DIY Homemade laser diode driver

So this is what I think is 200mA?

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Hmm yeah should be.. I don't know how that scale works though.
If I were you, I'd confirm with the other one set to 500ma.

anselm said:

Hmm yeah should be.. I don't know how that scale works though.
If I were you, I'd confirm with the other one set to 500ma.

Click to expand...

This measurement holds true using the Simpson meter.

Well, hooray!
Looks like your driver is set to go then!
So this is with the pot turned almost all the way "up" (low resistance), right?
Now just be careful to discharge any capacitors you might have in your driver before
connecting the laser diode.

anselm said:

Well, hooray!
Looks like your driver is set to go then!
So this is with the pot turned almost all the way "up" (low resistance), right?
Now just be careful to discharge any capacitors you might have in your driver before
connecting the laser diode.

Click to expand...

This pot is finicky, in the middle of the rotation is approximately where the lowest current can be found while at the end of both directions it is at it highest current. I've read about discharging but haven't attached the laser yet, but thanks for reminded me.

Hi, im new to this forum. I built this driver but i dont know what voltage to put into it can anyone help? i burnt out my diode trying 9v. what voltage should the diode itself get? and what would be a good LD to get? thanks

Kmssd said:

This pot is finicky, in the middle of the rotation is approximately where the lowest current can be found while at the end of both directions it is at it highest current. I've read about discharging but haven't attached the laser yet, but thanks for reminded me.

Click to expand...

This cause you connected both the ends together, instead to connect the middle (sweeper) with one end

Now here's a question that is perplexing me -

Can anyone isolate the cause of this \/ behavior in a DDL (with Lm1117 if it matters) driver?



Take it as a given that there aren't any hidden PWM circuits in my host. This is a pure DDL driver, with the only deviation being that I've been using LM1117 instead of LM317.

Do you have at least 10uF input and output caps the LM1117 requires for stability?

Might be going in and out of thermal shutdown.

Interesting - not input caps - just output.

Is that crucial?

Seen this way, it's hard to give a reply ..... can you post a pair of pics of the driver, and schematic ? ..... this may help in understanding the possible cause (can also be a self-oscillation from the LM, but without some more data, it's hard to say)

EDIT: it depend from the circuit, about the caps, but, as example, for the serie 78xx / 79xx regulators, is always suggested to have a 100nF input and a 330nF output caps for avoid self-oscillations, so it may be ..... BTW, in self-oscillation state, the regulator DO NOT regulate, so better you don't use it in that condition, or check with an oscilloscope for possible current peaks (before destroy the diode )

Luckily yes I can - already had prepared the schematic

Just note that the diagram says LM317. I used LM1117.

rhd said:

Interesting - not input caps - just output.

Is that crucial?

Click to expand...

Yes, especially if your input source is any distance away from the driver, like a length of wire. Also your output cap must have low ESR, if you are using an electrolytic cap they have high ESR ... put a ceramic 0.1uF cap in parallel to lower ESR.

Are you using two units from the same production lot ?

Paralleling devices that are not planned for be paralleled, is possible (sometimes) using units coming from the same production lot, but usually difficult if they don't match.

Also, add a 100 to 220nF plus an 2.2 to 10uF at the input , and another 330nF at the output.

And again, parallel directly also the ADJ pins and use a single resistor (the two in parallel, as example)

Conceptually, that schematic is a bit wrong, sorry.

EDIT: more specifically:

You there have inputs and outputs tied up together, but ADJ pins independents ..... and two independents resistors for current settings, each one with its own value (tolerances means that is very difficult that you get 2 resistors 100% identicals) ..... what happens now, here ?

When, as example, one of the two ICs increase the voltage for keep the current, it reflect on the second one, that maybe is already ok, then the second one feel more current than the programmed one, and try to lower it forcing its output lower, but this reflect back on the first one ..... and so on, endless.

This can end in a self-oscillation cycle, or in the burning of one of the regulators (if they don't go in protection in time), or in the burning of the laser diode (cause in oscillation cycle, the regulators don't regulate at all )

Check that the two LM317 are from the same producer and production lot (this means ALL the numbers and cyphers on them MUST be identicals, not just the part number) ..... then parallel them directly pin-to-pin ..... then connect them as if they was a single regulator, with a single resistor ..... and don't forget the capacitors, both at input and output .....

Hey. Thanks for the help.
I'm on a cellphone, so excuse the shortness of this reply.

Same production lot for sure. They were in blister packaging, right next to each other.

If I parallel the resistors, do I need to change the resistance value?

Two 1.5 ohm resistors in parallel gives you 0.75 ohm, that for the LM serie with 1.25V reference voltage gives you (1.25/0.75) = 1.666A.

Just be sure to dissipate the heat from the regulators with a decent heatsink, and remember that both the LMs and the resistors needs to dissipate heat (the resistors will dissipate almost 1W, and each one of the regulators will dissipate a minimum of 2W, and also more if you keep the Vin too much high) ..... and for the same reason, keep the Vin the lowest possible, like, 5.5V for the LD, almost 2.5V for the regulators dropout, and another 1.25V for the resistor dropout ..... from 9V, it must be ok, and ofcourse, anything over this will be converted and dissipated in heat, so, if for 9V your ICs needs to dissipate 2W each, for, say, 12V, they need to dissipate 4.5W each.

HIMNL9 said:

...

EDIT: more specifically:

You there have inputs and outputs tied up together, but ADJ pins independents ..... and two independents resistors for current settings, each one with its own value (tolerances means that is very difficult that you get 2 resistors 100% identicals) ..... what happens now, here ?

When, as example, one of the two ICs increase the voltage for keep the current, it reflect on the second one, that maybe is already ok, then the second one feel more current than the programmed one, and try to lower it forcing its output lower, but this reflect back on the first one ..... and so on, endless.

This can end in a self-oscillation cycle, or in the burning of one of the regulators (if they don't go in protection in time), or in the burning of the laser diode (cause in oscillation cycle, the regulators don't regulate at all )

Check that the two LM317 are from the same producer and production lot (this means ALL the numbers and cyphers on them MUST be identicals, not just the part number) ..... then parallel them directly pin-to-pin ..... then connect them as if they was a single regulator, with a single resistor ..... and don't forget the capacitors, both at input and output .....

Click to expand...

I disagree, even in the same production lot there can be variations in the voltage drop it regulates for, if one regulates for 1.24 volts and the other for 1.26, only one will take all the load if they are connected with all pins together.
Putting votlage sources parallel is indeed a bad idea, but they are wired as a current source which does work in parallel. The LM317 regulates it output pin, which is not directly connected to the other output pin, so the changes of one regulator won't affect the other much, certainly if the load is a diode. A larger capacitor will be enough to keep it stable. I can do this when I get home to see how stable it is.