LD driver configuring questions

[VDX]

...
I will try this. I know planters suggested this before. I just not sure how to calc what resistor I need to get.

... I'm measuring/adjusting my drivers with a 0,1 Ohm resistor (100 Milliohms) in place of the diode - then 10x the voltage drop will give your current value.

The calculation is pretty simple with Ohms law: - I (Ampere) = U (Volt) / R (Ohm)

For a given voltage drop of e.g. 0,25 Volts across a 100 mOhm resistor this will give -- 0,25 / 0,1 ==> 2,5 Ampere

Viktor

[catalanjo]

@ Viktor .."I'm measuring/adjusting my drivers with a 0,1 Ohm resistor (100 Milliohms)" ...precision low value resistors are readilly available one off in Germany, but not so easy to find one off in other parts of the world (tricky in Bogota for example, despite being a capital city). Also Maltes seems reticent about his geographic location,...so I have limited my suggestions to easily available components as far as possible.

I personally use 1 to 3 ohm resistor with a fan cooled diode string in series to adjust voltage to match the diode to be substituted.
The reason I do this is, is firstly due to availability but also it enables me to adjust optics bed first stage cooling to a simple measured power parameter, well before getting expensive LDs involved. (1W for every volt across resistor).

In absence of further info, I have to assume that your LD got zapped so fast (an instant before fail),that laserbee didn't even notice ........ which has fulminated the diode structure to such an extent that internal LD wiring has gone open circuit .... possibly even a "faulty" LD

A lasorb (close to diode) might well have sacrificed itself trying to save the diode in this scenario, and I suggest you try and get a couple before trying out any new diodes with your current setup. (however bad the postage is where you are)

Viktor has more experience of high speed zapping than most of us, as he modulates with high speed rectangular waves, perhaps he can chip in, with some idea as to how to check fast noise without breaking the bank. (eg. very fast scope).

Please keep posting ...like Planters, I am interested to find out exactly what happened here, ... it is a bit unusual, and merits further investigation.

Cheers

[Maltes]

... I'm measuring/adjusting my drivers with a 0,1 Ohm resistor (100 Milliohms) in place of the diode - then 10x the voltage drop will give your current value.

The calculation is pretty simple with Ohms law: - I (Ampere) = U (Volt) / R (Ohm)

For a given voltage drop of e.g. 0,25 Volts across a 100 mOhm resistor this will give -- 0,25 / 0,1 ==> 2,5 Ampere

Viktor

I see thank you. I will do this test. But what W should the resistor be rated for?

[catalanjo]

5W are readilly available and come encapsulated with ally heat sink. 10W ones (also mounted inside heat sink) are better, but a bit more difficult to find, 100W ones turn up in bargain basement sales from people like CPC/Farnell. (but you have to spend yonks looking for them)

Cheers
PS ... Watts = Amps x Volts ! (but might get HOT)....so bigger = better.

[VDX]

... you can heavy "overdrive" a resistor, when clamped into a big cooler - I've used two 6,2 Ohm enamel resistors stated as "5W-types" to heat a prototype hot-end in some ten seconds to 300 degC and used this hot-end for some weeks without degrading the resistors.

So get any resistor rated for 2W or 5W and clamp it into an aluminium cooler with enough thermal mass to get it running without glowing

To check for "high speed zapping" without a DSO or Oszilloscope you can try with a sound-card, which will give a humming or whistling sound if there's high speed oszillation below 20 kHz - but be sure to reduce the max voltage to 1V ... best with a two-resistor- voltage divider bridge ...

Viktor

[catalanjo]

Hi Viktor...hoped you would jump in .....but I was referring to >20k noise up to RF and the like !
Cheers

[planters]

A 3 ohm resistor is about right for the red diodes with their approximate 2.5 V drop and around 800 mA driving current. This resistor does dot have to be a precision resistor, 2ohm and 4 ohm will work fine as well.

It is unlikely your driver is sending a high voltage spike through the diode. They are designed not to do this. If your budget prevents you from getting an oscilloscope then you might want to get a driver or several from other sources as well as several diodes. The P 73 diodes are not very expensive and you will need a quantity at some point. This would be the diode I would recommend for your project in any case. It is robust, produces a lot of power, can be over driven easily and is inexpensive. It is the diode I use exclusively for my red modules.

Even a very basic oscilloscope is a very useful tool if you are building or repairing laser projectors. You might seriously see if you can get one.

[catalanjo]

I think Maltes is using Blue not Red.

@ "It is unlikely your driver is sending a high voltage spike through the diode" ...what do you thinks has killed his LD?

Cheers
PS. The precision bit, was concerning a 100miliohm resistor which Viktor uses to deduce current flow.

[Maltes]

Yeah I was testing on a NDB7875. A bit more expensive but not the price of green or 465nm blue.

But I'm going to use few P73 diodes for this project, maybe overdrive them at 1.2A to get 1W from each so the red power will get closer to the greens (NUGM01).

Okay, I *may* have created an ESD situation on the PCB of the driver which maybe didn't get the driver but somehow reached the diode and killed it? Yeah, I'll admit I wasn't wearing my wriststrap that day.
Still don't understand how turning the Gain pot creates an open circuit across the diode. It doesn't glow dimply or at all, doesn't draw current, not what you'd expect from a dead diode as you guys mentioned. Could ESD really destroy the internals of the diode in such a way as to cause an open circuit? I guess that will be the main suspect after I do the test with the resistor.

[catalanjo]

If the "gain pot" is dodgy, then it could quite easily change, from no gain to max gain, in a wink of an eye!
Which is why I called it the "primary suspect".
On the other hand when LD is open circuit it would be fairly normal for the driver voltage to jump suddenly due to it having beam suppression below a set threshold level (the other pot).

However without more measurement it's still impossible to tell ...........it might turn out to be, "something completely different" .... I don't think that static could cause LD to go open circuit ......it would die a different way and go LED

Cheers