Laser Driver/Modulator Circuit

[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.

 

[Roff]

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]

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]

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]

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.