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Old 11-01-2007, 06:09 AM #1
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Default Driver Circuits and Capacitor Protection

I've been thinking about the various circuits I see posted utilizing LM317 as a current driver for laser diodes. All looks great until I look at that capacitor sitting in parallel across the laser diode.

I see the potential for disaster there should battery / switch connection transients occur, charging the capacitor and then allowing it to discharge through the laser diode without any current limit.

Doing some research in SAM's laser FAQ, I found this little warning:

Quote:
CAUTION: There must NOT be any filter capacitance in the power supply after the current limiting resistor. This is to minimize the chance that a bad connection to the diode will result in excessive current should such a capacitor charge to a much higher voltage and then discharge through the diode without current limiting.
( http://www.repairfaq.org/sam/laserdps.htm under Battery Power for Laser Diodes )

Isn't this in effect what we are doing since there is not any resistance limiting current flow from a charged capacitor through our diode?

( http://stonetek.org/sales/pix/LD_Driver_Schematic.jpg )

(Just asking here and hoping those with a deeper understanding can explain this.)


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Old 11-01-2007, 06:41 AM #2
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Default Re: Driver Circuits and Capacitor Protection

There is no way this would be a problem unless there was a bad connection. The most common way I have seen people blow diodes when using the LM317 is by connecting their meters to measure current after they apply power to the circuit. What happens is the capacitor will be charged up and when they connect the meter "poof" goes the diode. This is one reason it is recommended to solder the capacitor directly across the diode. The capacitor will absorb spikes no matter where they might come from.

As far as battery power, I am having a difficult time understanding where he is coming from with the statement "should such a capacitor charge to a much higher voltage". The reason we use capacitors is to prevent voltage spikes. There have been so many people blow diodes because they don't use a capacitor. I learned my lesson too.
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Old 11-01-2007, 11:25 AM #3
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Default Re: Driver Circuits and Capacitor Protection

Quote:
Originally Posted by Gazoo
As far as battery power, I am having a difficult time understanding where he is coming from with the statement "should such a capacitor charge to a much higher voltage".
A current source will charge a lone capacitor at its output to supply voltage. Using 2 CR123s for example, this would result in the cap being charged to 6 volts, which is 2 to 3 times (i'd call that 'much') higher than what a typical red LD requires.

I just solder the cap onto the LD leads within the aixiz module usually - nothing can go wrong with that.
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Old 11-01-2007, 07:56 PM #4
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Default Re: Driver Circuits and Capacitor Protection

If Sam's warning is valid, wouldn't a small resistor in series with the capacitor eliminate his concern? (the resistor/capacitor pair soldered across the diode)

When the circuit is initially powered, it would seem as if the capacitor would begin charging until the voltage rises to the forward voltage of the laser diode (taking miliseconds) at which time it would discharge through the diode, momentarily increasing diode current above that provided by our current limiting LM317 circuit.

The capacitor should in effect act like a battery (one with extrememly small internal resistance.) When it discharges through the diode, current of the discharge is not limited by anything but the laser diode effective resistance itself.

Is this initial power-on discharge a concern for our diode's lifespan?
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Old 11-01-2007, 10:00 PM #5
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Default Re: Driver Circuits and Capacitor Protection

The capacitor will discharge on its own into the diode...no resistor is needed. The only danger is with what I previously posted, and using breadboards which can result in bad connections. As long as all of the connections are good, there is no cause for concern.

The best way to avoid the problem is to solder the capacitor directly across the diode. I have been doing it just like Benm does, and I have had zero problems. Remember why we are using the capacitor in the first place
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Old 11-01-2007, 10:14 PM #6
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Default Re: Driver Circuits and Capacitor Protection

Quote:
The capacitor will discharge on its own into the diode
This was my only concern... a capacitor discharging through the diode unlimited by an external resistor. That and trying to understand Sam's warning about capacitors placed after the final current limiting resistor.

Thanks for the explanation.
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Old 11-02-2007, 12:12 AM #7
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Default Re: Driver Circuits and Capacitor Protection

You place a schottky diode in series with the capacitor and a resistor in parallel with the schottky diode. During initial current application the capacitor charges through the schottky diode and ramps the dI/dt and dV/dt across the LD. The resistor in parallel with the schottky diode lets the capacitor discharge during power off. The resistor can be sized to limit any potential discharge current into the LD incase of an intermittent connection.
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Old 11-02-2007, 02:13 AM #8
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Default Re: Driver Circuits and Capacitor Protection

I like that solution. Thanks!
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Old 11-02-2007, 11:28 PM #9
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Default Re: Driver Circuits and Capacitor Protection

I didnt see the warning in sams faq, but had the idea that the diode can change its charicteristics (sp) when it starts to warm up, I dont know this for sure, but it souds like it is possible for this to happen, if the diodes resistance drops, even a little, the cap bieng there would be real bad for the diode. *I havnt put a cap on my diodes in the past, and I have NEVER lost a diode for a non mechanical reason. BUT Im not saying it cant happen! *this is out of 9 lasers I have built, and ran some of them within 50 foot of a running tesla coil (which made me extremely nervous about the diode) but the diodes I have still work! 3 of these are the 16x gb diodes. that cap there on the diode sure makes me nervous! why not put it on the other side of a resistor.... If anyone can add anything to this, I REALLY wanna know what the experts think... Is the cap a cause for concern if the diodes charicteristics change?

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Old 11-03-2007, 12:16 AM #10
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Default Re: Driver Circuits and Capacitor Protection

While we're on the subject (I don't think this is too far afield), Daedal chose 47uF because it was on his proto board at the time. What is considered the smallest capacitance for diode protection? I know someone stated they were happy with 10uF. How much of a pulse are we protecting against?
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Old 11-03-2007, 12:21 AM #11
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Default Re: Driver Circuits and Capacitor Protection

I dont know... they must be short though! U would have to hook the regulator to a storage scope, and watch the power to see if there were any spikes... I wish I had one!

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Old 11-03-2007, 12:26 AM #12
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Default Re: Driver Circuits and Capacitor Protection

Quote:
Originally Posted by Shilood
While we're on the subject (I don't think this is too far afield), Daedal chose 47uF because it was on his proto board at the time. *What is considered the smallest capacitance for diode protection? *I know someone stated they were happy with 10uF. *How much of a pulse are we protecting against?
I would not be worried with 1uF. In fact, a better solution would be something like 0.1uF low ESR and a 4.7uF in parallel. That will pick up the high freq transients and the slower ones as well.
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Old 11-03-2007, 01:02 AM #13
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Default Re: Driver Circuits and Capacitor Protection

hmm, I guess it matters if there is the possibility of a change in the diode, and how likely the diode is to be damaged without the cap there? * has anyone tried the lm317 driver without the cap? *:-? has anyone scoped this circuit to see how it acts on powerup, and power down? *:-? It obviously works verry well with the cap, and very likely needs the cap there to prevent spikes, I was just wondering if anyone has actually done the tests with a scope? :-? *HMMM, I wonder if this could be the reason a few blu rays have died with no explanation? :-? or maybe they were hit by that one cosmic ray that came all the way accross the universe to hit that one little diode a million light years from its origin? * *There is no doubt that the cap helps with spikes, just is the spike more likely than the diode changeing its characteristics? :-? maybe it depends on the diode, and on the environment it is bieng used in. still, I am supprised my diode didnt have a problem in the vacinity of a running coil, I do have a cap, it is a 2300uf, but its seperated from the diode by the resistor (20 ohms) in my circuit.

also, I really need to say that ddl's driver is VERRY well tested, and u cant beat a proven circuit, I would use his circuit before any other circuit, including the one im useing now, but the lm317 cant drive a big diode that needs 2.8 amps... and u know if it works on bluray diodes, it HAS TO BE VERY stable! 8-)

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Old 11-03-2007, 04:44 AM #14
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Default Re: Driver Circuits and Capacitor Protection

Quote:
I would not be worried with 1uF. In fact, a better solution would be something like 0.1uF low ESR and a 4.7uF in parallel. That will pick up the high freq transients and the slower ones as well.
Haha.. you read my mind. My 0.1uF tantalums are in now as well as my 2.2uF caps.
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Old 11-03-2007, 07:12 AM #15
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Default Re: Driver Circuits and Capacitor Protection

"Experts"? ;D That's a good one... Gazoo qualifies as an expert in my opinion... Benm does too... And Chimo... :P ;D :

Here's the deal...

To start things off... let's start with the capacitors' risk of killing the diodes by... discharging spontaneously? :-?

Umm... think about it... The capacitor is doing us a favor... It's filling up like a battery right before the diode anyway... it's in fact a PERFECT battery, or about as close to it as we can get in the real world! The output of a capacitor is exactly how a diode wants it... nice and clean with no spikes or sudden drop outs or start ups. A sudden start for a diode is the same as a slow-acting poison... they hate it... and sudden shut downs are just as bad... : A slow up/down ramp, even if it's the order of pico-seconds, is ALWAYS better... the capacitor in the LM317 circuit does that and more...

The LM317 circuit needs a capacitance... it's good for it... Any regulator requires a capacitance to make sure the output is nice and filtered and as, in fact, DC.

The discharging that the capacitor does is quite healthy too, mind you... It will charge up to some specific amount and then start to discharge the rest into the diode as fast as it is getting it in... in the process of charging up, it'll sake the first jolt, then slowly start to give the diode more and more... until it reaches full potential. At which point the current is pretty much just being filtered by the cap and then sent off to the LD.

Why would you use a current limiting resistor BEFORE the LD and after the capacitor when you are already limiting the current going into the cap? Are you expecting the cap to be a black hole for energy? It's not going to suck the energy and store it indefinitely... it's going to have to come out one way or the other... and the whole reason we are using the LM317 is to limit the current... so it's already nice for the LD, we just need to take out the spikes and whatnots.

Why 47uF? It was handy... and not for no reason... I picked it out of a stack of over 1000 different sizes. It's what I personally believe is a good number that distinguishes between a 'too small' and a 'too big'. It's the right size and fits just right. Your opinion may be different... and I encourage that. I have seen circuits that use as low as a 0.1uF capacitance for the LD... as a matter of fact, do you know those big and fat lasers that pump out more than a watt of power? Or the even beefier ones that all the laser shows use nowadays? These probably do not top 1uF ever... If you start using more than 0.1uF your output to the diode will be too slow and the show will be ruined as a result of all the slow start and slow fade out... BUT, on the other hand, the input to the circuit is heavily filtered... The better the module is, and the more expensive it is, the heavier the filtering before the circuit... If you have tried turning them on... they usually need a minute or so before they kick on... or you have to start the power supply in advance and just keep it on... it's because the capacitors have that long of a ramp-up.

The IDEAL circuit for a laser diode is one that runs from a capacitor... and endless capacitor... I have seen circuits where the input is so regulated that it is actually running off of a series of capacitors that charge one after the other. The lasers that top-end companies use to calibrate with are ones just like that... The lasers used in some military and medical purposes are also just like that... the process uses a stacked capacitor charge-discharge circuit where by the first capacitor is charged from a V-source, and after some time constant the circuit flips and lets the cap charge another cap, and then that cap charges yet another... and that last one discharges into the laser diode... the result? A perfectly filtered and uniform DC source

Now... to cover the full spectrum, I agree with using a couple different kinds... the better idea I would say is probably to add one before the circuit (10uF or so) and then add another at the back of the LD... this should cover the full spectrum... or even better and take it one step further... add both at the beginning of the circuit and have the input to the regulator (output from batteries or any other source) filtered, and then add a smaller 1uF cap on the back of the LD...

It's an endless set of choices and with limitless configurations... take your pick with what you see is your best choice and what gives you your best results. I presented my idea as a helpful and guiding post for other to come

If you need any help, let me know

--DDL
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Old 11-03-2007, 12:46 PM #16
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Default Re: Driver Circuits and Capacitor Protection

Quote:
The LM317 circuit needs a capacitance... it's good for it... Any regulator requires a capacitance to make sure the output is nice and filtered and as, in fact, DC.
True, the manufacturer suggests at capacitor of at least 1 uF on the output to ensure stable operation. Personally, i'd recommend placing on the input of the regulator as well, unless battery connections are short and very solid.

Just a single elctrolytic cap suffices here. Putting a small ceramic cap in paralel does improve HF perfomance markedly, but this is not required for driving LDs.

Omitting the output cap copletely -may- cause the LM317 to behave unstably and oscillate, probably destroying your LD in the process.
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