Laser Driver/Modulator Circuit - Page 2 - Electronic Circuits Projects Diagrams Free

silverchris · 29th April 2007, 12:08 AM

OK. Got the voltage messurement and it is 2.11 at the TL431. I guess It needs to be exactly 2.5? or could more then 2.5V work? Also does anyone know what the average current needed for a DVD rom laser? That is what I am using and I think 114mA is to high but I could be wrong....

ericgibbs · 29th April 2007, 07:41 AM

hi chris,
>> It needs to be exactly 2.5? or could more then 2.5V work?

The Vref 2.5 is a current limit voltage applied to the base of Q2, the current thru R9 creates a voltage on the emitter of Q2, limiting the current thru the LD.
If you start to reduce the Vref 2.5V, then you will have to start altering the value of R9 to compensate. If you increase Vref then then current will have to be greater thru R9,Q2 and the LD before the limiter works.

Ref the working current of the DVD/LD I cant help with that query.

silverchris · 29th April 2007, 07:59 AM

ah. I lost my zener for it while working in it so I am trying to work out a way to get a TL431 in there but my spair TL431 seems to be malfunctioning. I have a 2.2K resistor between Cathode and REf and a 1K resistor between Anode and REf so it should be around 8V but I get 6.73V or so

ericgibbs · 29th April 2007, 09:01 AM

hi,
Whats the value of R1, is it still 1K0 ?

Measure the voltage at the junction of the cathode and the 2K2???
I think you are limiting the TL431 of current to much from your +12Vsupply.

ericgibbs · 29th April 2007, 09:46 AM

hi,
As you have a 6V zener, you could use the attached cct for the +8Vs.

The RX 1K2 may require 'tweaking' to give an output of +8Vs, if you have a 1K0 pot connect that in series with a 470R.

The transistor T1 is a general purpose npn.

silverchris · 1st May 2007, 03:32 AM

Thanks. Unfortunately I have to put the project on hold for a few days due to some stuff coming up but I will let you know how it goes.

Roff · 1st May 2007, 08:42 PM

Quote:

Originally Posted by ericgibbs

chris,
Did a rough analysis of the cct, you must have a 8V7 zener on that regulator cct, else the 2.5Vreg CR2 will not be effective.
With a 6V zener on the base, you will have Vrail of approx +5.3V, the CR2 progzener has a current spec of 10mA, there is
a 560R in series with CR2 from the '5.3v' . So you have a voltage drop of 5.6v!!!
Check the CR2 voltage.

Without the Q2 base 2.5V clamp voltage, limiting the current, its most likely that the LD has died.

The reduced Vrail voltage will also adversley effect the control of the photodiode monitoring feedback loop.
The conduction current thru the monitor photodiode and R5, R4, R7 path will be lower, so the switch on voltage of Q1 will be
reduced, hence the current in the LD will be higher than required before the monitor cct responds.

Regards

Eric, maybe I misunderstood you. CR2 is not a constant current (10mA) sink. It will draw as much current as R3 provides, less the emitter current of Q1, which is basically the same as the base current of Q2.
The problem I see with VCC=5.3V is that CR3 (if used) will not allow enough current through the laser diode, even if Q2 were saturated. As mentioned in Sam's explanation, this is intentional. Without CR3, the photodiode can become forward-biased if VCC is low, causing Q1 and Q2 to be a latching positive feedback loop (similar to an SCR), instead of negative feedback as it is when the photo-diode is reverse-biased.
I agree that the best solution is to use a TL431 for CR1, programmed to 8.7V.

ericgibbs · 9th May 2007, 11:22 AM

Hi Ron,
Sorry about delayed response.

The CR2 device as I understand from the datasheet will sink 10mA, if its not configured in the programmable mode?

The way I see CR2 working is that it acts a base clamp voltage for the LD drive transistor, so that the voltage drop across the 10R shuts off the the driver when the maximum rated current is reached?

I cant see a CR3, do we a different drawing? [Ive attached the one Im using]
Regards

EDIT: attachment.

Roff · 10th May 2007, 05:08 AM

Quote:

Originally Posted by ericgibbs

Hi Ron,
Sorry about delayed response.

The CR2 device as I understand from the datasheet will sink 10mA, if its not configured in the programmable mode?

The way I see CR2 working is that it acts a base clamp voltage for the LD drive transistor, so that the voltage drop across the 10R shuts off the the driver when the maximum rated current is reached?

I cant see a CR3, do we a different drawing? [Ive attached the one Im using]
Regards

EDIT: attachment.

I'm looking at this schematic. Is this the wrong one? I couldn't get the second pdf you referenced to load.

ericgibbs · 10th May 2007, 07:21 AM

hi Ron,

Chris said it was this version,>> Laser Diode Power Supply 1 (RE-LD1)
He did say he was busy and would get back to the project shortly.

I have re-attached the LD1 Ver1, shortened the file name

Just did a test download of file,OK.

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29th April 2007, 12:08 AM	(permalink)
silverchris	OK. Got the voltage messurement and it is 2.11 at the TL431. I guess It needs to be exactly 2.5? or could more then 2.5V work? Also does anyone know what the average current needed for a DVD rom laser? That is what I am using and I think 114mA is to high but I could be wrong.... Last edited by silverchris; 29th April 2007 at 12:11 AM.

29th April 2007, 07:41 AM	(permalink)
ericgibbs	hi chris, >> It needs to be exactly 2.5? or could more then 2.5V work? The Vref 2.5 is a current limit voltage applied to the base of Q2, the current thru R9 creates a voltage on the emitter of Q2, limiting the current thru the LD. If you start to reduce the Vref 2.5V, then you will have to start altering the value of R9 to compensate. If you increase Vref then then current will have to be greater thru R9,Q2 and the LD before the limiter works. Ref the working current of the DVD/LD I cant help with that query. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/

29th April 2007, 07:59 AM	(permalink)
silverchris	ah. I lost my zener for it while working in it so I am trying to work out a way to get a TL431 in there but my spair TL431 seems to be malfunctioning. I have a 2.2K resistor between Cathode and REf and a 1K resistor between Anode and REf so it should be around 8V but I get 6.73V or so

29th April 2007, 09:01 AM	(permalink)
ericgibbs	hi, Whats the value of R1, is it still 1K0 ? Measure the voltage at the junction of the cathode and the 2K2??? I think you are limiting the TL431 of current to much from your +12Vsupply. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/

29th April 2007, 09:46 AM	(permalink)
ericgibbs	hi, As you have a 6V zener, you could use the attached cct for the +8Vs. The RX 1K2 may require 'tweaking' to give an output of +8Vs, if you have a 1K0 pot connect that in series with a 470R. The transistor T1 is a general purpose npn. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/ Last edited by ericgibbs; 7th July 2008 at 12:19 PM.

1st May 2007, 03:32 AM	(permalink)
silverchris	Thanks. Unfortunately I have to put the project on hold for a few days due to some stuff coming up but I will let you know how it goes.

9th May 2007, 11:22 AM	(permalink)
ericgibbs	Hi Ron, Sorry about delayed response. The CR2 device as I understand from the datasheet will sink 10mA, if its not configured in the programmable mode? The way I see CR2 working is that it acts a base clamp voltage for the LD drive transistor, so that the voltage drop across the 10R shuts off the the driver when the maximum rated current is reached? I cant see a CR3, do we a different drawing? [Ive attached the one Im using] Regards EDIT: attachment. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/ Last edited by ericgibbs; 7th July 2008 at 12:20 PM.

10th May 2007, 07:21 AM	(permalink)
ericgibbs	hi Ron, Chris said it was this version,>> Laser Diode Power Supply 1 (RE-LD1) He did say he was busy and would get back to the project shortly. I have re-attached the LD1 Ver1, shortened the file name Just did a test download of file,OK. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/ Last edited by ericgibbs; 7th July 2008 at 12:20 PM.