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Old 05-05-2008, 11:23 AM #1
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Default Protection from your Protection Cap

just to update, emphasize, and reinforce a few other posts I've made:

blown another diode,
even though you're using a protection capacitor?

==> put a resistor in parallel with that cap! <==


you can use a surface-mount, between the diode pins!

With most drivers, 1K or so in parallel will discharge the cap plenty fast but not materially affect the circuit.

You can figure the extra current it will draw: in parallel with a 4V laser diode, 1K ohms would draw 4mA, not that much more load for your driver.

And according to this RC Time Constant Calculator, a 1K resistor will discharge a 10uf cap to only 10% of its charge in 23 milliseconds. (and to 1% in 46 msec)

And here's a very reliable, sturdy way to do it:

The photo below shows my crude prototype - it's merely a small piece of 2-sided pc board, with 2 holes drilled through it. In one hole I've pressed a size 0603, 1.2K ohm resistor; in the other is a similar-sized 4.7uf capacitor. So the R and the C are each in parallel with the 2 sides of the board. While soldering the parts in, I tinned the whole board so that when connecting the diode it will take minimal heat.

All that needs to be done is to wedge the board between the 2 diode pins, then solder them with a quick touch. The driver circuit can then be soldered to the other end of the protection board.

This board is small enough - and sturdy enough - to be trimmed to your desired length using wire cutters.
Need more capacitance? Drill more little holes and plug 'em up.

Of course if designing your own board, if it's a direct-solder to the LD you can just add the resistor to the design.

[in the photo, the board is in place on the LD but not soldered to it. The parts in the foreground are 1.2K resistor, left; and 4.7uf 6.3V capacitor on the right. My board was roughly cut and poorly soldered but I hope it shows the potential for this approach]

I hope this helps save some laser diodes!
:-)
DanQ
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Protection from your Protection Cap-laserdiode_protect.jpg  


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Old 05-05-2008, 11:50 AM #2
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Default Re: Protection from your Protection Cap

how can a capacitor kill a diode when its hard wired to the diode like you suggest?
it might be a good idea to have a 1k or so resistor on the actual driver board, as the caps on the driver board would be the ones that damage your diode. having a resistor soldered directly across the diode is useless
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Old 05-05-2008, 11:51 AM #3
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Default Re: Protection from your Protection Cap

the topic title I should have used:

"Blown again? Why not resistor?"

then again maybe I shouldn't have...
:-X
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Old 05-05-2008, 11:57 AM #4
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Default Re: Protection from your Protection Cap

Quote:
Originally Posted by woop
how can a capacitor kill a diode when its hard wired to the diode like you suggest?
it might be a good idea to have a 1k or so resistor on the actual driver board, as the caps on the driver board would be the ones that damage your diode. having a resistor soldered directly across the diode is useless
Woop, it's useless only if no capacitor is going to be placed across the LD; or if that cap is always going to be securely in place. Of course, the resistor could/should go with the cap, whether it's on-board or on-laser. But since there are many, many posts recommending a cap be on the LD, most people are doing so... so that's where I've stuck the resistor here. (my homemade drivers all have both C and R onboard).

Several people have posted that they killed their diode by having a poor connection to the cap - whether poor solder joint, or flaky wire joint.

It just seems cleaner to me, to keep that cap discharged at all times when not lighting up that laser!

DanQ
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Old 05-05-2008, 12:01 PM #5
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Default Re: Protection from your Protection Cap

well if there is a flaky connection, people should learn how to solder, or use some flux to clean the joint beforehand.
adding a resistor is not a very good solution.
however the pcb idea between the leads is nice, and that idea in its self ensures that no damage will be done with flaky connections
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Old 05-05-2008, 12:11 PM #6
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Default Re: Protection from your Protection Cap

Quote:
Originally Posted by woop
well if there is a flaky connection, people should learn how to solder, or use some flux to clean the joint beforehand.
adding a resistor is not a very good solution.
however the pcb idea between the leads is nice, and that idea in its self ensures that no damage will be done with flaky connections
Yeah - I remember seeing someone - Daedal?? - who sells diodes solders the two leads together - it might make sense as an option to just install a protection board like this (cut down a bit), to protect against static, voltage surge, and mis-soldering by the end-user.

My mention of flaky connections is from what people say happened, not from what should be - there are a few people involved here now who don't have the skills that others do. Your mention that they should learn to solder has resonance with me, because imho a lot of folks here should be starting with a light bulb before making a laser :

DanQ
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Old 05-05-2008, 12:21 PM #7
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Default Re: Protection from your Protection Cap

Quote:
Originally Posted by danq
Your mention that they should learn to soldeer has resonance with me, because imho a lot of folks here should be starting with a light bulb before making a laser *:

DanQ
ditto

anyone reading this, if you have trouble soldering, GET SOME FLUX it helps a LOT..
even though solder comes with flux in the core, its way better to apply flux before soldering
soldering flux just makes the solder stick and flow to anything it touches, it is the best thing since sliced bread
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