Constant intensity lighting module for caboose lighting

A potentially better choice is this switching supply, it's smaller, cheaper, and much more efficient.

I recommend against the linear supplies due to the heat generated for even fairly small currents, not to mention the size.

Item number: 280962841170 on our favorite auction site.

You can spend a little more and dispense with the need for a bridge and supplemental capacitor, this is item # 261876960293.  Shipping for units past the first one is only 40 cents.

John,

For us non electrical engineers, can you provide more info about the items you suggested?

Can I assume that the first item you listed will handle LED's for marker lights. Will one need any diodes when hooking up an LED to these boards?

I assume that it is just a straight connection from the center roller and outside rails as noted on the boards input markings.

Ray

This might need some explanation.  T

The original post mentions using a "diode" to convert the AC track voltage to the DC voltage required by the module.  Just a diode operates only half-time while a bridge (4-diodes in a single component) converts full-time.  This means, relatively speaking, you need a larger capacitor when using "just" a diode.  Otherwise the output voltage may not be steady and you lose the benefit of the module's regulation function because you've starved the module's input.

In addition to GRJ's points for using a switching supply, I suggest you get one with as large an input capacitor as possible even if it costs a few pennies more.  This will help with flicker tolerance because a larger capacitor stores more energy in reserve.  Here's a module that has a 220uF capacitor (note "220" on the capacitor on the input side of the module).

The listings don't seem to specify that capacitor value so there is some dice-rolling; some listings even show different capacitor values for different photos of the same item! 

As I recall from another thread, the AC-input version (with the on-board bridge) uses a 470uF input capacitor.

Obviously you can supplement whatever capacitor is there with your own capacitor.  Experience shows that 100uF as shown in some module photos might be a tad low for flicker reduction in passenger cars.  It's a your-mileage-may-vary depending on how bright you want your LEDs, how clean your wheels/tracks/switches are, your personal tolerance for flicker, etc.

Search for "LM2596 module" for the wide variety of choices.  If the issue is connectors, there are versions with those too.

The OP's board and the first one I posted both require at a minimum a diode or bridge rectifier to convert the AC track power for the module.  The second one I posted has the bridge on-board and doesn't need anything added.

You don't need diodes, but I recommend series resistors for connecting diodes to constant voltage sources.  The reasoning here is that very small variations in voltage make huge variations in the LED current.  It's very easy to go from the nominal 20ma to the LED to twice that.  At that current, they will only last a short time, excess current is a killer of LED's.

Truthfully, for LED's I prefer constant current sources, I don't have to think about voltage or excess current.

If you just have a couple of LED's that you want to light in a caboose, it's a whole lot easier to use a couple of components and power them directly.

Here's a four component solution that does the trick and there's no risk of cooking the LED's if you pick the current set regulator resistor properly.  For low currents of 20-30 milliamps maximum, you can use a TO-92 part, for higher currents, the TO-220 part is more appropriate.

Another simple approach for LED's is the CL2 single-chip constant current regulator.

Good alternate solution.  Are there any off the shelf modules available to purchase?

Here is what I found on the "bay".  $2 each with free shipping.

DC to DC Step Down Adjustable Power Supply Module, Based on LM2596 Switching Regulator.

Module name: LM2596 step-down Regulator power supply module

Specification:

Input voltage: 4.5-40V

Output Voltage: 1.2-35V (adjustable)

Output current: rated current 2A, maximum 3A (need to add heat sink), if the output power greater than 15W, the proposed combined heat sink

Conversion efficiency: Up to 92% (output voltage higher, the higher the efficiency)

Static power consumption : about 6mA

Operating temperature: Industrial grade (-40 ℃ to +85 ℃) (output power 10W or less)

Output ripple: < 120mV

Minimum difference voltage :1.5V

(as is the step-down DC-DC circuit, so the input voltage is higher than the output voltage needed more than1.5V

in order to ensure stable output voltage)

Load regulation: &plusmn;1%

Voltage regulation: &plusmn; 0.5%

Size: 35 (L) X 32 (W) X 12 (H) mm (with potentiometer)

Dynamic response speed: 5% 200us

Short circuit protection: current limiting, since the recovery

bulb cars draw 10 times the power of similarly bright LED cars.  If the objective is a somewhat universal solution, then you should use a LM2596 (that's a IC chip number used and what to search for on ebay) converter module.  There are other chip-number modules too but the LM2596 type seems most readily available and lowest cost at this time. 

Note that these LM2596 modules can control Watts of power or over an Amp of current.  But if you are only powering a few LEDs and want constant brightness, then the simple DIY circuit GRJ shows works fine as you are controlling much less than a Watt of power (and much less than 1 Amp of current).

I comment about capacitance simply as an extension of "constant brightness".  In other words, do you want constant brightness lighting (no flicker) over switches, dirty track, etc..  If so then adding energy storage or capacitor(s) is a consideration to ride out the momentary power interruptions.  As it turns out, it is practical/economical (say, 25 cents of parts) to add/select the right parts to ride out momentary interruptions for LEDs...but impractical/spendy (bulky and several dollars of parts) to do the same for similarly bright incandescent bulbs. 

One other thing to think about...  This discussion is implicitly based on the assumption of stepping down constant track voltage - which is what you would do for typical command/control systems.  For those of us running variable AC power to the track, I don't think these boards will work well, even though the specs just cited by Stan say input voltage of 4.5 to 40 v (DC) AND a minimum difference voltage of 1.5 v.  I guess if you are powering a typical LED or two at 2.5 to 3 v, one might be able to get away with this board - especially because most AF transformers have a 'starting' voltage of about 5 v (I assume Lionel's are about the same...).  But, if you were planning to use one of the multi-led light strips (about which there are several recent threads), those operate at 12 v (DC), so the input voltage would have to be 13 - 14 v.  At that track voltage, there would be a lot of 'Flyers' flying...

As has been discussed in other threads, one way around this is to use a buck-boost circuit - of which there seem to be several on the 'bay site as well as some discussions of build vs buy circuits here on OGR...