Constant Lo Current For LED

[Torchy]

Following the recent discussions about LED currents/series resistors ...

The 1N4148 is optional (allows connection to AC and reverse-polarity protects). Drives LED at constant current of ~6 to 8 mA, supply 5 - 30 volts. Use any jfet, the NTE I had lying around. Good results with hi-brightness LEDs

Just a thought  

[petemoore]

Looks simple enough !

[Peter Snowberg]

To set the brightness, you can remove the 1N4148 and add a resistor between the JFET source connection and the junction of the gate and LED anode. Try 1K to start and change from there.

Take care,
-Peter

[R.G.]

Nota Bene: Ohm's law still applies...

The JFET constant current source still obeys Ohm's corollary that the power it dissipates is equal to the voltage across it times the current through it. JFETs can easily have Idss (drain current with gate and source shorted) of 5-20ma, which is great for running LEDs. The problem can come in with voltage.

The TO-92 package can safely dissipate about 100mW in free air. If you have a 2V LED and 10ma of Idss, you can only use 0.1W/0.01A = 10V across the FET, giving an allowable power supply of 10+2+0.7V = 12.7V. Not too bad, should work fine with a 9V supply.

If you have a 20ma JFET, it can only withstand 0.1W/0.02A = 5V, so the composite power supply is 5V + 2.7V = 7.7V, and a 9V battery can overheat the JFET.

The First Rule of Power Electronics is - Don't dissipate any more heat than you have to.
The Second Rule is - If you have to dissipate heat, do it in a passive component, as they're far cheaper.

Oh, yeah. The Third Rule is - Sometimes it burns up anyway. 8-)

R.G.

[puretube]

...has it ever been mentioned, that 2 forum members,
"maximee" and "puretube" live/come from OHM`s
birthplace/hometown : Erlangen ?  

[zachary vex]

use the forum fave, j201.  it'll keep current constant at less than 1 mA from 4 V to 70 V.  use an ultra-bright led.  skip the 1n4148.

[R.G.]

use the forum fave, j201. it'll keep current constant at less than 1 mA from 4 V to 70 V

... which brings up the Fourth Rule: Know your components.

JFET Idss varies all over the map, in yet another instance of why everything analog isn't JFET. Fortunately, the manufacturers tell us what Idss is for most JFETs.

Can one horse run faster than another? Of course it can. But which one? The numbers matter, and you just have to know them. Easter-egging is a terribly slow way to find them out.