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Old 12-12-2007, 11:33 AM #1
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Default Custom LD Driver

Howdy,

I'm trying to make a somewhat universal LD driver and this is what I came up with. It's based on Daedal's current-regulating driver, with an extra tidbit for voltage-regulation. I'm fairly new to electronics and the theory behind it, so if you don't mind using plain English...

The current is set using R2, which can be adjusted between 0 ohms and 100 ohms. Consequently, the final output current can be adjusted between (about) 10 mA to 90 mA (this is what I measured, not calculated...)

The voltage is set using R5, which can be adjusted between 0 ohms and 300 ohms. The final output voltage would then be between (about) 3.75 volts to 4.75 volts.

I was hoping I could get some input on the quality of this driver and what problems, if any, I would experience with it.

Main questions:

1. I feel like there should be more on the voltage-regulation part of the circuit. Schematics I've found for voltage-regulation with the LM317 include some capacitors around it, and I don't have any. As I said, I'm fairly new to electronics, so should I include a capacitor to avoid any spikes in the voltage coming from the LM317 on the voltage-regulation side of this circuit?

2. I've only tested this circuit using a multimeter to give me what the output voltage & current is with various settings for R2 & R5. I have not tried it with anything other than an LED. I keep reading that I need to try it under a load, but I don't know what this means. I've heard it suggested that I use 4 diodes in series to test it. How would this work? i.e. How do I know if it's working if plain black diodes are going to stay plain and black even if it is working?

Thanks a lot for any help!

Dave
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Old 12-12-2007, 12:03 PM #2
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Default Re: Custom LD Driver

Nice. What you do is to put 4 1N4001 diodes in parallel, then measure current to them and voltage across them with a DMM. I'm pretty sure you know how to do that having just designed and built that circuit. :P
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Old 12-12-2007, 12:48 PM #3
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Default Re: Custom LD Driver

But what am I looking for when I'm measuring the current & voltage around the diodes? Should the values stay the same or decrease by their rated amounts? Or perhaps more importantly, what should I be looking for if the circuit is *not* working?

Dave
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Old 12-12-2007, 01:59 PM #4
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Default Re: Custom LD Driver

Not sure if this helps, but at the electronics store I go to, they have adjustable capacitors, that look like potentiometers. I believe they are called trim-caps or cap trimmers or something like that. I think that may make it easier to fine tune what you are trying to do. Good luck.
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Old 12-12-2007, 02:10 PM #5
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Default Re: Custom LD Driver

For driving laser diodes with one LM317 set up as a constant current supply you are already regulating voltage. Why is there a need for another LM317 except to drop efficiency? :-?
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Old 12-12-2007, 02:56 PM #6
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Default Re: Custom LD Driver

Just build Daedal's circuit. There's no need for extra capacitors, especially if you're running from batteries. (Trimmer capacitors are usually in the picoFarad range by the way - great for tuning RF circuits but not this kind of thing.)
More specifically, you don't need a separate voltage regulator. The single LM317, configured as in that circuit, will take an input voltage up to 37V (well, oversimplifying here but that's the idea), and output whatever voltage causes the current through the load to hit the target you set with the potentiometer.
I prefer to use 4 LEDs in parallel as the test load.
You can either directly measure the mA with your meter in series with that load, or you can put a resistor in series with that load and measure the mV across it and divide by the resistance. If you use a 1 Ohm resistor, then the mA = the mV you measure.
Measuring the voltage across the load is not especially useful, unless you're just curious.

Once you're done testing with your test load, crank the current way down, substitute the real load (the LD), and gradually raise the current to your target.

This is all spelled out MANY places on this forum.
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Old 12-12-2007, 03:14 PM #7
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Default Re: Custom LD Driver

as far as the numbers your looking for, it depends on what your powering, and the input power.

example most people here have been using DDL's circuit for reds which you are looking for 2.5-3.0v and anywhere between 100ma to 300ma depending on how fast you wanna replace your diode. alot of people have found 200-250ma to be the sweet spot for diode life and burning power. I personally run mine (which is a SenKat diode) at 250-260ma.

Now if you are talking BluRay you want 4.5v or close to it at 30-40ma (notice only 1 Zero in those numbers) this is normally done with DDL's circuit and a 9v battery.

I hope these numbers help you in tuning your design.



I hope this helps your measurements
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