LM317/LM338 2x 16850 voltage not enough?

[bakulaw]

Hi! I've NDB7875 Diode and LM338 Regulator.I used .7(.666) Ohm resistor on LM338, expecting 1.7A Current.
Are 2x 16850 batteries enough if I want around 1.5A current? I tried using 2x 16850 and it's only around 600mA, I tried 3s Lipo and it's working, 1.55A. I just wanna be sure before I extend my host.

 

[DashApple]

You should need at lest 3-4V more than the forward voltage of the laser diode in use with a LM338/317 regulators , there is 1.25V over the sense resistor and you need at least 3V over the regulator .

Two Lipo's give 7.2V/8.4V , assuming the diode is 4.5-5 Vf , Plus the 1.25V ref , that's 5.75-6.25V , with the LiPo's at 8.4V max that leaves around 2V over the regulator , I would say that's below its minimum dropout at 1.5A so it wont regulate current ,

With a 0.7 Ohm resistor , assuming the LM338 has a 1.25V Ref it would be 1.78A , 0.666 Ohms would be 1.877A


When its run from 3 LiPos it will regulate but if your sense resistor is getting rather warm the current will drop as its resistance rises , or is the resistor is +- 5% ?

 

[bakulaw]

Yes my resistor is 5%, 6pcs 1W each in series and parallel to get the value .666. Still waiting for my 3W resistor. Thanks for the reply, Im gonna use 3x 16340 and 1% resistor hope it works.

 

[bakulaw]

Make sure you give the regulator a decent sized headsink too, and your resistor needs to be at least 5 watts.

1.87 ^ 2 * 5.8 volts = 21 watts of power dissipated in the regulator alone.

1.87 ^ 2 * 1.25 = 4.3 watts dissipated by the resistor.


I figured the power dissipated by taking the current output (1.25 / 0.666 ohms), squared it and then multipled it by the approximate voltage that's going to be dropped by the regulator. (You're putting in 12.4 volts. 5.5 volts will be used by the diode. 1.25 volts will be dropped by the resistor. so, 5.8 volts will be dropped by the regulator).


Be careful bud.

I though it's only IxV = P. What's ^ for? lol im bad at this. I'll post a pic, pls
look if you think my heatsink size is enough. Thanks!

 

[Hiemal]

I though it's only IxV = P. What's ^ for? lol im bad at this. I'll post a pic, pls
look if you think my heatsink size is enough. Thanks!

Wow, I'm so sorry.

I've been hopped up on pain meds when I wrote what I said; I don't know where I got the squared from at all.

Yes, your heatsink should be big enough, my apologizes.

 

[Cyparagon]

Minor warning: This style of regulator has a Vref of 1.25V. 0.666ohms programs the device for 1.88A (1.25/.666), not the 1.7A or 1.5A you're expecting.

 

[bakulaw]

Minor warning: This style of regulator has a Vref of 1.25V. 0.666ohms programs the device for 1.88A (1.25/.666), not the 1.7A or 1.5A you're expecting.

Thanks for the info. On seller's page suggested 2.4A, still safe if they're not lying

 

[Pi R Squared]

Thanks for the info. On seller's page suggested 2.4A, still safe if they're not lying

I am guessing the seller may be DTR and this is a 9mm 445nm laser? Or a very expensive 520nm green? I can guess from the suggested 2.4A. I just noticed in your two photos that it looks like the copper module with the diode is sticking out in front and not heat sinked at all. If you run it like that it will burn out real quick.

Alan

 

[bakulaw]

I put the copper module that way so that I can remove the diode when needed, I thermal glued it. I think it's still heatsinked since the casing before the copper is alluminum, but im not sure,I only tried it for a min and it's not hot.

Yes this is 9mm 445nm from dtr, Do you know if I can really run this at 2.4A if properly heat sinked?

 

[Pi R Squared]

I put the copper module that way so that I can remove the diode when needed, I thermal glued it. I think it's still heatsinked since the casing before the copper is alluminum, but im not sure,I only tried it for a min and it's not hot.

Yes this is 9mm 445nm from dtr, Do you know if I can really run this at 2.4A if properly heat sinked?

Yes that is the maximum you should run it at, if properly heat sinked, and it isn't right now in my opinion. I run mine at 2.2A and it is a fire hazard so be careful with that thing. Here is the one I put together if you want to have a look: http://laserpointerforums.com/f65/assembled-445nm-9mm-stainless-steel-host-pic-heavy-85997.html
It's so powerful it smoked the sensor on my LPM so it must be over 3W. What lens are you using? The G2 will give you more power.

Alan

 

[bakulaw]

Yes that is the maximum you should run it at, if properly heat sinked, and it isn't right now in my opinion. I run mine at 2.2A and it is a fire hazard so be careful with that thing. Here is the one I put together if you want to have a look: http://laserpointerforums.com/f65/assembled-445nm-9mm-stainless-steel-host-pic-heavy-85997.html
It's so powerful it smoked the sensor on my LPM so it must be over 3W. What lens are you using? The G2 will give you more power.

Alan

I replaced the stock plastic lense with the lense from dtr, I forgot the name. Nice pics, except for the Ultrafire lol. My Ultrafire 4000 real capacity is less than 500mAh.

 

[Pi R Squared]

I replaced the stock plastic lense with the lense from dtr, I forgot the name. Nice pics, except for the Ultrafire lol. My Ultrafire 4000 real capacity is less than 500mAh.

Yes I know, Ultrafire are well known here to be the lowest quality brand there is. I can verify they are always a fraction of their stated capacity of mAh. The ones with the blue lable seem to be the worst. See this thread if you haven't seen it yet: http://laserpointerforums.com/f67/warning-see-if-you-will-ever-buy-ultrafire-after-seeing-87108.html
Anyway about the lens, a G2 will give about 30% more power than a 3 element lens, a three element lens will give you some bat wings type of splash because of the rectangular beam and reflections inside the lens but it will have a smaller dot at a distance. The G2 lens is made in the US and has a quality anti reflective coating and is only a one element lens so there is less power loss, but the beam expands more over a distance.

Alan

 

[bakulaw]

Yes I know, Ultrafire are well known here to be the lowest quality brand there is. I can verify they are always a fraction of their stated capacity of mAh. The ones with the blue lable seem to be the worst. See this thread if you haven't seen it yet: http://laserpointerforums.com/f67/warning-see-if-you-will-ever-buy-ultrafire-after-seeing-87108.html
Anyway about the lens, a G2 will give about 30% more power than a 3 element lens, a three element lens will give you some bat wings type of splash because of the rectangular beam and reflections inside the lens but it will have a smaller dot at a distance. The G2 lens is made in the US and has a quality anti reflective coating and is only a one element lens so there is less power loss, but the beam expands more over a distance.

Alan

Wow 30% didn't know lense will give that much power. I thought upgrade is just to replace plastic so that it doesn't melt. I'll check the G2.Thanks!

 

[bakulaw]

Hi! I don't want to start another topic just for this simple question, so i'll ask it here.

Is it bad to put capacitor on the output of LM338 constant current? My ndb7875 stopped working, the last thing I did was add output capacitor.
Could it kill the diode instantly? I didn't even see it lit.

 

[The Lightning Stalker]

Hi! I don't want to start another topic just for this simple question, so i'll ask it here.

Is it bad to put capacitor on the output of LM338 constant current? My ndb7875 stopped working, the last thing I did was add output capacitor.
Could it kill the diode instantly? I didn't even see it lit.

Yes, if the capacitor was charged when you
connected it to the diode.

 

[Hiemal]

Make sure you have the LM338 hooked up properly along with the laser diode.

Your laser diode should still produce some light even if it was killed, which leads me to believe you may just have something incorrectly connected.