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# Can I change resistance values in laser diode driver circuit to be able to use it with another laser diode? Answered

I have Laser survey units,tens of them, each has IR original laser diode with this specification

((Ith= 17mA, Iop= 31mA, 798nm, Im=1.26mA)).

This laser diode is not available any more, I used to use a Sanyo replacement for it DL3144-008S with

((Ith=25mA, Iop=30mA,785nm,Im=2.0mA)).

The driver of laser diode is attatched, Iused to change the value of series resistor with photo diode to adjust the output power and it works great.

Unfortunately the new LD is also not available now the available one for me is 9GAIA red LD(ADL-65o55TL) with ((Ith=18 to 25 mA,Iop=25 to 35mA,655nm,Im=0.12mA))

the beam is very dim even with current 35mA!!! does it have any relation with monitoring current which is very low in the new one and if it has, if someone can advise me how to change values of the resistance to let the driver work with the new monitoring current, and if it is something else kindly advise

Thanks

## Discussions

You say, "the beam is very dim even with current 35mA!!!"

But I am wondering how you are measuring the current through the laser diode.

Do you have an ammeter in series with the uh... I guess it is the brown wire carrying current to the laser diode (LD)?

Or perhaps you are just measuring the voltage across R19, which is carrying most of the LD current. It is approximately 5.5 ohms, so with 35 mA flowing through it, it will have a voltage of 0.035*5.5 = 0.1925 V ~= 200 mV across it.

By the way, I do not see anything obviously wrong with this plan to change the resistor in series with the photodiode (PD).

As far as I can tell, that unnamed resistor (which is drawn as 2 resistors in parallel in one of your diagrams) converts the current from the photodiode to a voltage, and that voltage is seen on the yellow wire, which is connected through R17, a 20 Kohm resistor, to the inverting input of that op-amp.

I am guessing that making this current sensing resistor larger will make the voltage across it larger, which will cause the driver to reduce current to the laser diode.

Conversely, making the current sensing resistor smaller, will make the voltage across it smaller, and cause the driver to increase current to the laser diode.

Also you say you have done this before,

"...Iused [sic] to change the value of series resistor with photo diode to adjust the output power and it works great. "

which makes me wonder why you are asking this forum for help, if you have already done this sort of thing before.

Another thing I have to add, is I did some edits to that diagram Joe M drew, because the wiring for the photodiode (PD) looked wrong to me.

Anyway, I'll attach that drawing to this post, and link to the original scale version of it here:
https://www.instructables.com/files/orig/FA4/ODNY/...

By the way, do the laser diode and photodiode come together as one package?

I am kind of thinking they have to, if the subscripts stand for what I think they stand for, with Im being monitor current, and that being the same thing as photodiode current.

So if you get a laser diode plus photodiode package with Im ten times smaller than before, I think that means you want your current sensing resistor to be ten times larger than it was before. Otherwise the driver is going to be overcurrenting the laser diode as soon as you turn it on, because the signal from the photodiode current sense resistor is too small.

Thanks
-Each of the three mentioned laser diodes is a package( photo and laser diode together).
- I measure the current using Ameter across red wire
- The schematic you have drawn is right,it is perfect.
_ I said I used to change the series value with photo Diode and it works great that was with the SANYOO LD which is not available any more, The available one now is 9 GAIA with the very low monitoring current and I tried to change the value of this resistor using potentiometer instead of it, starting from 200 K ohm till 2 K Ohm it does not make any difference in the laser power or the voltage on the yellow pin which is fixed on 2.45 V.
- I tried also to change value of R17 from 20 K to 200 K, and still nothing change.
Thanks for your help and for the clear schematic and best regards .

Regarding what you wrote about puting a 200 Kohm pot, in place of the current sensing resistor, and the voltage on the yellow wire never changing, constantly 2.45 volts: That is what I would expect to see if the pot was inserted incorrectly; so that the resistance across it was always 200 Kohm, or maybe if there was open circuit (always infinite), in the place where the pot should be.

Also I was thinking, for the purposes of testing the driver circuit, you could put some different things in place of the laser diode and photodiode.

Specifically, what I had in mind was, 3 silicon diodes in series, in place of the laser diode, and also a small valued potentiometer wired as a voltage divider, in place of the photodiode plus current sense resistor. The goal of this setup would be just to check that the driver was responding correctly; i.e. reducing current to the 3 silicon diodes, in response to increasing voltage on the yellow wire.

The reason for using 3 silicon diodes in series, instead of a laser diode, is because the silicon diodes (like 1n4007 or similar)

https://en.wikipedia.org/wiki/1N400x_general-purpo...

is because you can dump huge (1 A is huge compared to 30 mA) amounts of current through them without damage.

The reason for using 3 silicon diodes in series, is to give about the same forward voltage drop as a red LED, about 3*0.6 = 1.8 volts.

I mean it would just be handy if you had some way of getting the driver running correctly, without having to risk overcurrenting your new laser diodes while doing it.

You shouldn't be trashing LDs in the first place. (Lazer Diodes)
I noticed you have no voltages for your LDs.
As long as the driver is set for the right voltage the LD shouldn't draw more current then the LD needs.
If the voltage is right the driver also needs to be able to supply the right current, if the driver will only supply half the current without burning out the driver the LD will be dull.
The LD schematic is not bad but the driver schematic I cannot read.

Thanks
For the the Original LD there is nothing mentioned in the service manual about its part number or the voltage specifications
For the Second LD (SANYO) the data sheet only has the reverse voltage( for LD 2V, and PIN 30V) but nothing about the operating voltage
For the available LD (9GAIA) the reverse V(for LD 2V, and for PD 30V) and operating voltage (2.2v typ, 2.5v Max)
I will try to attach a better image for the driver schematic today
Thanks and best regards

My lazers are three pin that is why the 3 pin box.
Ok between the red and the black you need 2.2 to 2.5 volts on the lazer.

The voltage is already 2.3 v but it is between the red and the brown( the tow terminals of LD ) , between red and black it is 5 v, between yellow and black it is 2.5v,
I think(but not sure) the critical item is the monitoring current it is very far from the first and second models Im but I don,t exactly know how to control it because I do not understand if the monitoring current is the current given by driver circuit to photo diode , or current given by photo diode to be sensed by the driver circuit
thanks.

I think I may have found another problem.

I did quite some experimenting with all sorts of laser diodes.
The one thing I learned it that they won't tolerate abuse at all.
This goes for all parameters!
Heat is one the fastest killers here.
Especially when using replacements for other diodes the problem of heat must be considered.
If the diode get above recommended operating temps the lifespan will drop like flies in the winter.
Most laser diodes will require a constant voltage while the current provides the "brightness".
Whatever is stated in datasheets is usally based on ideal thermal conditions.
In the beginning I trashed a lot of my laser diodes.
I used different driver disegins from dirt simple to quite complex but it did not help.
One major reason was that I almost always over powered the diode in terms of long time use.
Over time things heat up, which in turn affects the set parameters.
And even an oversized heat sink is only as good as it is fast enough to react and transfer heat away.
Especially is used in an enclosure this can become a problem with prolonged and ongoing usage times.
I realised that, in the long run, it is not only cheaper but way more effective to use larger than planned diodes.
For example if I need a 0.5W output continously for several hours then I used at least a 1W diode, driver and heat transfer solution.
But the diode is only powered to the specs I need for the purpose instead of being operated at the maximum.
This reduces not only heat but also stress and does the same way for normal LED's.
I prefer high brightness LED at half power over a normal one at close to max power.
The LED in my projection clock now runs for about 8 years straight...
A common misconception is that you always need a laser diode to match the orignal specs.
The only relevant spec is the wavelenght and beam characteristic.

If you are not 100% certain you driver is fully suitable for the diode then please consider replacing the driverwith a ready to go driver from China.
They are still of low quality for the cheap ones but you can get them to match most commonly available laser diodes.
If yours operates for long times in an enclosure then please consider a driver with a temp sensor for the diode.
The require to drill a 2mm hole into the heatsink that should be right where the diode is with as little material left in the housing wall to not go all the way through.
These types reduce the current if the set temp goes to high or if it happens too fast for some reason shut it off to prevent damage.
Slightly cheaper is to include a basic temp sensor with beeper output to alarm you so you can shut it off manually until cooled own enough.
But as said, using an overrated diode to start with can prevent a lot of problems later on.
For example if a 1W diode would reach a temp of 80°C after 30 minutes than a 3W diode running at an output of just 1W would be somewhere in the 40-50°C range or even below if the heatsink is matched to the 3W output.
Same story on the driver side.
Simple designs can get hot if the current limiting is not very efficient.
In almost all cases this causes the output current to go UP.
This is why good drivers are temperature compensated.
Either by feedback of the output current or by means of temperature sensors.

The thing that concerns me a bit is how you change the wavelength with the replacements diodes.
I don't know what you do with them but I do know that the difference is significant enough to have a severe affect if the original wavelenght is a requirement.
For example a sensor for 798nm might not even react at 655nm even if the intensity is much higher.
In terms of something as simple as remote control it could mean instead of 10m distance it will only cover 3m...
I strongly recommend you use diodes as close to the original 798nm as possible!
And as if possible then with a higher power rating but driven at the original specs.
The light output then might be slightly lower but you can compensate by increasing the output on the driver.
Depending on the specs of the diode you should not need more than 20% above the set rating to get well above the intended output.
Just to give you the worst possible example:
A diode like this and driven at the same light output as your 31mA original wouldn't even get warm and would most likely be able to continiously run until your grandkids retire ;)
Just noticed the link does not work for me, so here it is manually:
https://www.ebay.com.au/itm/Laser-Diode-200mW-798n...

Thanks for the precious information, in my country it is not easy to get laser diodes so I do not have the ability to choose from the wide range of laser diodes, I can get only what is already available I am taking about 5mW laser diode used in a a flashlight assembly which is a part of an equipment(which has no spare parts available ) which contains a main P.C.B a small part of it is the LD driver, some of the original laser diodes worked for more than 10 years, but some of them became very weak does not give enough power to cover the working range diameter, I used to replace them with SANYOO diode , that was O.K
but the available LD for me now is 9GAIA there is nothing to worry about from point of view of IR and Red diode replacement because the detectors and receivers of the system can work with both types of laser
I need any ideas to adjust the values of the components of the driver circuit to be able to drive the available LD or I have to to get rid of any unit that has a laser diode problem which is very very expensive choice
Thanks again and best regards