LED voltage regulators

Model Railroader General Discussion (Model Railroader)
1

looking to drive some LED’S I need lights in my MRC meters

is this the best way the LM334Z This comes in a very small package (TO-92), smaller than the LED. The LM334Z is a “programmable current source”, what this means is that the chip will allow only a certain current through it, irrespective of the voltage. “Programmable” only means that you can set the amount of current yourself. This is done with a single resistor. The unit can take forward voltages up to 40V, and needs no protection against reverse voltages (i.e. the locomotive is running in “reverse”) for voltages up to 20V. Both of these exceed the nominal 12V of model railroads by a very comfortable margin

one chip = less price

nature of the circuit means that it will equally well drive a 1.7V Red LED or a 3.6V White LED without any modification!

2

Assuming you are opening up the case to install the LEDs in the meters, why not just use a single 1000 ohm resistor and power it from the internal DC source ???

Mark.

3

that will take the 12V down to 3.6??

4

Resistors don’t limit voltage, they limit current - this is the important factor as LEDs operate off very low current. You can operate LEDs from virtually any voltage as long as you se the proper resistor to limit the current flow to the LED to the required rating. A 1000 ohm resistor is always a good starting ( safe ) point to start with. You may find that the white LEDs are way too bright at their maximum current rating for the meters and will probably need to use a much higher rated resistor to dim them down to a more tolerable brightness. A little experimenting comes into play at this point. Probably a value somewhere between 5000 and 10,000 ohms will be much more acceptable for your meters. You can add resistors in series to obtain the value you want, just add the total values of all the resistors in series for the total ohms resistance.

Also keep in mind LEDs are polarity sensitive. If it doesn’t light the first time, just reverse the leads. Installing it backwards won’t hurt it, but it won’t light either.

Mark.

5

I just getting back into RR so I’m not sure exactly a MRC meters is but I did just get my EE degree so I do know some thing about LED. They are run of current not voltages so adding a resistor would be the simplest. I think the LM334Z would also work but I never used one and it is could get pretty complicated. If I know more about the meter (circuit diagrams or just the voltage you are adding in the led and the led what kind) I could be more help.

Justin

6

hello justin

the MRC is a 9500 and it has two meters one for volts and one for current

7

I’ve used the LM334Z in locomotives to drive headlights. This chip does NOT regulate voltage! It is specifically a current regulator, which is the criterion needed for LEDs. Information and a description of the circuit and materials needed can be found here. You can get them from Jameco Electronics

It is a slick and very effective little (very little) beastie.

8

.66 mouser had them for .64

well if I order parts whats a good white led some are over $8

I seen a bluish one ?

K

9

Thanks Gmcrail, nice link and the LM334Z doesn’t look that hard to use. And I will keep this in mined if I am ever to add a LED to a variable voltage source like a train headlights, it will stop them from dimming at low speed/voltages. This would work for you too but you could get by with just a resistor for each LED.

V = I*R

V= the fixed DC voltage

I= the amps that your LED runs best at

R= the resistor you need to add

Justin

10

dont do it!!!

this is not working out you are right

leds are directonal and are not that good for a meter

I driller the pack I drilled the meters I worked for hours

so far nadda its not a good light

I did find some of the plastic fiber optic

plastioptic here is the mess

run dont walk away from this upgrade

I know MRC make’s a lightd one, just my luck

K

11

It’s kinda hard to tell from the pics, but it appears as though the meter casing and meter face are made from a translucent white plastic. IF that’s the case, mount the leds so they are aimed at the back of the housing causing it to glow - much like the dashboard lights in older cars.

Also, sand the surface of the LED to diffuse the light more, it will eliminate that focused spot of light.

If you are after the edge light idea, you are going to have to use bulbs to get the kind of illumination you’re after. A couple twelve volt bulbs in series wired to the 12v tap would give you a nice glow to the meters and not be blinding in the dark.

Mark.

12

WOULDN’T IT BE SIMPLER to just replace the bulbs?

  1. LEDs don’t illuminate as well as bulbs.

  2. They are uni-Directional. Bulbs are not.

  3. MRC used to use a non-standard fastener to seal their power packs. You’ll probably have to buy a special tool or drill them out .

  4. MRC will replace them if you mail your unit to them. How badly do you need lights?

13

well this unit never had bulbs I did add 2 grain o wheet but they quickly burnt out and melted the plasic, I tryied the leds from behind the meter and it just barly lit the meter yes I did get the long torx head tool

hold everything

I just added a 470 ohm and the light was awsom

two lights drilled into the meter

wow what was I thinking I just added a toggle to the power

this may work

it worked but took till 3am

K