that case; but many semiconductors fail shorted when their reverse voltage rating is exceeded, which would increase the individual voltages.

[jimhaleyscomet]

Since I am are trying to eliminate ballast heat also.....I am considering adding one or two more LED couples (for a total of 8 or 9  2V 30mA LED pairs in series) and running without the resistor and from track power (mine is 18VA / 17V max).     I have noticed that my all my LED's light up at about 5V AC, they get bright by 10V AC and seem fairly bright at 18V AC.  Since I have enough LED's to cover the voltage drop (and a tad more) then there should be no way to burn out an LED due to over voltage.  Plus I am running all of them at less than rated amperage so they should last longer no problem.  I can see if I were trying to maintain voltage across one LED (say 2Vnominal to 2.25Vmax) the tolerances could be a problem.  However, with 9 LED pairs in the circuit (nominal 18V), I can jump (9 x .25 V) about 4Volts without approaching burnout. Plus if one 2V nominal LED is actually 1.8V than the additional voltage drop is spread across all 9 LED's (effectively taking them from 2V to (2V+.2V/9LED) .02Volts each.  

Also, if an LED does fry due to excessive heat, won't that just shut down the entire string?  I believe they fry "open" just like the old Christmas strings.   

In conclusion...I believe I can run 9 2V LED pairs in an AC powered train car on a maximum 18VA /18V transformer without worry about frying the entire line.  This has worked very well on my bread board but I am definately an amature at this!  Please let me know if I am overlooking something!!!!! 

Currenty I am looking for a very cheap (pennies per car) way to mount all 9 LED's in a train car.  For buildings I am considering wiring 9 LED pairs in series and mounting the pairs to 2 terminal terminal strips but that is spendy at $1 per strip.  I am familiar working with wood but non flamable plastic might be better.

Jim H 

[lionelsoni]

Some day I'm going to try using a capacitive LED ballast.  At 18 volts, 60 hertz, I figure 5microfarads would provide about 13 or 14 milliamperes for each of two back-to-back LEDs, without any ballast heat.

[Anonymous]

I went through the thing pretty thoroughly,there are no other electronics on the string. Unless there is some kind of device in the socket but all I saw were 2 LED leads in it. I will cut one apart tomarrow and look. I remember the old Xmas lights 12 volt wired in a series of 10. I still use have some of those 12 volt bulbs somewhere. Anyway I wired LEDs as I described is series of 4, on 12 volts pulsed DC and have not had a problem. Lightbulbs come and go but those things just seem to hang in there over a year now of fairly continuous use. They come in especially useful in places where bulbs are hard to change.

In passenger cars and cabooses, I use an adjustable voltage regulator circuit and just set it at the forward voltage of the LED,usually 3.2 volts. This way I can use several LEDS in parallel and not use resistors also.

I agree the production tolerance would have to be pretty good to allow this. Im not sure about the diode numbers,they had a lot of white on them from the encapsulation material but they were the 1 amp size.

Dale Hz

[lionelsoni]

That sounds convincing, Dale; but it still scares me.  They must be able to make a very consistent LED, to be able to get a reasonable current at a predictable voltage.  Maybe they do some selection.  Even so, it seems like some users would have trouble, given the amount of variation in line voltage.

(1N4002 is 100 volts; so I think probably 1N4003.)

[jimhaleyscomet]

Well I connected my LED's in series.  I had eight (18V/2V per bulb) sets of two back to back (anode to cathode) plus a 100 ohm 1/4 watt resistor.  It works great with my K-line 30VA transformer (peaks at 18 Volts). 

I noticed that when I turned on the transformer there is a brief voltage surge.  So I added a switch downstream of the transformer.  I turn on transformer then the switch.  No power surge now.

Now I need to figure a way to add the flashing LED to the A/C power string.  I am hesitant to use them back to back as I am not sure the lighted led when flashed off will still protect the companion reverse polarity LED from over voltage.   

Jim H 

 

 

 

[Anonymous]

Hi Bob

Just took one of those strings apart made by our friends in China,had a UL listing no less. Had 4 diodes wired as a bridge,1n4002 as I remember on a little PC board lumped into an incapsulation on the string.. I busted it apart to see what it was. Wanted to use the LEDs for my layout trying to assertain LED voltage. 2.4 volts RMS would peak at a little less than 3.4 just about right for white LEDs.

Dale Hz

[lionelsoni]

I am quite skeptical that Christmas LEDs are wired directly to a rectifier.  According to Wikipedia, "LED-based sets use a current-limiting resistor to reduce the current supplied to each LED."

You may get away with wiring LEDs directly to a voltage source if you are lucky or can adjust the source; but I wouldn't.

[Anonymous]

Jim

The 7 volt LED described is either a lightbulb or an LED with a built in resistor. To find out reverse the DC polarity for a second,if it still lights it is a lightbulb.

To answer your other questions you need to know the forward voltage and draw of the LEDs. LEDs can be wired without resistors,a common example is an LED Christmas string. A bridge rectifier ( 4 200 volt diodes) is installed in the line and they are wired in series so they each share a set voltage. 50 on the string would give each LED 2.4  RMS pulsed DC volts when plugged into a 120 volt wall socket.

A common value for clear LEDs is 3.2 volts forward. On a 12 volt AC  power feed these can be wired in a variety of ways without using a resistor.  Using a  bridge rectifier and wiring 4 in series,each diode would receive about 3 volts (ignoring the voltage drop from the bridge) . Since the peak voltage is 4.2 volts the diodes would light OK. However the peak may exceed the maximum allowed voltage on the diode. Most LEDs are OK with this and this is not usually a problem.

To get around this problem and maintain maximum brightness a capacitor can be installed across the bridge charging the circuit to a peak 17 volts DC.  6 LEDs (in series + to -) could then be comfortably  placed without fear of over voltage. I light my buildings using this method with warm white LEDS.

To calculate resistor value when wired singlely you need to know the forward drop of the LED and the current the LED uses. The value is the input voltage minus the voltage drop of the LED divided by the current draw. Typical for white is 3.2 volts, 20ma. In a 12 volt DC feed application for example (12-3.2)/.02=440 ohms. Therefore you would use a 470 ohm half watt resistor, that being the closest larger standard value.

Dale Hz

[lionelsoni]

Jim, the forward voltage of an LED varies through the spectrum, from about 1.5 volts for red to about 3.5 volts for blue (and white, which is really blue with phosphors added).  If yours tolerate higher voltage without damage, they almost certainly contain internal ballast resistors.

You can operate LEDs in series; but you still need a ballast resistor.  The ballast resistor is there to control the current, which is otherwise exquisitely sensitive to the LED voltage.  Imagine a white LED, which might go from off to burned out as the voltage varies from 3 to 3.5 volts.  If you put a resistor between it and, for example, a 12-volt DC supply, the resistor will have about 8 or 9 volts across it, no matter what the LED's voltage is.  So you can select the resistance to give you whatever current you want the LED to operate at.  Suppose that is 10 milliamperes.  Then you could use an 820-ohm resistor, which will, by Ohm's law, draw 10 milliamperes at 8.2 volts.  Since it is in series with the LED, the LEDs current will also be about 10 milliamperes, which is what you want.  The voltage or the LED's characteristics can now vary a reasonable amount; but the current will not be far from 10 milliamperes; and the LED will be safe.

[Roger Bielen]

Jim, a good source of wiring LED's is "Easy Electronics Projects for Toy Trains" by David E. Greenwald, Kalmbach Books.  Myself, I've built small voltage reduction board with hardware from Radio Shack to power LED's and then connect to my 12VAC accessory feed.

The 7V LED may have a resistor, or resistance, already in the item.

[jimhaleyscomet]

FYI for everyone...I added a bunch of LED threads to the FAQ index.  See the thread if you want to run LED's off of a transformer wall pack.  I think I also have added every suggested link to the first post. 

First a little background. I am playing around with LED's.   I am using a Bachman DC controller (For HO trains).  I fried a few by overdring the voltage to see what happens for myself.  Folks talk about a resistor to give the circuit ballast  (see the FAQ links).  Since the voltage is so easy to modulate I can actually dim and brighten the LED's.  The circuit lights up at about 16V and has no problem operating at 20V.  If I add enough LCD's in series (say 10 LED's, 2volt drop operation each) do I need a resistor with the DC powerpack?  Or is there already a resistor in the powerpack?

Also...if I use a "blinking" LED it blinks the entire circuit.  Will that harm anything?

One final question...One LED takes 7V to light (unlike its siblings).  What is the deal with that? 

Thanks in advance for your help Bob and all the rest!

Jim H