Pulsed Laser Driver

[.3lite]

Uhm, i suggest you to be careful with that schematic.

In that configuration, for turn off the LD, you put in short circuit the LM317, and also if is true that in the current regulation configuration it's almost self-protected, i'm not too much sure about the lifetime of a similar schematic in long periods

Some of my pal on another forum did it to power his own DPSS green laser (LD - 200mW 808nm pumping xD) and it worked fine

 

[FireMyLaser]

Try putting the transistor in series with the current limiting resistor.

 

[HIMNL9]

I'm not saying it don't work ..... i'm only saying that, in this configuration, the transistor, for turn off the LD, short circuit it, putting in short circuit also the output of the LM317, and in this way, each time it go in short circuit, go in self-protection, and this is usually not the better way for use a regulator let it living long and happy

Other than this, if you're using this system with a bare LD, it can also go ok, but if, as example, you have a capacitor, like, 1 uF or more, in parallel of the LD, each time the transistor close, also have to adsorb all the energy stored in the capacitor (and this can also end in a premature dead of the transistor itself )

It can work, just, it's not the better thing to do ..... is a bit like put a switch in parallel to a lamp, and close it for turn the lamp off .....


@ FireMyLaser: i'm not sure if the LM317 agree with this solution, too .....

 

[FireMyLaser]

 

[.3lite]

Because, LM317 will try to keep current specifed by resistor (remember LM317 is a VOLTAGE regulator) by increasing voltage and after transistor opens it will kill the diode with higher voltage.

LM317 stabilize the current by regulating voltage on the diode.

 

[FireMyLaser]

My idea was that if the transistor is off, no current will flow (well except 10mA or so cus of the nature of the LM317). When the transistor is on it will act like a switch and currnet will flow, through the current limiting resistor, just like in a normal DDL setup.

 

[nikokapo]

If the average power isn't higher than the power at CW it wont be any brighter.
Lets say you have one 100mW cw diode and one 200mW pulsed diode (insert x kHz here), duty cycle 50%. Both will be the same brightness. Remember LEDs use dimmers based on this. The eye don't see just the on time, it sees both the on- and off time. But it happens so fast that we see an average output. If you try to slow down till you can see the pulses brighter, it will be flickering and be VERY annoying aka a puke-light. But also that's way to slow for pulsed operation anyway.

Thanks for the info, it was very helpful I thought we'd see the peak output and not the average (I guess the "off" periods dissapeared in my mind ).

Well, i made a test, right now ..... built on-the-fly a pwm driver, and took my labby heatsink, and 2 PHR from 2 sleds (same lot, so probably they are identical)

Putting both them in aixiz modules and driving the first one with 100 mA through the LM317 driver, hooked the second one to the PWM driver and an oscilloscope, and made some measures.

The PWM driver is powered at 12V, with the resistor in series to the LD at 27 ohm, that, considering 5V of FV of the LD, do 260mA of current (used a mosfet for the switch).

at 500 Hz, with 0,75mS on/1,25mS off, i get the same spots appearence (can't see the beam, no smoke here ) ..... then tried at 166 Hz, 33% duty cycle (2ms on/4ms off) and got the same spot appearence again (this mean, theorically, there's a little increase in visibility, cause i was with duty cycle 1 on 3, when for the same power, i have to go with duty cycle 1 on 2,6, with 100mA on the fixed one and 260mA on the pulsed one)

Anyway, the result is really subjective, and can change from person to person, and unfortunately i have no time for continue the tests at the moment, sorry (had to open the shop)

Maybe with some group of tests, made with different persons with the same circuit and diodes, we can have a better idea about how it work, and if someone notice it better than others, for different individual sensitivity (also to be considered that i made the tests only with PHR, maybe with red ones the results can give some diferent data)

If someone want to do that, i can draw a better circuit that anyone can duplicate, so all can use also the same circuits (as example, fixed values for frequencies, for have a sorta of "grid of values" that can be reproduced also from those that don't have instruments at home, and so on).

Yes, YES. This is awesome. As somebody stated before, there should be a "sweet spot" between duty cycle and optimized peak power vs. mA.

My idea was that if the transistor is off, no current will flow (well except 10mA or so cus of the nature of the LM317). When the transistor is on it will act like a switch and currnet will flow, through the current limiting resistor, just like in a normal DDL setup.

This ^

 

[HIMNL9]

My idea was that if the transistor is off, no current will flow (well except 10mA or so cus of the nature of the LM317). When the transistor is on it will act like a switch and currnet will flow, through the current limiting resistor, just like in a normal DDL setup.

Yes, i've understood that, is just that the LM317 is not too much stable, when you left the adj/ref pin opened ..... "theorically", with the pin opened, it must give 1,25 V at the output, but the internal configuration make me think that the impedence is a bit high (after all, the maximum current drawed from the regulation network through the adj pin is 100 microA), and this means, usually, sensitivity to the disturbs and interferences ..... and if this put the regulator in an oscillation cycle ?

 

[FireMyLaser]

That sound bad lol. Maybe it could be helped if you put another resistor in parallel with the transistor (gosh complicated), that has been carefully calculated with the other current limiting resistor in such a way that when the transistor is on the current will be 200mA, and when it's off the current will be 20mA. This way the pins will never be open.

 

[Laser_Ben]

Okay... This is a good idea, but it is a stupid one when you think about it. You will be seeing the average power of the beam, that means, if you have a 200mW CW laser, it will appear just as bright to your eyes as a 400mW unit with a 50% duty cycle. That is still way to high of a duty cycle to double the power out of your laser. Say you wanted to run a 10% duty cycle, that would mean your laser needs to be 2000mW or 2 Watts.

Pulsing is good for some things, but visibility is not one of them.

 

[FrostyTheDadMan]

The Mighty Micro Drive is in testing, US Patent Pending #61000221
You are at the guessing stage, That's where I was a year ago
You have a long hard road a head, It was not easy getting it this far So
Just sit back and relax and let LaserBee, Daniel, K-9 and myself finish. I promos their will be enough for every one.
Their about the size of a quarter and a adjustable output of 1.5 Volt to 6.5 volt and from 10 mA to 1 amp at a fixed frequency that is faster then the human eye can see, all from a 7.2, volt 300 mA rechargeable battery Pack
so you get No dots or dashes.
You will be able to run your PHR803T at a minimum of 170 mA in some cases up to 210 mA, with out sacrificing any diode life.

 

[FrostyTheDadMan]

Okay... This is a good idea, but it is a stupid one when you think about it. You will be seeing the average power of the beam, that means, if you have a 200mW CW laser, it will appear just as bright to your eyes as a 400mW unit with a 50% duty cycle. That is still way to high of a duty cycle to double the power out of your laser. Say you wanted to run a 10% duty cycle, that would mean your laser needs to be 2000mW or 2 Watts.

Pulsing is good for some things, but visibility is not one of them.

Man your eye cannot process the off time
real time is 30 frames per second and we are talking 25,000,000 times on and 25,000,000 times off per second that's 10 ns

 

[maxh]

Actually 25 MHz is 40 ns pulse width. 100 MHz is 10 ns.

I have a dimmer switch in the living room with an LED that turns on/off at 60 Hz. When I'm still and I look at it I don't see any flickering, but if I move my head or walk across the room, the flicker is quite clear even though it's double 30 Hz.

Your eye/brain can and will process the off time just as well as it processes the on time. That's why pulse width modulation works for adjusting brightness. The beam won't be any brighter than appropriate CW operation, and I'd venture to say that pulsed operation very well could put more strain on your LD for equal average output power than CW operation, resulting in a shorter lifespan. Just look at the data sheets. The maximum average power (taking into account duty cycle of course) for pulsed operation is typically lower than that for CW. 72% in fact for the datasheet I posted in this thread.

 

[nikokapo]

I'd venture to say that pulsed operation very well could put more strain on your LD for equal average output power than CW operation, resulting in a shorter lifespan. Just look at the data sheets. The maximum average power (taking into account duty cycle of course) for pulsed operation is typically lower than that for CW. 72% in fact for the datasheet I posted in this thread.

That is incorrect. Pulsed ratings are MUCH higher than CW ratings, if not I wouldn't even have created this thread.
I don't remember the exact powers but if you look at bluray diodes the pulsed ratings are around 200% higher than the CW rating. The thing is that people drive them as CW to their PW rated powers.

 

[FrostyTheDadMan]

Actually 25 MHz is 40 ns pulse width. 100 MHz is 10 ns.

I have a dimmer switch in the living room with an LED that turns on/off at 60 Hz. When I'm still and I look at it I don't see any flickering, but if I move my head or walk across the room, the flicker is quite clear even though it's double 30 Hz.

Your eye/brain can and will process the off time just as well as it processes the on time. That's why pulse width modulation works for adjusting brightness. The beam won't be any brighter than appropriate CW operation, and I'd venture to say that pulsed operation very well could put more strain on your LD for equal average output power than CW operation, resulting in a shorter lifespan. Just look at the data sheets. The maximum average power (taking into account duty cycle of course) for pulsed operation is typically lower than that for CW. 72% in fact for the datasheet I posted in this thread.

Ok I will give you the 100Mhz = 10ns
but 60 times a second is not 100,000,000 times a second there is a lot more zeros in mine.
Just about every dvd burning diode will pulse at almost twice the CW power that is enough to make up for the off time.
Like I told Dugin we will see soon.

 

[FrostyTheDadMan]

Actually 25 MHz is 40 ns pulse width. 100 MHz is 10 ns.

I have a dimmer switch in the living room with an LED that turns on/off at 60 Hz. When I'm still and I look at it I don't see any flickering, but if I move my head or walk across the room, the flicker is quite clear even though it's double 30 Hz.

Your eye/brain can and will process the off time just as well as it processes the on time. That's why pulse width modulation works for adjusting brightness. The beam won't be any brighter than appropriate CW operation, and I'd venture to say that pulsed operation very well could put more strain on your LD for equal average output power than CW operation, resulting in a shorter lifespan. Just look at the data sheets. The maximum average power (taking into account duty cycle of course) for pulsed operation is typically lower than that for CW. 72% in fact for the datasheet I posted in this thread.

Ok I will give you the 100Mhz = 10ns But 10ns on would be 50,000,000 cycles or 50Mhz @ 50% duty cycle, because each cycle has an on time and an off time,

but 60 times a second is not 50,000,000 times a second there is a lot more zeros in mine.
Just about every dvd burning diode will pulse at almost twice the CW power that is enough to make up for the off time.
Like I told Dugin we will see soon.
see their is so much involved and you all are just scratching the surface