Wendy: how can put 3x 3W LEDs (4V and 700mA) using your "practical current mirror" in a power supply of 12VDC?

thank you

 

Wendy: how can put 3x 3W LEDs (4V and 700mA) using your "practical current mirror" in a power supply of 12VDC?

thank you

If your 3 LEDs are in series then they will drop the full 12 V and you have no overhead to work with.

If they are in parallel then you just size the ballast resistors to give the desired current. You can run two in series on one string and the other on a second string. The ballast resistors will do a pretty good job of keeping the current in the two strings the same despite the very different Vce voltages that will result.

 

You can also scale the resistors so that you don't need the voltage regulator circuit to supply 700 mA. It could supply quite a bit less (like 70 mA or even just 7 mA) pretty easily.

You might even be able to put the regulator in series with one LED and then mirror that to drive the other two. I'd have to run some numbers.

 

Something like this?
But have to change the LM317 to a LM338 (heatsinks with a fan?)for this quantities of 3W LEDs, right?
and the 2N222 are enought or have to use mosfets in To-220 package also with heatsinks?

 

The 317 will servo the output voltage to keep it about 1.2 V above the voltage at Vadj.

Consider what the implications of that are for that circuit. Assuming the 317 can made that happen, does that accomplish the desired goal?

Also, how will it allow the individual strings to have different voltages while ensuring the same current in each?