DIY Homemade laser diode driver

[chimo]

Good writeup. I would like to add a little to help confuse people.

Everyone should keep in mind that the current will not remain constant for the laser diode if it is fed by a fixed voltage. As the laser diode die heats up (as a function of the power delivered to it), its resistance decreases. That must result in an increase in current if the voltage is held constant. The VI curve is quite steep and the effect is very dramatic (i.e. the diode will quickly enter the overdriven zone). That's whay the current sources are so nice - the current is held constant and the voltage regulator allows the voltage to "float".

If this causes too much confusion I’ll delete it.

My limited understanding is that a LASER Diode will draw a certain current at a given voltage. When using the 317 as a current regulator, it is still regulating voltage. The 317 always tried to maintain a 1.25 Volt difference between the Vout and Adj pins, but because we have a fixed value of resistance between the Vout and Adj pins the 317 raises the voltage until the resistor is dropping 1.25 Volts.

*Note* all LD forward drop voltages are numbers I pulled out of my…well, you can guess.

If one were to use a 5 Ohm resistor (or combination of resistors equaling 5 Ohms) between the Vout and Adj pins, the supply would be set for 250 mA. The resistor would have a voltage drop of 1.25 V and there would be 250 mA passing through it. Let’s assume that the LD at 250 mA has a forward voltage drop of 2.79 V, 1.25 V + 2.79 V = 4.04 V. The 317 will have to supply 4.04 V to maintain a 1.25 V drop across the 5 Ohm resistor.

Now let’s try with a 10 Ohm resistor, which would set the supply for 125 mA. Again, the resistor will have a voltage drop of 1.25 V, but at 125 mA the LD might have a forward voltage drop of only 2.62 V, 1.25 V + 2.62 V = 3.87 V. The 317 will have to supply only 3.87 V to drive the LD with only 125 mA.

So as I hope is now made clearer, the 317 is really regulating the current supplied to the LD by very precisely regulating the voltage.

 

[aaronX987]

On the other hand, you should always be able to reach the guts again, in case they need an adjustment. What current did you set yours to?

Well, I opened it up again and tested the voltage first. for the two cr123 batteries it was at about 2.5v and for a 9v battery it was at 2.89 I think, but nothing came above 3v. I am using 5 Ohms of resistance (2x 10 Ohm resistors parallel) which should give me 250 mA if I can get the right voltage I assume. Also What does the POT do, I thought it controlled voltage. I have mine up to the max. I guess in order for me to see if this works I need to get at least rechargeable cr123's.

 

[a_pyro_is]

Aaron, if you're running a SenKat diode, at 250mA a Vf of 2.89 V sounds pretty normal. Unless you're wanting to be able to adjust the current, just wire that 5 Ohm (Or two 10 Ohm parallel) between the Vout and Adj pins and leave it at a fixed 250mA.

2 fresh name-brand 123s should run it for a good while unless you have an extra diode of some sort in series somewere in your circuit.

But yes RCR123s will give you the V headroom to really get the full use out of your Batts.

 

[numberonekiwi]

hey thanks all for a better explanation and answer to my question my first driver I built is using the bd140 and bc557 with a 3ohm resistor and 1 ohm in series with the LD to measure current on the V scale 10mV reading = 10mA right? and a 8v4(9v) battery as Vdrop was across the transistor and now understand the LM circuit a little more

I will continue with my first build using this and my second will be using axis module ddl's circuit and 2x aa Lion 3.6v ea have got a donor case but unsure yet as to if it can be modded I was just am little unsure when one of the post said instead death for the LD with input to this circuit of 7v2 unless I read it wrong

Many thanks again

 

[IgorT]

Well, I opened it up again and tested the voltage first. for the two cr123 batteries it was at about 2.5v and for a 9v battery it was at 2.89 I think, but nothing came above 3v. I am using 5 Ohms of resistance (2x 10 Ohm resistors parallel) which should give me 250 mA if I can get the right voltage I assume. Also What does the POT do, I thought it controlled voltage. I have mine up to the max. I guess in order for me to see if this works I need to get at least rechargeable cr123's.

As Pyro said, 2.89 sounds just right for 250mA. This means the circuit works perfectly as long as the voltage is ABOVE 6V.

It also means your cr123 batteries are probably a little empty already and are not enough to feed your laser, since their voltage drops below 6V very quickly as they discharge.

Go rechargable, and you'll have a "constant" power output over their entire capacity, with the only variations coming from the heat.

 

[aaronX987]

Thanks for the feedback IgorT and Pyro. I ordered some rechargeable cr123's and that should sustain the voltage. Untill then I guess I can't measure the mA's since I dont feel like buying new non-rechargeable cr123's and 9v batteries just dont have enough juice.

 

[IgorT]

Thanks for the feedback IgorT and Pyro. I ordered some rechargeable cr123's and that should sustain the voltage. Untill then I guess I can't measure the mA's since I dont feel like buying new non-rechargeable cr123's and 9v batteries just dont have enough juice.

If the 9V battery was able to supply your laser with enough current, without it's voltage dropping too much, the mA's will stay the same..

If not, and it's voltage dropped below 6V during the test, the current could even go up, once you put the rechargables in there.

You should check this - the voltage of the 9V battery when it was supplying the 250mA. If it's over 6V the current will stay the same with the rechargables, since this is obviously what you set it to..

 

[rubberband]

Hi

i just built anouter DDL circut and tryed powering it with a 9V battery

ok i am getting no output from the driver but measuring form the vout on the lm317 a grand total of 1.23 volts but after the pot and resistors i measure no current or voltage.

what could be the problem?

Thanks
Mitch

 

[Da_Dude]

Hey, Nice guide.
If only I knew how to use it.
Um....
I know "Pot" stands for potentiometer, what does "ohm" mean?
Also, I googled, and saw thinks like "10K-Ohm "
What does the k stand for?
...
I am such a electronics noob... :-[

 

[aaronX987]

Da_Dude,
Don't worry, noobs make the world go round.
Don't google it, wiki it! That should be enough help. Ohm is a measure of electrical resistance and the K stands for thousand.

 

[Da_Dude]

Da_Dude,
 Don't worry, noobs make the world go round.  

Thanks. ;D
And, yes, noobs do make the world go round.
Here I am but a lowly noob.
In other forums, I am almost famous.
I modify N64 games for fun.
I program for fun...
Lot o' stuf.

 

[IgorT]

Daedal..

I've shown this circuit to my electronics engineer, asking a few questions about it, and he said, used in this way, it is perfect for driving the LDs and can handle voltages up to 37V without the current (and the voltage) on the LD changing.

If it get's too hot, the current could start dropping, so heatsinking is recommended at higher voltages.

This means it's perfect for battery applications, as long as the voltage is above 6V, and would make good use of battery capacity this way.


So for anyone thinking of going from two 3V batteries to two rechargable lithiums: You don't have to change anything in the circuit, if it is set correctly.
It is possible you set it wrong tho, if your two 3V batteries were slightly discharged when doing so. That's why it is recommended to set the current AGAIN, when you know for sure the voltage is above 6V. After that, the current won't change again, unless the voltage of the batteries drops below 6V, but that means you should recharge them anyway..

 

[a_pyro_is]

There has been a lot of discussion as to the best capacitor/capacitors across the LD when using the LM317 Constant Current circuit. Is there anyone out there with a scope who can graph the first few milliseconds using several different capacitors or combinations of capacitors? Maybe also send a few transient spikes and see how they handle it?

It was mostly a guess for me when I picked a low ESR .01uF and an electrolytic 10uF, But it seems to work so far. I did manage to kill one LD by a loose connection to my multimeter, but it had only a .01uF across the pins which was apparently not enough.

Hmm...this might need to get it's own thread if someone is willing.

 

[Daedal]

Daedal..

I've shown this circuit to my electronics engineer, asking a few questions about it, and he said, used in this way, it is perfect for driving the LDs and can handle voltages up to 37V without the current (and the voltage) on the LD changing.

If it get's too hot, the current could start dropping, so heatsinking is recommended at higher voltages.

This means it's perfect for battery applications, as long as the voltage is above 6V, and would make good use of battery capacity this way.


So for anyone thinking of going from two 3V batteries to two rechargable lithiums: You don't have to change anything in the circuit, if it is set correctly.
It is possible you set it wrong tho, if your two 3V batteries were slightly discharged when doing so. That's why it is recommended to set the current AGAIN, when you know for sure the voltage is above 6V. After that, the current won't change again, unless the voltage of the batteries drops below 6V, but that means you should recharge them anyway..

Thanx Igor

--DDL

 

[Daedal]

There has been a lot of discussion as to the best capacitor/capacitors across the LD when using the LM317 Constant Current circuit. Is there anyone out there with a scope who can graph the first few milliseconds using several different capacitors or combinations of capacitors? Maybe also send a few transient spikes and see how they handle it?

It was mostly a guess for me when I picked a low ESR .01uF and an electrolytic 10uF, But it seems to work so far. I did manage to kill one LD by a loose connection to my multimeter, but it had only a .01uF across the pins which was apparently not enough.

Hmm...this might need to get it's own thread if someone is willing.

Just tested the circuit on a scope... regardless of how much I pule the input or jack it up (went up to 50V input on a now declared unusabe LM317 for safety) the output was steady as a rock with no jolts or spikes... it continues to slowly ramp up regardless of input voltage

If I do manage to get a USB scope sometime in the near future... I'll graph it out properly and post it on here

--DDL

 

[a_pyro_is]

Thanks DDL, I was also wondering just how much a capacitor will protect from a spike after the 317. such as if someone had an ammeter in series but with a loose contact.