Passenger Car lighting circuit.

[Lemosteam]

I am attempting to light an old Atlas/Rivarossi boattail observation car.  Rather than use @peteski 's circuit board design that I used on my Polar Express cars, I would like to compress the package of SMD components into a single direct soldered small package.

The components are the same, I just want to feel comfortable knowing that if I solder them together, I will get the same (or similar) results.

In the roof I have glued in a flexible, two conductor pcb strip with LED's soldered at specific intervals, with positive and negative colored flex wire at the vestibule end.

At that same end I want to package the electronics hidden in that area.

The original circuit calls for, in this order from the power inputs, 1 bridge rectifier, a 1k ohm resistor, 6 caps, and a resistor for each separate LED.

I have a few questions:

Do I really need all six caps?

Will a clustered SMD soldered package generate enough heat to melt the car near the caps or resistors?

If "No" then:

What is the best way to arrange the caps, stacked with a + and - wire down each end, or side by side with the wires along each end?

Since the LED's are down the strip will one resistor for all there have a similar brightness as one resistor per LED?

[peteski]

Oh man, I revisited the Polar Express thread - that was 10 years ago already!! 

I dug up the original circuit diagram.
The LEDs, caps or the bridge rectifier will not get hot.
The 56 ohm resistor connected to the positive output of the rectifier will nto get hot.

I'm not a fan of using bunch of parallel-connected LEDs with a single series-connected resistor, but it is done. If all the LEDs are identical (hopefully from the same manufacturing batch), then they should evenly share the current (glow with the same brightness).  But the now-single resistor value will have to be changed since now the combined LEDs circuit will draw a sum of currents of each individual LED. That resistor will have to have a lower value than the original LED resistors. Let me know how many parallel LEDs you'll use and I'll let you know what the new resistor value will be.  That resistor can get warm, but likely not enough to cause any damage (especially if it is not in contact with the model's shell).

You can change the number of the caps in the circuit (or even eliminate them), but with less total capacitance, the circuit will be prone to more flicker (depending how reliable the pickup is, and how clean the track and wheel treads are).
You can physically stack the caps any way you want, as long you connect the caps in the pack in parallel (all positives together, and all negatives together).  I recommended the caps in the polar express based on their physical size, so they could be mounted on the light board in the roof without being visible through the windows.  You could replace them with even a single larger capacitance cap, but larger size Tantalum caps get pricey. 

Also remember to "burn-in" the tantalum caps before installing the circuit in the model to weed out any bad ones (which fail by burning up).  We discussed the process in the Electronics forum in the past.  Even safer would be to use tantalum-polymer version of the caps.  Those are less prone to spectacular failures.

[Lemosteam]

Hi Pete,

I understand but space is limited, there are three LED on the strip.

Here is what they look like, and yes they are from the same manufacturing reel. Can’t recall where I got them. I bought them for this flex pcb concept.

At a straight three volts DC they appear to be the brightness I am looking for.



Sorry for the crappy pic.

[peteski]

These look like yellow/amber (not white) LEDs.  3 volts?  Those should have around 2V across them. I would think if you feed them straight 3V (withotu any resistors) they would be extremely bright, get hot and eventually burn up.  What is the power source you're using?

I would need the current reading (not voltage).  Can you connect the multimeter set on 200mA range (most meters have that), in series with the power source and the LEDs, then tell me the current reading for the brightness you like.

[Lemosteam]

Sorry there are four LED's and they are yellow, not white.  Meter read 8.5 on the 200mA setting.

Also still need to know if I need all 6 capacitors or if less will do.

It's this little guy, 3V button battery that may or may not be fresh.

[peteski]

OK, that helps.  With 4 LEDs in parallel consuming 8.4mA each, that is 33.6mA total.  Use a 220 ohm resistor in series with those LEDs. Resistor should be rated for at least 0.5 Watt (1 Watt would be better).  This resistor will get fairly warm.  This is all calculated for about 12V track voltage.  Of course if the track voltage is lower, the LEDs will not glow as bright, but that is also true with the circuit in your Polar Express.

Why don't you build the circuit, and before installing it in the car connect the LEDs and track power using test leads. That way you can see how warm things will get.

Your LED tester looks like it uses a small lithium coin cell (like CR2032). Those, even when shorted cannot supply a lot of current.  Not enough to burn up the LED (the battery's internal resistance basically limits the current just as if an external resistor was connected in series with the LED).  When I was surprised that the yellow LED did not burn up when 3V was applied, I assumed that you were using a power supply capable of delivering couple of amps of current.

As for the capacitor question, I answered it in my previous post:
You can change the number of the caps in the circuit (or even eliminate them), but with less total capacitance, the circuit will be prone to more flicker (depending how reliable the pickup is, and how clean the track and wheel treads are).
You can physically stack the caps any way you want, as long you connect the caps in the pack in parallel (all positives together, and all negatives together).   I recommended the caps in the polar express based on their physical size, so they could be mounted on the light board in the roof without being visible through the windows.  You could replace them with even a single larger capacitance cap, but larger size Tantalum caps get pricey.

Also remember to "burn-in" the tantalum caps before installing the circuit in the model to weed out any bad ones (which fail by burning up).  We discussed the process in the Electronics forum in the past.  Even safer would be to use tantalum-polymer version of the caps.  Those are less prone to spectacular failures.

[Lemosteam]

I bought more of the same caps you recommended in your article.  I will have to buy a 1 watt resistor somewhere local.  Do I still need the 520ohm resistor before the caps?

[peteski]

I bought more of the same caps you recommended in your article.  I will have to buy a 1 watt resistor somewhere local.  Do I still need the 520ohm resistor before the caps?

Yes John, you still need that 52 ohm resistor (the resistor value code shows 520). You can use the same ones you used in the Polar Express.
Not sure where you will pick up a  220 ohm 1W resistor locally.  Do you have electronic suppliers nearby?  Radio Shack stores are all gone.  Also keep in mind that a 1W resistor will be physically larger than the surface-mount ones you have.

[peteski]

John, I just thought of a way you can use the parts you already have for that 220 ohm resistor.

Just connect 4 of the 1K ohm resistors in parallel. That will result in a 250 ohm resistor. That's close enough (the LEDs will be very slightly dimmer). And the wattage will be shared between all 4 resistors, so that will be safe.  I think the resistors you have SMD1206 size are rated 0.25W each, so the total wattage will be 1W.

[Lemosteam]

John, I just thought of a way you can use the parts you already have for that 220 ohm resistor.

Just connect 4 of the 1K ohm resistors in parallel. That will result in a 250 ohm resistor. That's close enough (the LEDs will be very slightly dimmer). And the wattage will be shared between all 4 resistors, so that will be safe.  I think the resistors you have SMD1206 size are rated 0.25W each, so the total wattage will be 1W.

Perfect!

[Lemosteam]

This is as small as I can get the package. The positive and negative conductors are 0.25mm thick leftover etch strips on the positive and negative sides of a six pack of caps. The 250 ohm resistor pack made from four 1000k SMD in parallel. I added a micro connector so I could service the roof if needed. The 56ohm resistor and bridge rectifier are up front.








Here is the package in the vestibule end of the car. Had to remove some restroom walls.

[tehachapifan]

Nice work!

Dude smokin' in the corner has some killer bell bottoms!

[peteski]

Looks good John!

[Dwight in Toronto]

That’s a really nice little “cube” of a package - well done!

I constructed my own LED strip power supplies a couple years ago - one for each of the 12 coaches in my Kato Morning Daylight.   However (and not knowing any better), I used an electrolytic capacitor which was rather large-ish, and a 5V voltage regulator component as well.

Although my approach resulted in beautiful flicker-free lighting, this cube innovation is a significantly more compact package.

Can someone repeat exactly what capacitors those are in the six-pack, please & thanks?

Also, I remember asking @peteski  why a voltage regulator is not required, but I don’t recall his explanation, and I can’t find the thread.  Apologies Peter, but might you care to enlighten once again? 

[Lemosteam]

HI Dwight, It's Pete's design so I won't comment other than to say that the caps I used are these:

100 UF 20V Tantalum Capacitor, Digikey # 718-1744-1-ND. And  as you might expect they are sold individually priced based on the number you purchase STARTING AT $1.21 USD apiece.

https://www.digikey.com/en/products/detail/vishay-sprague/293D107X9020D2TE3/2259901?s=N4IgTCBcDaIOwEYAcBaBcAsG0oHIBEQBdAXyA

And by the way this is a pic of it lighted with the Kato DC powerpack at about 40%