Question about modulated driver idea

[ElektroFreak]

I've been mulling over this idea for awhile, mostly for TTL modulation, and now that the lasorb is available I think it might be possible, but I'd like any feedback from the great minds here..

Most people are familiar with the common LM317 current regulation circuit used as a simple laser diode driver.

I've been formulating a simple way to modulate the circuit, but so far the results have allowed for very limited modulation speed due to the protection capacitor across the diode.

In a standard LM317 current regulation circuit it is possible to modulate using the adj. pin. If a lasorb was used instead of the filter capacitor, would the modulation speed increase enough to be a viable modulation circuit or would the LM317 circuitry itself still limit modulation speed too much for show use? That's my question.

By my logic, using a circuit like this even analog modulation would be possible. Is my logic accurate in this regard?

I obviously haven't built such a circuit yet, but I'd like to know what you guys think..

[FourDee]

An LM317 just does not regulate that fast ... I mean sure you could make it work to some extent, but it doesn't cost much more to build a decent driver so why bother

[ElektroFreak]

An excellent point, although now I wonder just what the rise-fall time is on an LM317 (time to bust out the 'scope)..

More than a cost issue, it is a simplicity issue. I was thinking "it couldn't be that easy".. and it's probably not. Depending on any extra parts that are needed, it seems that it could work but you might end up putting just as much into that as you would another, better design. On the other hand if it were that easy, it would probably be beneficial to someone somewhere..

[mixedgas]

An excellent point, although now I wonder just what the rise-fall time is on an LM317 (time to bust out the 'scope)..

More than a cost issue, it is a simplicity issue. I was thinking "it couldn't be that easy".. and it's probably not. Depending on any extra parts that are needed, it seems that it could work but you might end up putting just as much into that as you would another, better design. On the other hand if it were that easy, it would probably be beneficial to someone somewhere..

The problem with 317 is if the potentiometer wiper lifts during tuning it surges to full current. This is well documented both in Sam's FAQ and Winfield Hill's book "The Art of Electronics".

There is a way around this by using a parallel resistor across the adjust pot.

As for the 317 being slow:

SEE:

http://web.telia.com/~u85920178/tx/317-tx.htm

Just kidding with ya, but many people have made 317 audio amplifers.



Steve

[ElektroFreak]

If I were introducing a modulation signal on the adjust pin rather than using a pot would the issue still occur with the LM317 surging to max current? I would think it wouldn't but I'm not really sure. I can't remember ever hearing before that someone had directly modulated the adj. pin to vary the LM317 output. If the thing will handle audio frequencies, then it should definitely be able to handle high-speed laser modulation. Using a transistor connected to the adjust pin would actually be the way to go. Obviously can't put 5V to the adj. pin, but if a transistor was used in the adjust portion of the circuit then a low voltage modulation signal could be applied to it.. I guess I'll have to try to put something together and see how it goes.

[mixedgas]

If I were introducing a modulation signal on the adjust pin rather than using a pot would the issue still occur with the LM317 surging to max current? I would think it wouldn't but I'm not really sure. I can't remember ever hearing before that someone had directly modulated the adj. pin to vary the LM317 output. If the thing will handle audio frequencies, then it should definitely be able to handle high-speed laser modulation. Using a transistor connected to the adjust pin would actually be the way to go. Obviously can't put 5V to the adj. pin, but if a transistor was used in the adjust portion of the circuit then a low voltage modulation signal could be applied to it.. I guess I'll have to try to put something together and see how it goes.

SEE this:

http://www.circuit-projects.com/misc...-by-lm317.html

as you sink more and more current out of the adj pin, the Vout goes up. You'd need some means on limiting the drive from the 317 and some bias on the transistor.

Steve

[mixedgas]

SEE this:

http://www.circuit-projects.com/misc...-by-lm317.html

as you sink more and more current out of the adj pin, the Vout goes up. You'd need some means on limiting the drive from the 317 and some bias on the transistor.

Steve

two 317s in series? one as max current and the other as the modulator? wierd idea but I've seen worse.

Steve

[ElektroFreak]

Sounds like it would work.. I'll have to see what I can work up in the way of a schematic..

[drlava]

Here's a high speed shunt style driver I posted around a 317:
http://www.photonlexicon.com/forums/...ghlight=driver

[ElektroFreak]

Nice! Thanks....