Laser Driver/Modulator Circuit

[silverchris]

Hello all,
I have built a circuit from Sams laser FAQ. This one I am having trouble regulating the current. Or at least I think I am. R5 seems to adjust nothing to me. So I am wondering if the circuit has a problem or I am screwing up somewhere? I would very much appreciate any incite.
Thanks,
Silverchris

 

[silverchris]

Hey guys sorry to bump this but it is driving me insane. As far as a can tell I have followed the circut exactly except for subbing in a 6V zener for CR1 and a 10 ohm resistor for CR3. Is the circuit faulty or my bulding faulty. Please advise me.

 

[ericgibbs]

hi,
As you have subbed a 6VZ for a 8V7Z, you have reduced the voltage to the driver circuit by 25%.

Also CR3 has to be a 3V9Z, not a 10R, else the laser drive will not work correctly.

If you are short of zeners of the correct voltage rating, you could use 1amp rectifiers in series with your existing zeners.
Allowing 0.7V forward drop per rectifier.

Regards

 

[silverchris]

I actually have a very low to no supply of zeners. The 6v zener I have found was scanvenged from a old power supply. The schematic said I could use 3.9 ohms IIRC but the lowest resistor I have is a 10 ohm one here is the sams laser faq page I am going from https://www.repairfaq.org/sam/laserdps.htm#dpssg2 Could I sub 5 diodes for CR3? MY problem is that the current is to high. by about 60-80mA and that R5 does nearly nothing to adjust it.

 

[ericgibbs]

hi chris,
Five rectifiers with a Vfwd of approx 0.7V each will give you 3.5V.
It won't be as stable as an actual zener, but it will enable you to test your cct.

The 8V7 zener on the base of the transistor is giving an output voltage of approx 8V at the emitter, by using a 6V2 you have reduced the voltage to the LD rail to approx 5.5V, which will effect the power and function of the rest of the cct.

I'll look at the link.

 

[ericgibbs]

hi,

>> Laser Diode Power Supply 1 (RE-LD1)

From the link is this the cct you are referring to?

I see there is a 3R9 resistor in place of the 3V5 zener, but what about R9 the 10R shown on the circuit?.

I would recommend that you use the original zener diodes, the laser emitter will be easily damaged if overdriven.
Did you see the notes on the link ref, permanent reduced light output, if over driven for a short period.

 

[silverchris]

I am using SG-LD2 and SG-LD. I saw about the damage and the laser diode I have been using is probably damaged by now but I have some other ones from dvd drives for spares. I am thinking there should be less then 40mA going through them.

 

[ericgibbs]

hi,

'Note #6' on the original cct diagram,would be the way to test, before commiting the laser emitter.

Refer note #5, on current measurement

 

[silverchris]

Ya I was trying that but I have no photodiode around except for in the laser module. BTW how much should R5 change the current? I am messuring the current across R9 like it says in that PDF and I get 106mA all the time no mater what place r5 is adjusted to. also how high do you think r4 and r7 should be for 30 or so mA? 5-9K ohms or so?

 

[ericgibbs]

hi,
Looking at the notes for those resistors values, says, it a function of the monitoring photodiode, I have no details of that device.

I would agree a higher value for R4/7 would increase the effect of Q1 in reducing the LD drive current, try a 4K7 in both R4 and R7.

 

[silverchris]

I will give it a shot tomorrow and report back how it goes. Thanks for the help

 

[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

 

[silverchris]

Think I could replace the zener with a TL431 set correctly?Till I can place a parts order to get the proper stuff?

 

[ericgibbs]

hi,
The TL431 if set to give the same as the 8V7Z, so the +Vrail is +8V should be OK.
EDIT: providing the Vsupply to the Q4 transistor collector dosn't 'sag' too much when you are driving the LD.

Did you get the chance to measure the "2.5v" voltage?

If you have some lower voltage zeners in your bits box, remember you can connect them in series for the 8V7.

 

[silverchris]

Didn't get to messure it but I know when loaded it sags to 2.0V or less IIRC I have a couple of 14V zeners but that is higher then my supply voltage which is 12 from a SLA battery

 

[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....

 

[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.

 

[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

 

[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.

 

[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.