Practical use of current limiter

Since you have multiple LED circuits, you'd need multiple CL-2 chips. You can't just put one on the P/S output, it would limit the total current to 20ma, probably not what you had in mind.

I also think a 1 ohm resistor isn't offering much protection for your LED's, I'd be using at least a 50 ohm one, even at 3.3 volts.  There is no current limiting in an LED, and you're probably working them pretty hard.

As for the CL-2, I buy them at Digikey.

Thanks!  Guess I've been lucky on the single LED's W/ 1 ohm resistor.  Most have been in service for 3-4 years.  Let me give a hypothetical.. I am wiring a 3 story O-Scale house with a string of 4 LED's wired in series on each floor.  Assume approximately 12 DC volts needed per sting.  Could I use one CL-2 on the 12V input to all strings or would I still need to put a resistor in each sting.  I am trying to understand if the CL would be a better choice than resistors on each string.

Thanks,

Bruce

Yep, one CL-2 for the series string would be fine.  You can also use one resistor in the string, and it's cheaper.  Since you have a fixed voltage, it's not an issue to figure out the resistor.  Note that for 12 volts of DC, three series LED's and a current limiting resistor is about all you can afford, you need a little headroom above the LED operating voltages.  White LED's run around 3 volts.  Put a 150 ohm resistor in series with the three and you'll be running them all at 20ma, the max for most common LED's.  For less brightness, just increase the value of the resistor, 300 ohms will give you about 10ma for three in series and a 12VDC supply.

Bruce

The advantage of the CL2-N3 chip is that it eliminates resistor calculation and if input voltage varies it will still protect the LEDs. It is useful in passenger car lighting for conventional operation. Here are some circuits comparing use of a resistor for command operation and the chip for conventional operation.

http://www.jcstudiosinc.com/BlogShowThread?id=487&categoryId=426

If voltage input is sufficient it will also allow maximum lighting intensity and still protect the LED whether wired to the LED individually or LEDs in series.

The 1 ohm resistor is probably sufficient on the 3.3 volt DC supply since the input voltage of 3.3 equals the forward voltage of the LED. Most white ones are 3.2 volts,3.4 maximum forward.

12 volts would drive 3 LEDs in series with the chip or a 120 ohm resistor.

Dale H

As John says, the key is you have fixed-voltage (12 or 3.3) so a 5 cent resistor is the way to go vs. 44 cents each for the CL2 at Mouser (I couldn’t find the CL2 at DigiKey). The CL2 is at its best when the available voltage varies such as Dale’s example of passenger cars in conventional.

If you have a meter handy, measure the DC voltage across the 1 ohm resistor.  This will tell you the current into the LED. 1 ohm makes the math easy – the voltage in Volts is the current in Amps.  So if you measure 0.02 V across the resistor, then the LED is seeing 0.02 A (or 20 milliamps).  If you measure anything more than 0.025V (25 mA) I suggest increasing the resistance.

As the guys suggest, for your house, I’d go with 12V and 3 LEDs in series plus a resistor.

The CL2 requires about 3V across it to deliver the 20 mA. So you can’t use a CL2 with your 3.3V bus to drive a white LED or really any LED for that matter. By the time the CL2 takes its 3V “tax”, there’s only 0.3V left for the LED which means you’ll have something closer to a DED (dark-emitting-diode).

Now here’s the nerdy part. The CL2 in series with 3 white LEDs may not deliver the full 20 mA with a 12V bus. The following chart from the CL2 datasheet shows this voltage “tax”.  To deliver 20 mA, the voltage across the CL2 must be about 3V or so – otherwise it supplies something less than 20 mA or nothing at all if the voltage is too low.  At 20 mA, this leaves about 9V or only 3V per LED – cutting it a bit close. Of course the magic of the part is it continues to control (limit) the current to 20 mA over an extremely wide voltage range.  To contrast, I inserted the behavior of a 150 ohm resistor. Current starts flowing at a lower voltage than the CL2 but takes off to the stars with no looking back!

Thanks very much for all the replies.  The explanations are quite helpful.

Bruce

I discovered I got the CL2 chips at Mouser: Mouser Cl2N3-G

Right up to the time the resistor releases it's magic smoke, then current flow drops to zero!

Now John, we both know the LED is just as likely to fail as the Resistor depending on the resistor wattage.

Then again, if they are using Radio Shack parts, the resistor may be 1/2 or 1 watt and the LED will take a lot of damage before the resistor goes.

I've never felt the need to use anything but 1/4W resistors for 20ma LED circuits, they don't even get warm.