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1A LM317 driver with current selection switch

alessnilsen

Active member
Hi! I wan't make a driver for my Nichia NDB7412T, I have a 7.5V 4.5A power supply laying arround.

I wan't use a 1.2R resistor to get 1A from the LM317 but I ever used a power supply of 9V ~ 12V for this.

I'm afraid if 7.5V is enought for my laser diode and LM317, I wanna get 1W from this laser.

Is 7.5V enought for it or I will need a new power supply?

Thanks!

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Cyparagon

Well-known member
Re: Question about LD and driver/power supply

Yes, that will work, but 7.5V is about the absolute minimum. Worst case scenario, your current comes in at slightly less than an amp, and if that's the case, you can "fudge it" by switching to a 1R0. Probably good enough for you use.

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alessnilsen

Active member
Re: Question about LD and driver/power supply

Good, how do you calculate that it will work?

I done a dummy load to test the driver under 7.5V and the result was desapointing!

With 4.65V of drop voltage, I got 500mA, with 3.65V of drop voltage, I got 705mA! Since my LD has a 4.0~6.0V working voltage, I'll not be abble to get 1A from the LM317.

I must get a 12V 2A power supply for my driver!

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Cyparagon

Well-known member
Re: Will LM317 work on 7.5V 1A for blue LD?

It should work. There are several posibilities here:

You're measuring current wrong

alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD?

What results should I get with dummy load?

oahu99

Member
No, it does make sense that the setup doesn't work.

The lm317 has a voltage drop of about 3.5 to 4 volts just on its own.

So after the voltage drop of the regulator the voltage will not be high enough for the laser diode.

You will probably need a power supply over ~10 volts to fully power the laser.

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alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD?

It make sense now, if the LM317 can't raise the voltage there's no way to regulate the current with 4~6V voltage drop!

I will get a 12V 2A power supply and wait for the parts (LD, heatsink, etc) arrive.

Thanks!

Photonz

New member
Re: Will LM317 work on 7.5V 1A for blue LD?

No, check my signature. I did this exact build with an M140 diode and only 7.4-8.4V (two 3.7V Li-ion batts)

The voltage drop of the LM317 is only 1.25V I'm pretty sure.

DashApple

Well-known member
Re: Will LM317 work on 7.5V 1A for blue LD?

Well at a diode forward voltage of 5V and 1.25V across the Current set resistor the maximum the regulator could drop would be 1.25V for a supply from 7.5V

Looking at the LM317 V drop chart , at one amp it will drop just under 2.3V - 2.5V minimum .

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FireMyLaser

Well-known member
Re: Will LM317 work on 7.5V 1A for blue LD?

LM317 drops around 3V in constant current mode, even more at higher currents. Assume that it will drop 4V in worst-case scenario. 1.25V is just it's reference voltage.

alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD?

That's my new measurement of the dummy load:

5.12V -> 260mA
4.60V -> 485mA
3.77V -> 780mA

Tested with a 1R2 2W resistor. I still confused about those affirmations, will it work or no?

alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD?

I built my driver and tested again, the current starts to drop when output voltage < 4V! So, the answer is NO!

It uses a LM317, but now with a 19.1V @ 3.42A laptop power supply.
Since at this voltage the LM317 will produce a lot of heat, I decided to use a heatsink of a old Pentium 2, and works very stable!

Here measuring the currents with a dummy load (at 5.5V)

I added a resource to change the current, it uses a 1X1,2R and 2X1,5R resistor, giving me 289mA (low power mode) and 933mA (full power).
This is done shorting the 2X1,5R resistor through a switch. Will it damage the LD?

:drunk:

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IWIRE

New member
Re: Will LM317 work on 7.5V 1A for blue LD? Driver assembled.

It looks pretty nice, looks like you tried to do a nice clean job :beer:
I just built one using an LM338. At a diode voltage of 6v and supplying 4.5a mine will require a minimum of 9.5v. I found some pretty cool battery packs here All-Battery.com - Rechargeable batteries & Chargers - Li-PO Batteries for RC Cars . I'm thinking of going with the 11.1v pack.

alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD? Driver assembled.

It looks pretty nice, looks like you tried to do a nice clean job

Thanks man!

But the LM317 I used was defective, when I changed the current from 200 to 900mA, all started to fry, in the dummy load I readed 5A!!!

The 1R resistor burned my pants :wtf: , the resistors inside the driver fried too, one was 3.3R, now is 2R! This fried a little part of the board, so isn't too clean as was, but I was able to rebuild it completely and is working! :beer:

IWIRE

New member
You stuck with it and made it work. That is the important part. The first 338`s I got from fleabay. Big mistake. I got some from Digikey and they work fine.
I hope I don't have to use them and drives become available again.
But if not I can keep building until I get a drive built or some become available Never give up !

alessnilsen

Active member
Re: Will LM317 work on 7.5V 1A for blue LD? Driver assembled.

I'm thinking of going with the 11.1v pack.

That is cool! The 11.1V pack will fit better for your needs. Maybe 11.1V @ 5000mAh? That's will give you about a hour of run time (through my inaccurate math ).

My will run on outlet, run it with batteries will be too expensive for me, but nothing prevent me from getting a step down driver (buck driver) to do this.

You stuck with it and made it work

Yes, the tricky part is that the burnt resistors released a lot of heat that loosen off the copper pads, some of the copper pads fall off from the board. The result was not too beautifull. I hate when this occurs!

I paid U\$12,00 (R\$25,00) for a LM338! Now it have a broken terminal.

I done various mistakes with those thing, for example, use a trimpot to control the current (yes, burnt trimpot AND the LD), use a LM337 as a regulator (it was not working, changing the input voltage changes the current).

:shhh: