Help Using LM317T3 Lighting Circuit

Change the value of the resistor, the lower the value, the brighter the lights.  Don't go below about 30 ohms or you'll get overheating of the regulator.  If you go lower than around 100 ohms, I'd consider making sure the regulator isn't against a plastic car body.

Practically speaking, you probably purchased the 300 ohm surface-mount-resistor in some quantity.  Rather than removing the existing 300 ohm resistor, you can simply add a 2nd 300 ohm resistor "in parallel" which effectively halves the resistance (to 150 ohms) and doubles the current.  Brightness in LED strips is exactly proportional to current... doubling current doubles brightness.

If that is not enough, add a 3rd 300 ohm resistor.  This makes the effective resistance 1/3rd of 300 (100 ohms) and triples the current (triples the brightness).  And so on.

As GRJ says, you don't want to drop the effective resistance too low or else you might run into heating issues.  There are workarounds to deal with heating issues if you find you need yet more brightness.  If so, let us know exactly what your configuration (kind of track voltage, number of 3-LED sections in use, etc.).

hi you might also just try an adjustable power supply there cheap and just adjust the potentiometer on the board for the brightness and you wont need resisters that get hot from dropping voltage for your  led !check out these cheap  power supply's!

Alan  

https://www.ebay.com/sch/i.htm...able+power+supply+5a

https://www.ebay.com/itm/LM317...6:g:YhkAAOSwx2dYIKPI

https://www.ebay.com/itm/LM317...5:g:Uo4AAOSw6eZaj-IP

Alan

Thank you Stan and Gunner for the fast replies.   Yes Stan, I have several 300 ohm SMD's so I'll try multiples and post my results.   Gunner, I also ordered a small amount of 75 ohm SMD's and I try those as well.   Alan, I have already purchased the "RTR" Buck Converters (yes, they are very economical) but sometimes I just like to "roll my own".   It's kinda like therapy at 3:00 AM when I can't sleep.

Chief Bob (Retired)

Greetings Everyone,

 OK, here's what I did.  I had a few 150 Ohm SMD's so I took your suggestion Stan and paralleled them to come up with 75 Ohms.  I am using a strip consisting of 12 LED"S for comparison.  The circuit with the 75 Ohm set-up was noticeably brighter that the same strip using the 150 Ohm set-up (side by side comparison).  So, there was some indication that if I used a larger amount of LED's there may be a "work-around" regarding resistance values using the LM317T regulator.  I would love to hear about it.   Many thanks to those who replied with helpful suggestions.

 Chief Bob (Retired)  

BTW.  I forgot to mention that I'm using approximately 18 VAC track power provided by a Lionel 180W Power House.

Chief Bob (Retired)

Remember the more current you draw, the hotter the LM317 is going to get!  It's something to consider going forward.  With around 30 ohms, the bare LM317 will start to get fairly warm.  Not a problem it it's in free air, but if it's on or close to a plastic shell, it could be trouble.   Also, the higher the input voltage is to the regulator circuit, the more power is being dissipated in the LM317.

With 18 VAC in, you'll have about 26 VDC going into the regulator, and since the lights are operating at 12 volts, you have to drop the other 14 volts in the LM317.  Let's take that example with 42mA using 30 a ohm resistor.  The LM317 is dropping about .6 watts.  With a Thermal resistance of 50C/W to ambient, the temperature rise in the LM317 will be around 30C.  If the interior of the car is already at 30C, the regulator case will be at 60C.  It'll run fine at that temperature, but it will deform plastic.

gunrunnerjohn posted:  It'll run fine at that temperature, but it will deform plastic.

Well, we certainly don't want that now do we?

PUFFRBELLY posted:

...

I had a few 150 Ohm SMD's so I took your suggestion Stan and paralleled them to come up with 75 Ohms.  I am using a strip consisting of 12 LED"S for comparison.  The circuit with the 75 Ohm set-up was noticeably brighter that the same strip using the 150 Ohm set-up (side by side comparison).  So, there was some indication that if I used a larger amount of LED's there may be a "work-around" regarding resistance values using the LM317T regulator.  I would love to hear about it. 

Annoying to be sure, but there is some math.

You can estimate the current going into the LED strip = 1.25 / resistance.  150 Ohms is 1.25/150 -> 8.3 mA.  75 Ohms -> 16.7 mA.

With 12 LEDs, that's 4 sections of 3-LEDs each.  The current is divided between the sections.  With 75 Ohms, the 16.7 mA is split 4-ways so each section of 3-LEDs is receiving about 4 mA.

The 3-LED sections can safely handle about 20 mA each so you can crank up the current from where you are with 75 Ohms.  But this is just what the LED strip itself can safely handle.  As to what the LM317 circuit can safely handle without deforming any nearby plastic is another matter.  GRJ provided the relevant math above.

So as you continue to lower the resistance, the LEDs get brighter but the LM317 gets hotter and hotter.  At some point, it's simply a better idea to swap out the LM317 with one of the eBay 99-cent (or less) DC-to-DC converter modules.  You'd still use the choke, bridge, and capacitor...but replace the LM317/resistor with a DC-to-DC module.  This would allow adjustability (small screwdriver) and it would NOT get hot.

Bottom line, if you can get enough brightness using the LM317 and resistor without melting plastic, then you're done.  Otherwise, there are (at least) 3 options:

1) Attach the LM317 to a piece of scrap metal to act as a heatsink.  This will spread/dissipate the heat.  Cost would be as low as a #4 machine-screw and nut to fasten to the hole in the LM317.  Note: you cannot directly fasten the LM317 to the metal frame of your car.

2) Add a resistor between the capacitor and the LM317 to dissipate some of the heat.  Most economical would be a 1/2 Watt resistor.  These run a few pennies each.  If it comes to this, one of use will come up with a suitable value after you mess around with what you have to come up with an approximate resistor/brightness that works.

3) Replace the LM317/resistor combo with one of those DC-to-DC eBay regulator modules - the ones that are 99 cents free shipping.  You would still use the choke+bridge+capacitor.  Advantage here is there's a screwdriver adjustment so you can change brightness.

Stan and John,

Thank you both for all of the excellent information.  I understand the heat and the subsequent damage to plastic car bodies, so, I think I will either stick with the 75 Ohm SMD or go with the DO to DC modules.   Trying to figure all of the theory and math will just make my head explode. Again, thanks for all of the help!

Chief Bob (Retired)

FWIW, I've gone down to around 30 ohms on my passenger car LED modules, and I've never had any issues with overheating of the LM317.  Yes, it gets quite warm at 45ma out, but since it's not against any plastic body, it's never been a factor.  I really mention the heat just because you might stick the LM317 package against the plastic body, that would likely be a problem.

How about an alternative solution that also makes it easier to whack your modules together?  I published this board in the past, you can get three blank boards for $2.65 from OSH Park, that's less than $1 a board.  The parts cost will be probably on the order of another dollar or a bit more.  Being an all thru-hole design, it's pretty easy to populate and you have a neat module with all the parts in place.  Stick it to the top of the car or some other hidden place with double-sided foam tape and you're done.

Here are the files all ready for building the board, just drop this file at OSH Park and you can order the PCB blanks.

Constant Current LED Driver Thru-Hole Gerber.zip

Here's what you end up with, probably easier than sky-wiring the stuff.

Thanks again John!

Chief Bob (Retired)