Using 5V Led Strip for Pass Car Lighting with the DIY Module, in Conventional

ADDENDUM: Please see the 4th post down for updated information!

I have long thought that 5V led strip lighting would be best for passenger car conversions if you are running conventional, because they should light at a much lower track voltage than 12V strips. My curiosity got me though as to whether there might be more to it. Since all my conversions are used in command environment, how would I know?.  I decided to do a little bench testing just to find out how well 5V strips would work at conventional track power levels. I used a 14" piece of of 5V and the same of 12V led strip, side by side, 21 leds total for each, each powered by its own lighting module.

Some Background: 5V led strips are comprised of individual 2835 leds, each with a series 100R resistor; compared to 3 series 2835 leds with one 100R resistor per section for 12V led strip.  5V led spacing is 3 leds every 5.0 cm which closely matches standard 12V 300 led spacing.  A 14” long strip (to fit a 16” car) contains 21 leds, same as 7 sections of 12V strip leds 14” long. 18” cars would use 24 5V leds, or 8 sections of 12V strip (again 24 leds).  So as far as the number of leds required per car the 5V are the same as the 12V. Just for interest the regulated voltage to a 5V led strip is only ~3VDC compared to 9.15VDC for a 12V strip, for similar light output.

But here is a surprise: 5V leds need about 3 times the current compared to 12V strip for similar light output. At the full standard DIY module output of 50 ma (2.5ma per led) they are noticeably dim compared to 12V strip. A 10-105 ma lighting module lights them up way better at 60-80 ma (3-4 ma each). I adjusted the 5V module to 75ma to get the same brightness as the 12V module set at 25ma. Who'd have thunk it?

So this means that for decent conventional operation in the track power range of 5 to 10 VAC, we should be using lighting modules with a 100ma or even 125ma output. No big deal, the only change required is switching the R1 resistor from 24 ohm to 12 or 10 ohm respectively.

I was also worried about the heat generated by the 317 at these elevated power levels, but happily it stays reasonably cool, because it is only dissipating low track voltage down to ~3VDC to power the 5V strip. This would of course be very different if you later switch to command operation with 18VAC track power!

In Conclusion:

1. For conventional operation 5V led strip should be used instead of 12V. The big advantage is they are fully lit at about 5.6V track voltage and higher, and the cars will look right even at low track power.
2. If you use standard 12V strip in conventional mode, any track voltage less that about 10V will be noticeably dim, and below about 7V the leds are essentially dark.
3. 5V strip requires about 3 times the current of 12V strip (~75 ma) for the same light output. Because of this lighting modules with R1=10R and R2=100R (or 200R) should be used, with an output range of 10-125 ma.
4. Because of the much higher currents needed for 5V strip, the standard 470uF filter cap allows the leds to dim very quickly when power is interrupted. Consider stepping up to 1000uF caps for better flicker resistance.
5. If/when transitioning to command operation later, nothing would need to change, but the heat produced by the 317 will be way higher. This is because it is now reducing the rectified track voltage of about 24VDC down to 3VDC, at loads up to 125ma. Tab temps of 100C to 125C are likely. So the 317 tab will need to be attached to something metal like the car frame, vestibule end wall, or a generous size heat sink, and nowhere near anything plastic. Use an insulating kit if mounting to the frame!!
6. To avoid high 317 temps when converting to command operation you could switch the led strips to 12V at that time. No changes need be made to the lighting modules when converting, and no heat sinking is then needed.

Here is a pic of the two strips at 5.5VAC track power. Upper strip is 12V, bottom is 5V. Note the 12V is completely dark.

Here are the same 2 strips at 11 VAC track power. It might be a bit hard to tell, but both are about equal brightness. The 12V strip is getting 25ma, whereas the 5V strip is getting 75ma. You'll have to take my word for that!