LED Drivers

I think you are looking for Supertex CL2 (20 mAmp) or CL25 (25 mAmp) or similar Constant Current, LED Drivers.  They are available in the TO-92 transistor package from many vendors (e.g., Mouset). The data sheets describe how they are wired.

Data Sheets:

http://www.supertex.com/pdf/datasheets/CL2.pdf

http://www.supertex.com/pdf/datasheets/CL2.pdf

Vendor:

http://www.mouser.com/search/refine.aspx?N=4294901019&Keyword=cl2

That's it

Thanks, Ken.  Those are the parts I was looking for, which is of course the goal.  Now I just need to order a bunch and start using them in place of resistors.  Looks like the surface mount package might work well for some tight applications, and I'm thinking they won't get as hot as resistors might.

Parts & Pages...

Here is the link to the page I posted, though as I recall the resident DCC expert had issues with the idea of using the current limiting IC as a constant brightness component.  I'll try to find that discussion for you.

http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm

I got mine from Mouser Electronics.

They have them in 20mA and 25 mA configurations in a TO-92 package.

689-CL2N3-G

689-CL25N3-G

and in an SOT-89 (TO-243?) surface mount package

689-CL2N8-G

689-CL25N8-G

The Argument against current limiting IC's

I've quoted below, the meat and potatoes of Bruce Petrarca's comments from another discussion on these components.  Perhaps this is not relevant to your proposed application, but I thought to provide the info just in case.  This also reminds me that I was going to test the firing of an LED with one of these IC's to see how much more voltage is required.  I'll get right on that.

See the entire discussion here:

https://forum.mrhmag.com/post/leds-for-conventional-dc-locomotives-12192126

Power Dissipation

The power dissipation for a given voltage and current will be the same for the constant current devices and resistors. So they can get just as hot as equivalently rated resistors, but their advantage is that they provide a constant illumination.

The TO-92 package will dissipate 0.6 Watt, so it will definitely perform better than a 1/2 W resistor. This is significantly better than the 1/8 W and sometimes smaller resistors we find on some N and HO decoders. But for 20 to 24 VDC Large Scale installations, I prefer to use the TO-252 packages which will dissipate 2 W.

Voltage can be an issue for DC systems which may run at below 5 V, but for DCC systems the available voltage should always be above 5 V.

Test Results...

OK.

I went down to the workshop and wired up a basic 3mm red LED with a CL2 current limiting IC.  I connected the circuit to my variable power supply circuit, 0-10 volts DC, and connected my multi-meter to the circuit.

I started from 0 volts and slowly adjusted the pot. raising the voltage being fed to the LED.  I saw the first glimmer of light at 2.02 volts and had full illumination at 2.79 volts and then the illumination remained constant right up to 10 volts DC.

I could try this with a DC power pack, but I assume the results would be similar.

I guess the current limiting IC's do not pull as much power as some thought?

LED Circuit Testing

Keven,

The results you would get with a Power Pack and your current test setup would vary depending upon what type of Power Pack you use. Cheep to moderately expensive packs which are no more than variable voltage, step down transformers and a rectifier produce a rectified AC waveform: See Full-wave rectification at http://www.ask.com/wiki/Rectifier.Although an RMS meter will provide some relative data, you will need an oscilloscope to measure the voltage as most DVMs will not correctly measure these waveforms. The addition of a storage or more properly a filter capacitor after the rectifier in slightly more expensive supplies will produce a DC waveform with some 120 Hz ripple, and the DVM will be able to measure the waveform with reduced accuracy. Now add more filtering and regulation as available in some high end supplies and you have the same setup that you have now.

But if you try to test with a PMW  or PMW-Analogue pack you will need an oscilloscope for the measurements. You will have to report the results from PMW packs in terms of duty cycle at some peak voltage instead of DC Volts.

LEDs are specified as current driven devices. So to better understand the operation of LED's and LED circuits I suggest adding a current (DC Amps) meter in series with the resistor or current limiter you are using. I think you will find the results interesting.

LED Circuit

Hi Ken,

Obviously I am not an E.E. I am just a hobbyist, so judging by what I've read, you'll know better than I if my results are flawed.  Sadly, I do not have an oscilloscope and have not interacted with one since high school!

I'll try the test with my meter in series to measure the current.  I'm interested to see what that reveals.

I'll also try this out with a power pack.  I have one that I think I can switch off the PWM  functions on the output, so I'll see what I get with that as well.

Pulse free PS

Use a 12 volt car battery or two 9 volt batteries in series. Those will be PWM free.

Bernd

No flawed results

Kevin,

I am sorry if you felt I was dissing you for flawed results. I actually find your test data very interesting, especially regarding the turn on voltage. Your 2.02 VDC turn on voltage is substantially less than the 5 VDC predicted in the other referenced thread, and I was a little surprised that the maximum was as low as 2.79 VDC. What you have shown is that, other than ignoring some of us "experts", it a good idea to test and understand the performance of a circuit before soldering it in place. And, although when testing LED circuits with other packs you may not get accurate data, you will be able to evaluate the performance of devices you may want to use in a target environment. Keep testing.

I have, as have many of our brethren, pulled apart many well crafted devices after failing to adequately bench test individual components and sections of circuits. Worse is that the devices never go back together as neatly.

Regards,

Well, maybe some flawed results...

Hi Ken,

No, no, I did not mean to give the impression I thought you were dissing my testing at all.  In fact, I was expecting to be proven wrong, and may yet prove myself wrong.  It's happened before, lol!

I will keep testing, and I am starting to question my initial results so don't take those to the bank just yet!  When I tried my test again I saw 4 volts at first illumination, which is more in keeping with what Bruce predicted.  I think I know what went wrong, but not exactly sure why yet.  More testing...

In further testing I learned a couple of things.

1. With momentum turned off I can measure the output of my Troller power pack with at least some accuracy.  Yes, I have two of the few still working Troller packs in existence!

2.  Measuring voltage and current simultaneously with only one meter is a real pain.  I used to have two but one got fried in a magic smoke experiment!

For now, duty calls.  I'll return to testing another day, maybe after I get a second meter?