DIY Homemade laser diode driver

Let me preface this question: I know nothing about electronics.

That said, would it be possible to better control resistance by putting, say, the prescribed 100 ohm pot in the circuit then a 10 ohm pot to help fine tune the resistance? By setting the 10 ohm pot at about 5, then getting the 100 ohm pot adjusted fairly well, wouldn't the 10 ohm pot be able to go up or down just a little to help calibrate it even more accurately?

This may make no sense at all or not be possible, but let me know what you guys think.

Oh yeah, I forgot - I'm asking about single-turn pots since they're harder to adjust...

The main reason for the 10 ohm resistor is to keep people from frying their diodes. Yes, you could put two pots in the circuit, but the circuit was designed for beginners in mind. Personally I like using the 3 watt 25 ohm rheostat I bought from Radio Shack. I have no other resistors in my circuit.

Gazoo said:

Personally I like using the 3 watt 25 ohm rheostat I bought from Radio Shack. I have no other resistors in my circuit.

Click to expand...

Huh...there's an idea. I could just replace the resistors with a wire. That opens the possibility of frying the diode, but also lets me utilize the LD's full potential. I'm also kicking around the idea of replacing one of the 10 ohm resistors with a 3.7 ohm to bring the total resistance down to 2.5 ohm.

I would really like to fine tune the whole circuit so that turning the pot all the way down gives me max. current to the LD without damaging it, and I can turn the pot up for alignment without goggles purposes.

wilheldp, first of all... I like your avatar ;D

As about the resistors, don't use a wire because the LM317 won't work. Use a smaller bunch of resistors. Get a 2.2ohm, that's what I use. That gives you a max current of 568mA!!! That's enough to fry the diodes instantly. Practice safety and make sure the LD doesn't disconnect from the capacitor no matter what you do.

--DDL

Daedal said:

wilheldp, first of all... I like your avatar  ;D

As about the resistors, don't use a wire because the LM317 won't work. Use a smaller bunch of resistors. Get a 2.2ohm, that's what I use. That gives you a max current of 568mA!!! That's enough to fry the diodes instantly. Practice safety and make sure the LD doesn't disconnect from the capacitor no matter what you do.

--DDL

Click to expand...

How are you measuring your current? I couldn't get my multimeter to measure the current on my circuit anywhere...I tried putting it in between the positive lead of the circuit and the positive of the LD, but got no reading.

I got 212 mA by measuring the voltage drop across my 5 ohm resistor bank (it was 1.06 V). If I replace that with a 2.2 ohm resistor, that brings the current up to 482 mA (1.06/2.2). Where do you get 568?

One more thing, does anybody have a maximum recommended continuous current for this LD? I'm not going to cool it, so the only heat sink is the standard brass Aixiz module. I certainly don't want to blow this thing up, but I want to get maximum burning capability out of it.

wilheldp said:

[quote author=Daedal link=1185701612/240#244 date=1189612099]wilheldp, first of all... I like your avatar  ;D

As about the resistors, don't use a wire because the LM317 won't work. Use a smaller bunch of resistors. Get a 2.2ohm, that's what I use. That gives you a max current of 568mA!!! That's enough to fry the diodes instantly. Practice safety and make sure the LD doesn't disconnect from the capacitor no matter what you do.

--DDL

Click to expand...

How are you measuring your current?  I couldn't get my multimeter to measure the current on my circuit anywhere...I tried putting it in between the positive lead of the circuit and the positive of the LD, but got no reading.

I got 212 mA by measuring the voltage drop across my 5 ohm resistor bank (it was 1.06 V).  If I replace that with a 2.2 ohm resistor, that brings the current up to 482 mA (1.06/2.2).  Where do you get 568?

One more thing, does anybody have a maximum recommended continuous current for this LD?  I'm not going to cool it, so the only heat sink is the standard brass Aixiz module.  I certainly don't want to blow this thing up, but I want to get maximum burning capability out of it.[/quote]

1.25/2.2=0.568

You should have had a 1.25V drop across your 5 ohm resistor bank for a current of 250mA. Perhaps your Vin was to low to maintain regulation. What were you using as a source?

You are using the best method for measuring current (measuring the voltage drop across a known circuit element). If you insert the DMM in the circuit in series, it will affect the circuit operation and you are actually measuring its effects as well on the circuit.

chimo said:

1.25/2.2=0.568

You should have had a 1.25V drop across your 5 ohm resistor bank for a current of 250mA.  Perhaps your Vin was to low to maintain regulation.  What were you using as a source?

You are using the best method for measuring current (measuring the voltage drop across a known circuit element).  If you insert the DMM in the circuit in series, it will affect the circuit operation and you are actually measuring its effects as well on the circuit.

Click to expand...

I'm using a 4-AA battery holder from RadioShack as my power source. I guess that makes a difference since the original circuit was designed to run off of 2-CR123 batteries, eh? I measured that 1.06V drop across that resistor several times, and it never fluctated, so I'm pretty sure of the number. Maybe I'll just do some more testing with LEDs to fine tune the circuit.

wilheldp said:

[quote author=chimo link=1185701612/240#246 date=1189617412]1.25/2.2=0.568

You should have had a 1.25V drop across your 5 ohm resistor bank for a current of 250mA.  Perhaps your Vin was to low to maintain regulation.  What were you using as a source?

You are using the best method for measuring current (measuring the voltage drop across a known circuit element).  If you insert the DMM in the circuit in series, it will affect the circuit operation and you are actually measuring its effects as well on the circuit.

Click to expand...

I'm using a 4-AA battery holder from RadioShack as my power source.  I guess that makes a difference since the original circuit was designed to run off of 2-CR123 batteries, eh?  I measured that 1.06V drop across that resistor several times, and it never fluctated, so I'm pretty sure of the number.  Maybe I'll just do some more testing with LEDs to fine tune the circuit.[/quote]

What type of batteries? Alkaline or rechargeables? With battery sag, you may be running into a problem with the LM317's drop out overhead requirements. Try measuring the supply voltage to the LM317 under load. The LM317 requires an input voltage that's higher then the output voltage.

Here's an example:
Let's say you were using NiMH cells for a supply voltage of 4.8V (4x1.2)
Let's say the dropout of the LM317 is 1.6V at 250mA out (it will actually vary between LM317s and with various output currents).
For a 250mA output, your sense resistor (5ohm bank) would drop 1.25V.
Let's say that for a drive current of 250mA the LD will have a Vf of 2.8V (this figure will vary as well).

Now for the math - let's find out what voltage is available for the LD after the various drops:

Vbatt (battery) - Vdo (LM317 dropout) - Vsense (sense resistor) = what's left for the LD
(for NiMH) 4.8 - 1.6 - 1.25 = 1.95V This is likely not enough for the LD to run at 250mA!
(for Alkalines under load) 5.6 - 1.6 - 1.25 = 2.75V Cutting it very close!

You may want to add a couple of cells for your battery configuration.

You will need a minimum of 6 volts for this circuit to be stable and do what it is meant to do.

As far as continuous operation, I do not recommend applying more than 160ma's to the diode. It will be a good burner with this amount of current.

Sorry but I am unexperienced at electronics - Just reassure me that if I was to put 9v into this and connect it to a dvd laser diode it would keep the diode lasting for a long time?

Thanks,
John

just tie 2 10ohm resistors together to make 5.4ohms and get rid of the pot and put the resistors in its place and you'll get around 250ma

idiotjohny you'll fry your diode without a doubt especially without a regulated current the current of the batts may be to strong and fry your diode

idiotjohny said:

Sorry but I am unexperienced at electronics - Just reassure me that if I was to put 9v into this and connect it to a dvd laser diode it would keep the diode lasting for a long time?

Thanks,
John

Click to expand...

John, if you are thinking of hooking up a 9V battery, then it's a bad idea because 9V batteries don't last long. Also, if you are thinking of using a power supply, then you should just try to find a 6V one. The point here is that once the voltage drop across the LM317 is dropped, the voltage is not decreased further. The voltage going out of the LM after a 9V supply is 6V... that's not good for a red LD at all.

--DDL

ZZZZZZZZZAP! I think I fried my diode.

I added another 10 ohm resistor to my bank making the overall resistance 3.33 ohms (3-10 ohm in parallel). I cranked the pot all the way up, and the brightness didn't seem all that much more than it was before. While I was getting my multimeter probes situated to take voltage drop (current) measurements, the diode suddenly dimmed. I'm pretty sure that the positive and negative leads on the diode touched and caused some kind of failure...I hope it's not the diode, but I can't think of anything else that would have failed. I changed the battery in hopes that the short circuit had just drained the batteries really quick. After the battery change, the diode appears to be lasing again, but it isn't nearly as bright as it was before.

Any hope that another circuit component (capacitor, regular diode, resistor, pot, or LM317) failed, or is my LD hosed? I don't see any other shorts in the circuit, and it appears to be drawing 294 mA (0.98V/3.33ohm).

:'(

Ok... This seems like the classic case of Capcitofriness..

What it sounds like is that the capacitor charged up and zapped your LD. This happens when the LD disconnects form the circuit and then reconnects before the Cap is dicharged.

Sadly, there isn't anything that can be done but to get another LD. In the mean time, you can use that fried diode as a test subject for now and for later projects to come

Also, if the + & - leads on an LD connect... it just causes a short and the circuit will just drain current. There is a current regulation through the LM37, so all that will happen is the current will just run through the leads and then drain your battery.

--DDL