Question for gunrunnerjohn

Is the 12.5K resistor in series with the LEDs? If so they should be very dim if not out.

I assume you are using the 317 for constant voltage? What have you set your output voltage for? 

 

Pete

The 12.1K resistor is between the output and the adjust terminals, 317 wired for constant current regulation.  LED's in series wired between the adjust and common

Can you maybe sketch out the circuit for us to see?

 

Rod

I think there is a minimum current for regulation...something like 10 ma or maybe half that but not guaranteed. So try 1.25/250=5ma

Most common LEDs are rated for between 10 and 20ma. Try recalculating the resistor for 10 ma and see what happens. You will be well in the safe zone.

 

Pete

These LED's are from a battery powered Christmas light set.  I measured the current in the set at 0.8 mA, really 0.0008A, I measured it twice.  Here's my circuit not much to it.  It appears as though I have hit the low current limit

 

 

You have it wired wrong.  The 12.1K should go between the OUT and ADJ terminals, and the LED's should be connected to the ADJ and common.  Here's a typical LM317 circuit for constant current, note that the output is from the ADJ terminal and the common or - terminal of the input bridge.

 

The LEDs should be connected between the adjust pin and ground. I just breadboarded this using a 120 ohm resistor which should regulate at 10ma and it does according to my Fluke meter. The LED is lit but not too bright which I expected. If you use an 80 ohm resistor the output should regulate at 15ma (.015).

To calculate the output resistor the formula is I=1.2/R

 

Pete

 

Pete

I had it wired correctly I had it drawn wrong.  Just did some testing in the shop.  The minimum current I could get out is 1mA

That makes sense, you need to have some current to allow the internal circuit in the LM317 operating power.

 

I confess, I've never tried to get extremely low current out of these, I'm normally using them to light LED's.

 

I guess if they are still too bright, you could dump some current with a parallel resistor. I think having the LEDs in parallel though would result in uneven illumination from the two of them. But if you had them in parallel, some equal value resistors in series with each one of them would even up the current to them.

Since you already have the LM317, perhaps put it into voltage regulator mode which requires 2 external resistors.  These should be chosen so that they alone draw the minimum operating current.   Now with a regulated fixed voltage add your 2 series LEDs plus whatever resistor is needed to get your 0.1 mA.

 

 

I just tried an LED with .1ma, and I'm curious what you're trying to accomplish?  When I did this, while I could see the LED lit, it certainly wasn't bright enough for class lights, at least IMO.  I'm surprised you find 1ma too bright.

 

I thought that the LEDs were too bright in the first post because the 317 was not regulating due to not running minimum current. Never tried it, just wondered if that happens.

Hard to say Chuck, but I know they were pretty dim with .1ma.   I normally run the class/markers in the 4-5ma range with my upgrades.  I don't like them full brightness like the headlights, that is too bright as a rule.

 

The LED’s I’m using for the marker lights are from a battery powered Christmas light set.  I measured the current at 0.8mA, really 0.0008A, and the LED’s were really bright.  I’ll try to get a picture tonight

Before you chisel that in granite, try this simple experiment.

 

Take a 9V battery and one of these marker lights.  I'm assuming they're either red or green, those will have a 1.5 volt operating voltage.

 

Use a series 75k ohm resistor (or a value close like 68k ohm will do) and see how bright they are.  I'm betting you'll find there's some issue with how you're measuring the current and they won't be bright enough for markers.

 

There are low current LED's, but I've never personally worked with them.  I buy my LED's in bulk, and I haven't scavenged Christmas lights for them, maybe they low current types?

 

I am guessing these are not conventional LEDs, maybe organic?? The guy who invented them, works at the same place I do. When I get back from York I will have to ask him.

 

Pete

It's hard to say what he has, without having one in hand or specifications, I can only guess.