how to switch a LM317 voltage regulator / LED on and off?

[davivid]

another thing I've noticed with this new circuit is its drawing slightly more current 0.72A as opposed to 0.69A. Do I need to adjust it? or does the new circuit draw more current to operate. This project calls for high reliability, so I really cant strain/damage the LED's (Typ IF=700mA) .

[BillO]

That's the 4% I was talking about and is the expected amount.  You can increase the 1.8 ohm resistor to 2.0 and increase the 1K to 2.7K which will drop current back down to about 693ma, or just reduce the 1K to 270 which should put it right at around 703ma.

Where the 1.8 ohm is R_L and the 1K is R_Adj the formula becomes:

I_Out = (1.25 + (R_Adj x I_Adj))/R_L

I_Adj is usually 50 uA

Is 700ma the nominal current or the absolute maximum?

[davivid]

Oh right sorry, I didn't realise that's what you meant.

The only specs I have for the LED is: Forward Voltage = 3.8 and Typ IF=700mA, which I assume means Typical? What does one do in these situations? run it at 700mA, or be cautious and drop it a bit lower?

[BillO]

You could run it at 700 without worrying.  Do you have a heat sink for it or does it come with a heat sink?  It will need to be kept cool somehow.  LEDs hate to get hot.  They should have a temperature specification too.

[davivid]

Great. I reduced the 1K to 270, and now it reads 0.7A.

I have mounted the LED on a heat sink, which keeps it really cool. I now just need to upgrade the 1.8 ohm resistor since its only rated for 0.6w.

[BillO]

Good.  Sounds like you've got the hang of this.

[davivid]

And don't forget the heatsinking for the LM317, it'll draw about 5.5W and the 1.8 ohm resistor will dissipate 0.9W remember.

Does that mean the LM317 heatsink should have a Thermal Resistance of 6.6 °C/W?

Thermal Resistance = (60 - AmbientTemp) / Power
Thermal Resistance = (60 - 20) / 5.5
Thermal Resistance = 6.6

Those things are huge!

[BillO]

That's a bit of overkill.

With a supply voltage of 7.5 @ .7 amps, the LM317 will be dissipating:
W_D = 0.7 x (7.5 - 3.8 - 1.25), or about 1.7 watts.  Let's call it 2 to be safe

They recommend a maximum T_J of 125°C for an LM317, but let's keep it cool and on the safe side at 85°C to factor in junction to case R_T.  And we'll put ambient at 25°C, again to be safe.  Then we have:

R_T= (85 - 25) / 2, or about 30°C/W should do nicely.