Help Converting to LED for my passenger cars

Mikeaa, GRJ showed a picture.  I don't have a link handy, but go to Alibaba.com and search for "buck ac to dc converter".

It is larger than one can fabricate, but it fits fine in a car with translucent windows.  

That Harbor Freight meter is cheap, but it won't accurately measure ac voltage under certain circumstances.

Most inexpensive meters have issues with chopped waveform AC power.  If you want better results, you need to look for meters that offer True RMS capability.

GRJ, didn't you post a picture of a can of worms?  Get out the can-opener!

All meters, irrespective of price, will not accurately measure voltages under certain circumstances.  Or in some cases the measurement does not represent what you think and/or what is useful for troubleshooting. This is increasingly the case with modern O-gauge electronics. Your chopped AC waveform is a classic example.  Here are a few more:

- Put just a diode (rather than a bridge) to convert track AC to DC.  Measure the voltage after the diode using AC or DC. This conversion is used on many simple train LED circuits. Show me a meter (true RMS or not) that accurately measures the voltage.

- Measure the pulsing DC voltage used in MTH PS2 to drive the constant-brightness lamps. The bulbs are 6V and brightness stays constant as you change track voltage.  Show me a meter (true RMS or not) that displays anywhere near 6V or that displays a steady reading as you change track voltage.

- In conventional track control, push the bell or whistle button which puts a DC offset on top of the AC voltage.  Show me a meter (true RMS or not) that accurately measures the voltage.

And so on.

It can get quite confusing understanding the capabilities/limitations of something as "simple" as a multimeter.  But if just starting to dabble with electronics via a passenger-car LED conversion project, the "free" Harbor Freight meter will make the relevant measurements....in my opinion.

Stan, you are correct as we've come to expect.

The good news is, other than some academic discussions, it's rarely necessary to have precise measurements of the things you mention.  Many times you really need a comparative reading with a known good reading.

As far as the DC offset with the whistle bell, I use a meter on the DC scale and get a good approximation of the magnitude of the offset.  I don't have to know  the absolute value, I just have to have a relative reading so I can compare it to other readings.

For the simple diode rectifier, I just measure the AC and compute what the effective voltage on the other side will be.

Stan, I do not disagree in the slightest with anything you said.  I have a boodle of those HF cheap meters.  Tkey're great to have handy.  But users should beware that they can give wild readings on some circuits.  Within the last 2 months I was doing something related to trains, and they gave readings that were so far off I knew from experience they were off.  I tried an ancient RadioSHack (Micronta or Archer) digital meter and a fairly new Ideal, and they gave a reading more in thje realm of reasonableness.

The comment above about using a 6 volt capacitor reminds me of another relevant consideration, of which some readers might not be aware.  If using a rectifier fed by an AC circuit, the AC voltage reading will not be the peak reading, and any capacitors or diodes on the DC side of the rectifier should be sized to handle the peak, not a DC or AC voltmeter reading. 

For example, in a 120-volt house circuit, the peak is up in the 165-175-volt range; for a 20-volt train circuit, I'd guess it's up between 25 to 30.  (I know GRJ has at hand the actual calcs.)  WHen building an AC to DCV converter, I'd use a cap rated at 30VDC

Capacitors fed from rectified AC using diodes or bridge diodes will charge up to the peak voltage of the AC waveform, not the RMS value.  So, given an 18 volt RMS waveform, the peak is 1.414 times the RMS value.  To be totally correct, you can subtract the diode drop from the value, but that will vary based on the load.

If you just stick a diode and capacitor on an 18 volt RMS pure sine wave, you'll get pretty close to 25 volts charge with no load.  As the load goes up, the voltage across the cap will obviously go down.

I use 35V caps for supplies as they are common and give me a reasonable margin of error.  Even if the RMS voltage hits 24 volts, the peak is still 34 volts, and I rarely expect to see 24 VAC powering the stuff I do.

GRJ & I actually agree on something!

Golly, we'll have to fix that, I'll go back and delete my comments!

Dale, GRJ, Stan, RJR,

Thanks for all the responses!  You guys are the greatest!  I'm trying to soak all this in and the more I read, the more I'm beginning to understand. And thanks to the OP for posting this thread!

I will likely get a few of the AC-DC Buck Converters to experiment with for lighting buildings, etc, but since I want to light some S Scale cars, I think I will order some of the individual components as suggested to keep things more compact.  Thank you Stan for making up that list.  I'll start with that and maybe look at adding some higher value resistors since the LED strips will likely be only 12 inches long.  Also with the cars being smaller, the lights will not have to be as bright.  I really like the ability to adjust the brightness.  Correct me if I'm wrong, but my understanding is that this circuit will work with AC or DC input and can be used with any of the control systems.  Will there be any issues if I use DCC?

Thanks, Mike A.

If you're lighting buildings, fyi putting the strip LEDs under eaves of stations, or the roof of station platforms, really makes them a center of attraction, rather than just a few lights inside.

No issues with DCC. 

When using un-modified off-the-shelf 12V LED strips, there is an issue with conventional control systems - at lower track voltages the LEDs may not light up or be too dim.

WRT the trimpot.  While probably hard to do in a passenger car, I tip my hat to the designers of some O-gauge engines where you access the audio or smoke level trimpot controls by opening a tender hatch or whatever.  It would be most clever to come up with a way to adjust brightness with the car buttoned-up and powered on the track.

I've done that on a couple, but it was somewhat of a PITA.  Every model car has it's own design, and having access to a control pot is unique to each brand and how you mount the pot.

If you come up with a clever solution Stan, I'm all ears.

One advantage of DIY is ability to customize functionality to exactly what you want.  Nothing says brightness adjustment has to be via a trimpot.  For example a simple switch can select one of two resistors so you have a low-high brightness switch like the bright-beam automotive headlight switch.

So rather than a 25 cent trimpot, you could use a 5 cent switch to select between one of two 2 cent resistor.  If you then mount the switch thru the floor of the car it would be fairly simple to reach under the car to flip the switch and change brightness.  Obviously this only gives 2 adjustment levels but then again how many does one need?!  For not much more you can get a 3-position slide switch and choose between 3 resistors for low-med-high.  Or maybe make one of the positions "OFF" so you have Off-low-high.  And so on.

Or, if you search the archives I previously showed how to change brightness of the LED strips by using a magnet to trip a switch in the car to change the resistor value.  So the passenger car lights turn bright when arriving at a station then turn dim when leaving a station.

I seem to recall an idea to use a photoresistor (resistance changes with ambient light) to adjust the brightness in a passenger car.

In other words, once you decide to roll-up-your-sleeves the possibilities are endless limited only by imagination.  LEDs on strips run about 2 or 3 pennies each.  So parts costs can be quite affordable.

Stan, if you copy the auction number from the eBay auction, it helps.  It's not a direct link, and so far, has been allowed.

It just seems these LED threads drag on for months and the ebay auction numbers turn to pumpkins sooner than later.  But you're right - if I'm already cutting and pasting images anyway it ought not to take much longer to copy the auction numbers...

The early readers get the benefit.

I have found alibaba to be a good place to  hunt down these gismoes.  Usually yields a list of dealers in China with varying prices, and delivery time hasn't been bad.

I tried the suggested search for the AC-input regulator on alibaba and ended up having to request a quote to get a price. Maybe that's no big deal and perhaps would save some money in the end, but for eBay users here's a Buy-It-Now listing for $3 per module when you buy 2 - free shipping.  ebay auction number 261521744701.

I've always found Alibaba somewhat useless as I don't want to ask for a quote, I want a price quote.

Try its subsidiary, aliexpress.com

I've purchased through Aliexpress, but that's not what you said.  If you go to Alibaba, it's a waste of time for the most part.

Here is what I use for each passenger car.  Many other similar products are also available.  For ac power pickup from the track a diode will also be necessary at the input.

Constant intensity lighting module for caboose lighting

Here is an inexpensive constant intensity lighting module that will be used for caboose and passenger car lighting purposes. It is a Dc to Dc converter module that has a screwdriver voltage adjustment.  It can be used for DC bulb or LED circuits only. The cost per module was $1 each on Ebay.

The module has screwdriver terminals for inputs and outputs.

For each LED that is powered a series resistor is also required. The LEDS that I have purchased include leads and a built in resistor.

Since it's a DC input module, I suspect you will need at least a bridge rectifier in addition to this module...

GRJ, you are a suspicious character, but this time your suspicions are well-grounded.

I've purchased through Aliexpress, but that's not what you said.

Mea culpa, mea culpa, mea maxima culpa.  All I can do is humbly beg foregiveness, and point to the words of a great lady:

"Mistakes are part of the dues one pays for a full life."

          ---Sophia Loren

This would be used for someone running D.C? So your track input would be D.C correct?

There's better options than that module anyway, it's pretty huge compared to the superior solutions available.

For an extra 80 cents, you can have this more compact buck/boost converter that will allow you to run conventional or command and maintain constant brightness of the LED strips.  Auction # 261813576458

The much lower profile allows it to be more easily hid, typically you can stick this to the roof of the passenger car with double-sided foam tape and it will be out of sight.  The big module recommended sticks down about 1.5" and would be seen hanging in the windows.  Another advantage of the buck/boost supply is it's a switching supply, so it doesn't get hot, the linear supply previously posted also gets pretty warm if it has to handle any significant current.

You still need the bridge rectifier I mentioned, but this will run with track power from around 4-5 volts up to 18 volts and output a constant voltage.  For the 12V LED strips, the standard buck supplies need 8-9 volts on the track to light the strips.  That means at low voltages the lights are out until you get rolling.  For some locomotives that means they're rolling at a pretty good clip before the lights come on.

It would be an easy choice for me.

How do I identify what kind of diodes are being used in a bridge rectifier, and what the voltage should be coming out of it?

The bridge rectifiers are rated for how much voltage they can safely carry. 50 volts is enough in most applications for our trains, (Peak voltage is at most 35 so there is an adequate margin of safety) though higher rated ones are often the same price.

They also have an amp rating. For passenger cars and LEDs,1 amp is fine. Sometimes 4 amp ones are easier to mount,depending on what you are doing.

All bridge rectifiers consist of 4 diodes in a package connected in a certain way so that full wave DC comes out of them..here is one of many google pics

Dale H

So the voltage coming out of the rectifier doesn't drop any? I thought I read somewhere that it would drop depending on what type of diode was used. Somewhere in the neighborhood  of .4 or .7 volts. So if I  was putting in 18volts it should drop .7 per diode X2 = 1.4 so I would end up with 16.6. Did I get that wrong?

The bridge itself drops a little over a half volt,the same as a diode. This is insignificant in most applications,especially when a capacitor is added.  There are special diodes with little or no drop but that is for other electronic applications. (Low voltage logic and switching circuits for example where a diode drop would be significant)

18 volts AC in would come out 17.4 pulsed DC. With a capacitor about 24 volts filtered DC. You wont miss the .6 volts. It is usually ignored.

Just to clarify, Each diode rectifies half the sine wave so the total drop is just one diode,not 2.

Here is a utube video that illustrates flow

LINK

Dale H

Stan What is that board in the picture and how or where cab I get a bunch of them? I want to keep everything as small as possible. I have done some research and can not find them anywhere.

You can get tons of prototype board material on eBay for cheap.

I prefer the fiberglass board, here's a good price, $1.41 for 5pcs: eBay - 261424055403

I take these and cut them to size on a bandsaw.

As GRJ suggests, eBay is the place to go.  I showed an example earlier in my post showing parts breakdown at under $1 per module via eBay.  I included enough board material to make 10 modules.  I didn't include the eBay auction number which is 140702746015.

You can get them at US mail order and even Amazon but for several times the price.

It occurs to me that we are not answering your question. Part of the reason you may have trouble finding it is because I don't think there's a universally used name for what to call it!  I found this explanation on Wikipedia for "perfboard" or perforated board.  The article also gives some assembly tips on how to use it.

https://en.wikipedia.org/wiki/Perfboard

Your photo shows a phenolic board; in general you can identify phenolic boards by the tan-brown color.  In your photo there are no solder-rings or pads on the board though you can find phenolic boards with these pads.

What GRJ and I recommend is FR4 board aka fiberglass epoxy resin. The FR4 boards shown at the bottom have solder-rings on each hole and on both sides of the board.  FR4 is more expensive than phenolic but is more durable and I find easier to work with.  I find the solder rings/pads adhere better to FR4 than to phenolic.  As shown in my photo above, Phenolic material can crumble. 

These boards are 1/16" thick and a bandsaw or hobby saw can cut them.  They are easy to drill thru for mounting holes.