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Powering an LED from DCC track power

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Powering an LED from DCC track power
Posted by dbduck on Sunday, June 22, 2014 11:34 PM

This may have been asked before, but I have not been able to find an answer through the search feature.

Can the AC leads of a bridge rectifier be connected directly to the track buss to provide DC voltage to which LEDs with the proper sized resistors can be connected? I understand I would have to measure the resulting voltage in order to calculate the proper sized resistors since it probably would not result in a 12 volt source

I am thinking something like this:

http://www.allelectronics.com/make-a-store/item/fwb-64/6-amp-400-volt-bridge-rectifier/1.html

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Posted by mlehman on Sunday, June 22, 2014 11:54 PM

I've tried a similar one from Radio Shack and it didn't play well with LEDs. Not sure exactly why...

However, I've had great luck with the FWB-15: http://www.allelectronics.com/make-a-store/item/fwb-15/1-1/2-amp-400-piv-full-wave-bridge-rectifier/1.html

RS equivalent is 276-0268.

I use these for lighting cars. The circuit is drawn out at: http://cs.trains.com/mrr/f/88/t/213765.aspx?page=2

...about halfway down the page as I use the RS 276-0329 LED strip segments that require 12 volt input. Don't forget your resistor to suit for protection and dimming. The cap can be as little as 47 uF to dampen flicker in mobile apps. For a stationary app, cap is not needed.

That said, plain ol' LEDs, one each, need down around 3 volts. You can add resistance to get that, but it's easier to do a lower voltage circuit. I used this one to build power supplies for structure lighting to run at 3 volts: http://www.spookshow.net/lowvcircuit.html

You can use the first circuit to do lighting like you want. I have this at one place on the layout where I needed a light to help illuminate a staging track that I see via CCTV. For that, I took a spare auto licence LED light array, built a bridge rectifier using one of the big square (5 amp?) RS bridge rectifiers. The auto lighting section is a good place to look for suitable lamps, as they are set up for 12 volts and require no additional resistance. Thing is a lot of rectifiers (not so much the LEDs) running all over the place off your DCC will suck a lot of juice. A handful is probably not an issue, but if you need a lot of lighting under the layout, consider another, separate power supply like that I used for my structures.

Mike Lehman

Urbana, IL

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Posted by dbduck on Monday, June 23, 2014 12:01 AM

Not going to use it to power a lot of LEDs

I have 12 volt regulated power supply for that..I was just wondering if I could..if I needed to power an LED in a remote location without having to run my DC buss in that direction

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Posted by Mark R. on Monday, June 23, 2014 12:35 AM

A single diode is all that is really needed.  To connect an LED to an AC supply you need to place a diode in inverse parallel with the LED and then provide a series current limiting resistor.

Consider the following circuit:

LED ac connection

A suitable diode for D1 would be a 1N4148.

Mark.

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Posted by dbduck on Monday, June 23, 2014 5:54 AM

 I guess my question really is: Will the connection of a rectifier or a diode cause any damage to the DCC booster or have any effect on the "encoded" information sent down the rails

Sorry if this seems like a rather dumb question, but I dont want to do something that would/could damage the DCC system

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Posted by MisterBeasley on Monday, June 23, 2014 6:47 AM

A LED is iself a diode.  If you put a LED in series with a resistor and put it across your DCC bus, it will work.  In reality, it will be "flickering" on and off, but the flicker will be too fast to notice.

This may not be "best practices," but for an isolated LED it should cause no problems.

It takes an iron man to play with a toy iron horse. 

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Posted by rrinker on Monday, June 23, 2014 7:41 AM

 No, a diode across the rails or a bridge rectifier will not 'eat' or distort the DCC signal. If you use a resistor such that the LED gets 10ma, 100 LEDs would draw 1 amp, 500 LEDs would draw 5 amps, so it will eventually add up to a significant amount of your booster power if you add too many LEDs. Still lights up many more cars than incandescent bulbs would for the same current draw.

 The issue with just the LED and resistor is that most LEDs can;t stand a large reverse voltage. A typical silicon rectifier diode can stand 100 or more volts 'backwards' - where it's acting like a door and blocking the flow of current. Exceed the reverse voltage limit and it's like bashing the door in, and the diode no longer acts like a diode. LEDs have reverse voltages not much higher than their forward voltage, maybe around 5V or so for a red LED and about 3.5 for a white LED (they white LEDs are generally more delicate - yes, the forward votlage of a red LED is often about 2.1 or so). Subjecting the LED continuiously to several times this will shorten the life, but that means it might last 50 years instead of 100. Limiting the current to well under the LED's limit helps. Best practices though would be to either use two LEDs back to back or a regualr diode antiparallel with the LED, thus the maximum reverse voltage the LED sees is the forward voltage of the other diode, 2.1 or 3.5 for an LED, or .7 for an ordinary diode.

                           --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by CSX Robert on Monday, June 23, 2014 12:19 PM

mlehman
...Thing is a lot of rectifiers (not so much the LEDs) running all over the place off your DCC will suck a lot of juice...

No, the bridge rectifiers will not draw any current, what you power with the rectifier will draw current, but the rectifier itself will not draw any.

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Posted by mlehman on Monday, June 23, 2014 1:25 PM

Draw is probably the wrong word. It will not be 100% efficient, so will expend energy just heating the room when a load is put on it.

Mike Lehman

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Posted by Mark R. on Monday, June 23, 2014 1:52 PM

A bridge rectifier or diode will drop VOLTAGE. A single diode will drop 0.7 volts and a bridge rectifier will drop 1.4 volts between the input and output.

If you feed 12 volts AC through a bridge rectifier, your net output is 10.6 volts DC.

Back in the days of DC and bulbs, that's how we used to run the 1.5 volt bulbs in our engines for constant lighting by exploiting that 1.4 volt drop by installing the bulb across the leads of the bridge rectifier.

Mark.

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Posted by Mark R. on Monday, June 23, 2014 1:59 PM

As for the requirement of a second LED or diode parallel to the LED in reverse fashion, the resistor in the circuit is already limiting the voltage / current to the proper level, The reverse duty cyle of AC won't light the LED (not that you can actually see it) but the resistor in series will also keep the inverse current well within tolerance of the LED.

I have four signals on my DCC layout that give polarity indication going into reverse sections. The green LED is attached to opposite rails on either side of the gap, and the red LED is attached to the same rail on either side of the gap. These LEDs have been running straight off of DCC track power with no inverse protection for over 15 years and have not damaged any of them.

Mark.

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Posted by richg1998 on Monday, June 23, 2014 4:50 PM

The NCE Power Cab uses a red led and a 1k resistor to monitor the AC out of the Cab. I traced out the circuit on my Power Cab. No reverse diode in the PC board where the Cab is plugged into the panel.

 Rich

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Posted by richg1998 on Monday, June 23, 2014 4:53 PM

Mark R.

A bridge rectifier or diode will drop VOLTAGE. A single diode will drop 0.7 volts and a bridge rectifier will drop 1.4 volts between the input and output.

If you feed 12 volts AC through a bridge rectifier, your net output is 10.6 volts DC.

Back in the days of DC and bulbs, that's how we used to run the 1.5 volt bulbs in our engines for constant lighting by exploiting that 1.4 volt drop by installing the bulb across the leads of the bridge rectifier.

Mark.

 

Even with a bridge rectifier, you will still need a resistor for an LED.

Rich

 

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Posted by narrow gauge nuclear on Tuesday, June 24, 2014 2:28 AM

Randy got it right.  You do need a reversing diode of the 1N4148 type across your LED.  The resistor only limits the voltage to the LED in the forward direction.  In the reverse direction the LED junction will see the full reverse track voltage as there is no current flow through the LED.  The 1N4148 will allow reverse flow through it and limit the often distructive or damaging reverse voltage across your LED to .7 volts or so.  LEDs are not made as rectifier diodes and must not be subjected over long terms to reverse voltages over about 5 volts.  White LEDS can often take a tiny bit more reversal as a rule.

Yes, you can get away with it, but long term use as a light source with reversals over 6-10 volts will destroy the LED junction in time as it degrades.  Don't use the reversing diode and be prepared to go back into your model and replace your LED.

 

Richard

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Posted by cmrproducts on Tuesday, June 24, 2014 7:23 AM

While this theory is all well and good!

I have YET to EVER place a reverse diode across an LED and have problems!

And I have been doing electronics for the past 30 years and built and sold electronic circuits for years.

And the circuits on my home layout and at the Club have been working too for the past 15 plus years with no problems!

Unless the LED MFG have reciently change the components - I can never remember the circuits from years ago in the Electronics magazines EVER showing a reverse Diode!

Is this another Urban Myth?

While Theory is great - REAL WORLD doesn't always prove this out.

BOB H - Clarion, PA

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Posted by rrinker on Tuesday, June 24, 2014 7:55 AM

 They were also careful to run the LED on DC only. It's no myth, it's right there on the spec sheet for most any LED. Here's one for an example: http://www.everlight.com/datasheets/333-2SDRT-S530-A3_datasheet.pdf

 Maximum reverse voltage: 5V. Maximum reverse current at 5V Vr: 10 micro amps (.0001A)

 Back in the early days of LEDs, they were REALLY careful with them, because a basic red LED cost $5 or more. I remember shelving some project ideas when I was a kid because they used a few LEDs and the LEDs alone cost more than the entire sum of other components used in the design, and this being before I was old enough to cut lawns in the neighborhood for spending money, I just couldn't afford to build them. Even in collegel, my senior electronics project was a digital oscilloscope that used an LED display matrix. 160 LEDs, at that time it cost around $80 for them. The entire rest of the circuit cost maybe $20. Now that you can buy 100 for a couple bucks, no big deal if a few fry. Almost as cheap as resistors these days.

                      --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by wjstix on Tuesday, June 24, 2014 8:03 AM

Evan Designs makes LEDs that come pre-wired with diodes and resistors so they can accept AC or DC up to 14 or 16 volts. I've used them straight from DCC power in a few out-of-the-way buildings and they work fine.

http://www.walthers.com/exec/search?category=LEM&scale=&manu=266&item=&keywords=&words=restrict&instock=Q&split=30&Submit=Search

 

Stix
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Posted by cmrproducts on Tuesday, June 24, 2014 8:03 AM

Randy

You are correct!

But this blanket statement of NEEDING a Reverse Bias diode for LEDs is false!

On AC - Yes - Maybe (as I have done a number and they are still running) but for the general rule - they need one!

DC - NOT needed!

We need to get into a habit of specifying - instead of the blanket statements as NOT everyone on this forun (or most other forums) is an Electrical Engineer and they believe every word that gets printed (or is it - they DON"T believe a word we say)! ;-)

BOB H - Clarion, PA

 

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Posted by woodone on Tuesday, June 24, 2014 10:57 AM

Guess that I may be cofused . I have several test tracks that fit on my work bench. I use a RCA plug to the test track from my DCC system (NCE Power Cab)  which let me swich from one test track to another. I put a lED on each test track with a 1K resistor. Resistor is on the positive leg of the LED and hooked to one rail. The LED's other leg is hooked to the other rail. I thought that the power coming from the DCC system was a Square wave AC at about 13.6 volts.

The LED's light up when there is power on the rails, even in the program mode.

It lets me know that there is power on the rails while I am doing DCC installs.

To date they still work, and this is after over 8 years of using this way.

I will say that the time of actual use is very short. Has I use the test track on the work bench for a short time.  

Some day I quess they will fail?

 

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Posted by narrow gauge nuclear on Tuesday, June 24, 2014 11:35 AM

Naturally, on DC no reverse diode is needed.  All Standard LEDs are designed for DC use only.

I am a retired Electronics engineer of 51 years.  I saw the LED come in and learned their strengths and weaknesses as well as advancements made in them over the years.  LEDs are important light sources as indicators in all modern electronics and not as sources of general illumination.

An engineer realizes that as a critical indicator of circuit operation and function, an LED is critical to the operator or user of that item and a burned out indicator can create a number of issues in not alerting a user to issues within the electronics.  Thus, a good engineer never designs an LED indicator into an AC circuit, period!

Model rails are not EEs and many just want the light for illumination, so do with them what you will.  The EE will and can only advise in their proper connection based on real experience in electronics, which was his chosen field.  EE's are not subject to old wive's tales or appocraphal stories related to their profession.  Too much is at stake. 

Advice was asked here and has been given.  Asked and answered.

All of this doesn't mean that a corporate bean counter can't compromise a product at the production stage.  I have seen items using the LED as a diode by itself in AC circuits.  I have even seen them used as real power rectifiers in circuits requiring a few tens of milliamperes in the cheap and mass produced small products on the market.  Thus, they provided DC for circuitry and acted as a power on lamp!  Bean counters can force engineers to alter circuitry to do stupid things like this.  They save money and sell a small product cheap....So cheap, that they know the customer will just buy a new one when it goes belly up.

A classic example of this is the variable speed trigger assemblies of cheap chi-com drills that fill the Harbor Freight stores here.  The "vari-speed" triggers fail frequently due to underrated, Triacs and quadracs coupled with the failure to put in protection components commonly found in well designed products.  They are working at a price point to get you to buy.  They could care less about their reputation.  They will just rename the company with the same old assembly line and workers.  Pennies can often make the difference.

Cutting out components that are only there to protect other components are the first to go. 

This model rail is one that will spend the extra 3 cents to protect an LED buried in his rolling stock with a IN4148 (tiny) or a 1N4001 If I have the room.

 

Richard

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Posted by Mark R. on Tuesday, June 24, 2014 11:55 AM

Richard - Maybe you can explain this ....

In my sitation I mentioned above, using an LED and resistor only attached to my DCC rails as a polarity indicator, it has not failed under continuous operation for fifteen years .... am I just lucky ?

Granted, if I had known then that adding this extra diode as a protection device, I most certainly would have. But at the time, I was not aware of this and just hooked them up based on the assumption it should work .... and it did .... and still is !

That being said, is there something different at play with running them on the higher frequency DCC "AC" signal as opposed to the more typical AC sine ?

My thought was that the calculated resistor value would limit the current / voltage to a safe level regardless of the direct of current flow. (?) Obviously not the case, but still wondering why my LEDs are still working when in theory they should be dead by now.

Mark.

 

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Posted by CSX Robert on Tuesday, June 24, 2014 5:35 PM

cmrproducts
But this blanket statement of NEEDING a Reverse Bias diode for LEDs is false! On AC - Yes - Maybe (as I have done a number and they are still running) but for the general rule - they need one! DC - NOT needed!

I don't think anyone was making such a blank statment.  The whole point of this thread is "Powering an LED from DCC track power", which is AC.

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Posted by narrow gauge nuclear on Wednesday, June 25, 2014 6:40 PM

Many may get away for years with out a reversing diode.  A great deal depends on the manufacture of the LED, the forward current (lower is better), the applied peak to peak voltage and the length of continuous operation.  I too have seen this gotten away with for long periods.  The folks who make the LEDs constantly warn of this reversal problem as seen in all their data sheets.

I have seen a 1N4007 diode used successfully in a 1200 volt circuit.  The diode is only rated for 1000 volts.  In general, ratings are the manufacturer's warranted safe operating ranges for zero failures based on controlled electrical conditions.

 

Richard

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Posted by richg1998 on Wednesday, June 25, 2014 7:37 PM

CSX Robert

 

 
cmrproducts
But this blanket statement of NEEDING a Reverse Bias diode for LEDs is false! On AC - Yes - Maybe (as I have done a number and they are still running) but for the general rule - they need one! DC - NOT needed!

 

I don't think anyone was making such a blank statment.  The whole point of this thread is "Powering an LED from DCC track power", which is AC.

 

Then you have the few who say, DCC is not AC. i have seen the arguments. You got to love it.

Rich

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Posted by Mark R. on Wednesday, June 25, 2014 8:55 PM

If you look at the scope of a DCC signal, there's no arguing it's an alternating current - it just happens to be a square wave at a much higher frequency.

 

Mark.

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Posted by richg1998 on Wednesday, June 25, 2014 9:31 PM

Mark R.

If you look at the scope of a DCC signal, there's no arguing it's an alternating current - it just happens to be a square wave at a much higher frequency.

 

Mark.

 

Spoiling a perfectly good argument with Facts. lol

It was usually by a couple people with a "little" electrical knowledge. The square wave confused them.

Rich

N

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