Get your LED's While they're Hot.

This idea is interesting. When you buy leds for passenger cars. They each come with a circuit board. Does a sting of leds have 1 such ciruit, or can these be used "as is"?
Sorry, I'm clueless.

quote:

Originally posted by Marty R:
This idea is interesting. When you buy leds for passenger cars. They each come with a circuit board. Does a sting of leds have 1 such ciruit, or can these be used "as is"?
Sorry, I'm clueless.

Marty,

You could use them as is and plug them into a wall outlet but i strip the LED's out of the light strings and use a resistor with each LED for engine and building lights.It's a little more work to do it this way but at 67 cents a hundred I'm willing to do it.

David

The LED strips were a lot less work for my passenger cars.

Ditto.
The strips are a flexible base with adhesive that can be cut to the proper length (on a border of a group of 3) and stuck to the underside of the roof. All the resistors and series wiring for 12V use are provided. You will also need to add a rectifier, a CL2 20mA current source provides the appropriate amount of current for a passenger car, and a capacitor if you want to minimize dropouts on dirty track.

Two el cheapo christmas LED's wired in parallel but back-to-back (anode to cathode) with a 680 to 1000 ohm one half watt resister in series with the pair. Connect accross any voltage source from 12 to 18 volts AC, even track voltage. Will easily replace a lamp in a passenger car. No rectifier, capacitor, or regulator needed. Each LED serves as the rectifier for the other.

Does the brightness vary with voltage? Yes, but not much and not as much as a lamp. Do they blink on dirty track? Yes, but so do lamps. Cheap, works great, last a long time.

Al

quote:

Originally posted by HOSO&NZ:
Two el cheapo christmas LED's wired in parallel but back-to-back (anode to cathode) with a 680 to 1000 ohm one half watt resister in series with the pair. Connect accross any voltage source from 12 to 18 volts AC, even track voltage. Will easily replace a lamp in a passenger car. No rectifier, capacitor, or regulator needed. Each LED serves as the rectifier for the other.

Does the brightness vary with voltage? Yes, but not much and not as much as a lamp. Do they blink on dirty track? Yes, but so do lamps. Cheap, works great, last a long time.

Al

I've been using the same thing as you but with a 1/4 watt 750 ohm resistor
Comes on at about 6 volts and I've tested them to 22 volts .So far so good.

67 Cents a hundred add a resistor and some wire and you have a LED with a total cost of 8 to 10 cents a piece.You can't buy them off the Bay or anywhere else for that.

David

quote:

quote:
Originally posted by HOSO&NZ:
Two el cheapo christmas LED's wired in parallel but back-to-back (anode to cathode) with a 680 to 1000 ohm one half watt resister in series with the pair. Connect accross any voltage source from 12 to 18 volts AC, even track voltage. Will easily replace a lamp in a passenger car. No rectifier, capacitor, or regulator needed. Each LED serves as the rectifier for the other.

Does the brightness vary with voltage? Yes, but not much and not as much as a lamp. Do they blink on dirty track? Yes, but so do lamps. Cheap, works great, last a long time.

Al

Does this diagram depict the above quote?

quote:

Originally posted by Bobby Ogage:

quote:

quote:
Originally posted by HOSO&NZ:
Two el cheapo christmas LED's wired in parallel but back-to-back (anode to cathode) with a 680 to 1000 ohm one half watt resister in series with the pair. Connect accross any voltage source from 12 to 18 volts AC, even track voltage. Will easily replace a lamp in a passenger car. No rectifier, capacitor, or regulator needed. Each LED serves as the rectifier for the other.

Does the brightness vary with voltage? Yes, but not much and not as much as a lamp. Do they blink on dirty track? Yes, but so do lamps. Cheap, works great, last a long time.

Al

Awesome, I'm a simple guy. That makes it easy.

Does this diagram depict the above quote?

quote:

Originally posted by Marty R:

quote:

Originally posted by Bobby Ogage:

quote:

quote:
Originally posted by HOSO&NZ:
Two el cheapo christmas LED's wired in parallel but back-to-back (anode to cathode) with a 680 to 1000 ohm one half watt resister in series with the pair. Connect accross any voltage source from 12 to 18 volts AC, even track voltage. Will easily replace a lamp in a passenger car. No rectifier, capacitor, or regulator needed. Each LED serves as the rectifier for the other.

Does the brightness vary with voltage? Yes, but not much and not as much as a lamp. Do they blink on dirty track? Yes, but so do lamps. Cheap, works great, last a long time.

Al

Awesome, I'm a simple guy. That makes it easy. Could you do 3 this way? How can you tell +/- on Christmas lights?

Does this diagram depict the above quote?

I recommend you do NOT try to wire LED's in parallel like that. Unless they're exactly matched, one will be brighter than the other, and in extreme cases, one will not light at all. In addition, your diagram doesn't include a diode to protect the LED's against reverse voltage, that's necessary. The absolute maximum reverse voltage for most LED's is in the 5 to 7 volt range, the peat voltage you'll get from 10 VAC from a transformer is over 14 volts! The resistor limits the current, but you'll find that greatly shortens the life of the LED's, from years to months.

This is a usable circuit in a pinch where space is limited.

One downs side is that the LED itself blocks reverse voltage somewhat (there is a lesser path of resistance each way) rather than a diode. Whether this shortens the life or not depends on the LED design,but in theory it works without shortening the life. Most LEDs can block 5 volts in reverse.With Christmas LEDs often we do not have the specs so we can only assume.

Another issue is there is no way to install a capacitor to reduce flicker. Each LED is half waved so it gets only 60 cycles instead of the 120 cycles it would get with a full bridge rectifier. So maximum brightness is only for half the time. With 2 red marker lights this probably would not be noticeable. However if used in a lighted car brightness would vary with transformer voltage in conventional and flicker would also be an issue.

This type of circuit is usually used to run to LEDs off 120 volts AC where a .05 NP capacitor is also in series to limit current.

Dale H

quote:

Originally posted by Dale H:
This is a usable circuit in a pinch where space is limited.

One downs side is that the LED itself blocks reverse voltage somewhat (there is a lesser path of resistance each way) rather than a diode. Whether this shortens the life or not depends on the LED design,but in theory it works without shortening the life. Most LEDs can block 5 volts in reverse.With Christmas LEDs often we do not have the specs so we can only assume.

Another issue is there is no way to install a capacitor to reduce flicker. Each LED is half waved so it gets only 60 cycles instead of the 120 cycles it would get with a full bridge rectifier. So maximum brightness is only for half the time. With 2 red marker lights this probably would not be noticeable. However if used in a lighted car brightness would vary with transformer voltage in conventional and flicker would also be an issue.

This type of circuit is usually used to run to LEDs off 120 volts AC where a .05 NP capacitor is also in series to limit current.

Dale H

That's why I've been touting the buying of the GE brand LED Christmas lights
They have the spec's on the box Size,voltage, forward current
The Chinese generic brands don't have this.

As I posted they were 76 cents a box at my wal-mart on close out for a 100 light string.

You can make simple LED lights just using a resistor . If you use a DC supply you don't need a diode and you don't need to be concerned with reverse voltage.

I would like to remind you guys that a lot of folks here just want simple solutions to simple questions . They can barley recognize a resistor but when you start adding a diode and a capacitor and making schematics are you doing it for their benefit or to show everyone that you know how to.

LED's are a cheap way to solve a problem of continuously burning out incandescent bulbs.It should be fun and easy for someone to get started not confusing as heck . The guy in the LED primer thread that posted the pictures is what it should look like.

Keep it simple not everyone's uncle was George Westinghouse.

David

Gunrunner, Please take another look at the drawing. Each LED acts as a rectifier for the other, in this case a shunt rectifier in stead of a series rectifier. That is to say that the LED will clamp the reverse voltage of the other LED to what ever it's forward voltage is. The resistor limits the current for each half cycle of the applied AC.

Dale, I have not made a technical study of the charistics of LED's but leakage current would not suprise me especially with cheep LED's. These LED's normally are connected in just such a series parallel arrangement with a single resistor in series with the whole string. I know this as I have taken them appart. I have used this circuit for many years without trouble. The only problem I have had is with LED's that have very different forward voltage drops and different forward current ratings. This causes a problem with brightness. Years ago when LED's were just starting to become popular I read an article that recomended using a diode in the shunt rectifier mode for LED's instead of the series mode as this would provide better reverse bias protection. Yes I have used these to light passenger cars and they work very well. Yes they blink and yes they vary in brightness with voltage BUT SO DO LIGHT BULBS. If this is objectionable then by all means go the full regulator and filter route, but if this is not a concern then this methode is quick and cheep.

One other problem this avoides, although it is not a problem in this application, is the question of 3 versus 4 terminal power supplies. In most train applications we use 3 terminal power supplies. One power input point, one power output point, and one point that is common to both. If you use a bridge rectifier, as is recommended in most projects, you create a 4 terminal supply. One point for the input, one point for the output, one return point for the input, and a seperate return point for the output. The input and output return points can no longer be connected together. Why is important? You cannot use a bridge rectifier to power an accessory and then trigger the accessory from the track without isolating the accessory completely from the track.

Al

I mention the diode because a transformer was illustrated, which certainly suggests AC. As for making the circuit complicated with a diode, let's not get carried away. FWIW, I used to use the LED's without the diode on AC, and I was losing some well before their time. Since adding the diode, none have died.

Sorry for the "complexity" of a diode, but IMO it's a necessary addition when running from an AC source.

quote:

Originally posted by gunrunnerjohn:
I mention the diode because a transformer was illustrated, which certainly suggests AC. As for making the circuit complicated with a diode, let's not get carried away. FWIW, I used to use the LED's without the diode on AC, and I was losing some well before their time. Since adding the diode, none have died.

Sorry for the "complexity" of a diode, but IMO it's a necessary addition when running from an AC source.

John,
Please don't take my comment personally.I'm saying you know what to do ,I know what to do but try to post diagrams and responses in a way that those that don't know all of this will know what to do.

If you do feed these with a simple DC transformer it simplifies things a great deal
Just a Bachmann HO DC transformer that you can pick up for 10 or 20 bucks will power several hundred LED's and I'd rather spend 20 bucks for a transformer than what several hundred diodes would cost.

Simple AC and LED's don't play well...So don't use AC

David

Strings of Christmas tree lights are wired like the 2 LED lights
in parallel across the transformer. A string of 50 lights will have
25 LEDs in series in parallel with the second 25 LEDs in series in the opposite
direction. Half the LEDs light on half the 115v sine wave and the other half
of the LEDs light on the other half of the sine wave.

quote:

Originally posted by DPC:

Simple AC and LED's don't play well...So don't use AC

David

Hard to do when you only have AC coming in to the locomotive.

quote:

Originally posted by pa:
Strings of Christmas tree lights are wired like the 2 LED lights
in parallel across the transformer. A string of 50 lights will have
25 LEDs in series in parallel with the second 25 LEDs in series in the opposite
direction. Half the LEDs light on half the 115v sine wave and the other half
of the LEDs light on the other half of the sine wave.

Correct, the side that is lit limits the voltage across the other string, so no diodes are necessary. That's the same as connecting two LED's in opposite directions with a resistor across AC, the one that is lit limits the voltage to the forward voltage of the LED, hence no additional protection diode is necessary.

quote:

Originally posted by pa:
Strings of Christmas tree lights are wired like the 2 LED lights
in parallel across the transformer. A string of 50 lights will have
25 LEDs in series in parallel with the second 25 LEDs in series in the opposite
direction. Half the LEDs light on half the 115v sine wave and the other half
of the LEDs light on the other half of the sine wave.

There are different methods

One I took apart had 2 diodes,each one half waved an equal number of lights,no resistor.

Another one had a bridge rectifier made out of 4, 1n4007 diodes,no resistors.

Another problem with these lights is distribution of light as they are unidirectional. They look like points of light and are not that good for general lighting. The ones I use have a 140 degree view angle.

Dale H

The circuit with the back to back LEDs will work perfectly well and is often used in consumer products.

Here's a brief description why.

The two important specifications of any diode are the forward current (If) and peak inverse voltage (PIV). There is also a voltage drop across a conducting diode.

For example the ubiquitous 1N4007 has a forward current of 1 AMP and a PIV of 1000V, therefore under normal operation when conducting, there should be no more than 1 AMP passing through the diode and when it is off, there should be no more that 1000V across the diode. The voltage drop is .7V

For LEDs the forward current is specified in terms of the current for normal light output. The PIV is not normally specified, but can be assumed to be at least 50V. For LEDs the voltage drop is about 2V

In the drawing in previous posts, on the positive half cycle of the AC, the left diode conducts and when the diode is on, there is 2V across the diodes and the rest of the voltage across the resistor. The right diode, then will never have more that 2 V across it, and thus will not burn out.

On the negative half cycle the right diode conducts and the left diode never has more than 2V across it.

Interestingly, in many commercial products powered from the line (like battery chargers), this circuit is used with the 110 connected in place of the transformer. However to eliminate the heat generated by the resistor dropping essentially 120 V at all times, a capacitor is used instead of the resistor. (BTW, I do not recommend this, except for sealed products).

LEDs do not really care whether they are powered by AC or DC, just limit the current.

Lad

The following post is technical in nature. Skip it if you aren't into understanding electronics.

Lad, two corrections to your post.

First, a typical LED's maximum reverse voltage is about 5V, not 50V. This data sheet for a typical LED illustrates this point. This is why we should provide a means to keep the reverse voltage low.

Second (and unimportant in this discussion), a conventional silicon diode typically drops closer to 1.0V at full load. The commonly mentioned .7V value is only appropriate when the diode is lightly loaded.

In addition (but seldom mentioned), the reverse voltage across an LED that is in series with a diode is not necessarily zero volts. The diode does not block all of the reverse voltage. Both the LED and the diode are reversed biased, and in this state they act as capacitors that are connected in series. The voltage divides INVERSELY according to the capacitances. If the 1N4007 has a larger capacitance than the LED, more reverse voltage will appear across the LED than the diode!

For the LED data sheet referenced above, the LED's capacitance at zero volts is typically 45 pF. The 1N4007 datasheet shows 15 pF for the diode at 4V reverse voltage. That would indicate that if we had 25V reverse voltage across the diode and LED (rectified 18V AC), the diode would have about 19V drop and the LED 6V.

If we change the diode to a 1N914 or 1N4148 (common small-signal diodes) with a capacitance of 4 pF, the LED will now have only 2V reverse voltage!
(The voltages in my examples were in error. Corrected 1/5.)


CONCLUSION: An LED is actually better protected by using a reverse diode in parallel with the LED rather than in series. The downside is that the power dissipated by the resistor doubles. The back-to-back paralleled LEDs provide this same protection.

Interesting point about the parallel diode Dale, I never really considered that. Given that you're typically only drawing 10ma or so for the LED, the power probably isn't a factor. I rarely run LED's at 20ma, they always seem to bright for the task for me.

Dale M

I have a related question. The circuit described half waves each LED as would a single diode in series. A full bridge rectifier would give the diode 120 cycles per second while half waving would give 60. In theory the LED is only getting half the juice that way and should burn only half as long per second.. Maybe the human eye does not see this but on the other hand there is no free lunch in physics.

In another post I linked some LED sights advising series resistor should be sized for peak voltage,not RMS voltage. In your answer you disagreed and said RMS voltage could be used and that peak only need to be used when a capacitor is also used. Well then if the LED is half waved pulsed then is not the RMS voltage also half? Then by this logic could a resistor of half value be used?

http://led.linear1.org/how-do-...alternating-current/

http://ogaugerr.infopop.cc/eve...312914027#3312914027

Dale H

The LED responds to the average current, though there is also a peak current rating on them. As for the light output, remember that your eye's response is not linear as far as judging light intensity.