Modulating a constant current driver

For years I've been using CW drivers and have never needed to modulate one... until now. It needs to support a modulation frequency of up to 10kHz, as well as a variable pulse width. While the venerable DDL (Daedal) driver - virtually world famous and ubiquitous - is great, I am not sure about how well it would take to being wiggled at 10kHz.

I have attached a schematic of a simple two-transistor circuit that functions as a constant current regulator, and is modulatable. Theoretically. I'm going to be using the PHR diodes on this - I only need about 200mW output in the 635nm region.

Can those of you with greater experience with the intricacies of LD driving take a quick look and let me know if I'm headed down the right path, or am I doomed to cooking my LDs with this circuit?

The source will be battery, 6V, probably 4 x AA cells. I will not be driving the diode at max CW, probably somewhere about 80% max I at most.

I will probably also be using 2N2222As instead of the BC547s, since the 547s have a max Ic of 100ma, and the 2222s have a max Ic of 600ma, which can support the 220ma drive current for which I am designing.

Thanks for your wisdom and help!

Dave

Attachment 26187

Daedal has another driver for loc reds that modulates very well. I have 2 or 3 sitting here with me; unfortunatly the red diodes attached to them died...

Not sure if the fact that the diodes attached to them died makes for a very impressive testimonial! :-) Is the Daedal driver you reference a public-domain schematic as was the original, or is it something being marketed and therefore not available as a schematic?

My diodes died because of me. There were not protected by LASOBs and I connected the modulation signal wrong on one and the second got grounded badly while I was moving it... I do not think it is available by schematic but you'd have to ask him.

Glad to hear the circuit wasn't the issue! Sorry the diodes died, though!

I'll contact Daedal direct, thanks for the lead.

Dave

pretty sure the flexmod3 is good to 100khz or something stupid high

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Originally Posted by allthatwhichis

There were not protected by LASOBs

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Ok, it is bugging me, and I have to ask this noob question. But what the heck is this LASOBs and what is the application of this device?.

http://lmgtfy.com/?q=lasorb

:)

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Originally Posted by flecom

http://lmgtfy.com/?q=lasorb

:)

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FNG's......... Yup going to need a few of them lasorb's around....

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Originally Posted by flecom

pretty sure the flexmod3 is good to 100khz or something stupid high

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The FlexMod3 is probably your best option as I am not sure if Mo is still building anything, although he may provide you with the drawings to build your own. drlava has the Flexmods in stock now though and it provides a lot more power compared to the loc driver from Mo.

One thing I'm finding in my research is that LDs don't like to be switched fully on/off when modulated, and so it's better to switch the drive current between max and a lower value that brings the diode below the lasing threshold, but not actually off. Is this true to what you know?

Yes, keeping the diode powered, but just under threshold improves response time, and therefore linearity, when going from dark to powered.

Thanks for the confirmation... I'll add that into my design, unless Daedal comes through and I get a much better schematic that's just as simple! :-)

I have modified the original schematic I posted after breadboarding the circuit and adding another resistor to provide a minimum current so the diode is never fully off. I have tested it with a 1.0 ohm resistor in place of the LD to measure current, and the calculations hold for various values of R. I will add a reverse bias diode for LD protection, but this circuit SEEMS pretty good so far.

It appears the calculation for Max I ( I = 0.7/R ) is dependent of the transistors Vbe. The calculations seem to suggest that for the 2N2222A, that 0.7 value should actually be 0.66 - but then again it could be due to the resistor values being off slightly... not sure which yet.

I'll post my results again when I have a fully functioning LD with modulation applied at the desired current (or after I cook my LD, whichever comes first... :-) )

Dave

I changed the location of the minimum current setting resistor and added the reverse spike protection diode, and I'm PWM'ing my diode with a propeller (a microcontroller if you aren't familiar with what a propeller is). I've run it to 10,000 Hz and have a nice square waveform going in, and a fairly square response as read on a photodiode.

Am I missing anything here - does anyone see any demons lurking in my schematic waiting to bite my faithful diode? :-)

Thanks for your thoughts.

Dave