hi all, i would like to modify a laser driver circuit to allow reducing laser brightness for alignment purposes. Usually these things have trim pots which can be adjusted, but mine (cheap!) appears to use fixed resistor values to set the different current limits for the 3 diodes. its all smt components so hard to see exactly whats going on, and although im handy with a soldering iron, i'm no electronics expert, and dont want to break the thing if possible.

my idea is to connect a small pot in parallel or series with the current limiting resistors to allow varying the output without going over the current current limits . im not sure if that would involve reducing the resistance or increasing it.

i just want to be sure im messing with the right resistors!

here is a photo of the board, as close as i can get..

on the right you have the power and ttl inputs, and on the right the outputs for the r/g/b laser diodes and fan.




to my mind r19, r16 and r13 near the "big" central diodes look like good candidates, being different values for the three channels, but i wanted a sense check, if anyone fancies casting an expert eye..

i was also informed that normally laser drivers have a very low ohm resistor in series with the diode output , which is useful for measuring the current going to the diode by measuring the voltage across this known value... however near the outputs, the only resistors i see appear to be 10k.. again maybe more expert eyes might see something ive missed.

also, what gives with c1? appears to have a resistor in place of the capacitor , but only on one of the 3 channels.

any help most appreciated.

 

Welcome to AAC.

What is the part number on the six pin IC?

 

Hi,, thanks for the reply. unfortunately I have no idea. I was hoping that it might be something standard and recognisable.. Maybe used commonly in led and laser drivers. Its definitely a constant current regulator, but as to part number, I have no clue.

 

Hi,, thanks for the reply. unfortunately I have no idea. I was hoping that it might be something standard and recognisable.. Maybe used commonly in led and laser drivers. Its definitely a constant current regulator, but as to part number, I have no clue.

It does have a number. Put a small drop of correction fluid on top of one, then wipe it off with reasonable pressure to read the laser etched numbers on the part.

 

I shall try as soon as I get home! Interesti g trick,

 

ok so i tried that trick..great! i didnt have correction fluid so i tried with white acrylic... with just the right camera angle it made the number very clear.. they are pt4119

https://datasheet.lcsc.com/lcsc/1912111437_PowTech-CR-PowTech-Shanghai-PT4119EE89E_C385316.pdf


hyowever, now i am more confused since it seems (from their example circuit at least) that the resistor to set current should be connected between Vin and CSN pins. however following the traces as best i can on my board it appears there is a component labelled as a capacitor in that location (c1,2 and 3) i do note that c1, at least, seems to be a resistor... the other two have no markings and are that beige colour i associate with capacitors.. ive no idea if this is a valid observation or not.

 

This is the relevant part of the datasheet:

The DIM pin can be driven by an external DC voltage (VDIM) to adjust the output current to a value below the nominal average value defined by RCS. The DC voltage is valid from 0.5V to 2.5V. When the DC voltage is higher than 2.5V, the output current keeps constant.

 

This is the relevant part of the datasheet:

Interesting! And thanks so much for the pointers. So in theory the ttl inputs can allow some kind of analogue control if the voltage on them drops below 2.5v? I wonder if in the case of my device the ttl input components allow this. I do know that in absence of a ttl signal (cables unplugged) the diodes go full-on, which suggests a deviation from the datasheet schematic, or maybe that's default behavior, and I have to go above 0.5 v to get back to reduced output current.?

I also know that you pay lots more for rgb lasers with analogue brightness control on the drivers, for some reason..

 

Ok so, thanks for the constructive pointers, and particularly the correction fluid trick! As noted, the diodes are full-on when no ttl connection is attached, so I assumed the dim pin was being pulled up to 5v. This was confirmed by measuring a precise 5v across the ttl connectors. It was a small leap (for me!) to try connecting a 100k pot across the ttl inputs.. Success! I can now manually dim each of the 3 diodes (and as a side effect, a nice infinite colour palette on the output

 

Ok so i now have a new question. if i wish to continue using the ttl signal which is fed into the driver, but have the ability to dim it, i am not sure how to approach this.

the dim pin is internally pulled up to 5v. The ttl signal is either 5v or pulls the pin down to ground.

i thought about adding a pot in the signal path to reduce the ttl signal below 2.5v to allow dimming, however this does not work, as the pin is internally pulled up to 5v, and adding a resistor in the ttl signal path means its not pulled down to 0v when the laser diode should be off.

i tried using a pot as a voltage divider between the ttl +ive and gnd, but again, when the pot is in the middle, the internal pull up keeps the dim pin at 5v

finally i tried using a pot to "short" the ttl signal... risky since i wasnt sure if the ttl source would like being shorted when the pot was at minimum resistance. i added a 150 ohm resistor in series to be safe. this didnt appear to do anything, so i tried a direct connection of the pot across the ttl without the resistor. This kinda works, but with a 5k pot, its basically only turning off the dode when i reach the lowest resistance, i.e. totally shorting the ttl signal. . is it just a case of using a much lower value pot? is it generally ok to short a ttl signal or should i always keep a low value resistor in there to avoid this?

i have no idea if what i have written is legible.. ! any ideas on how to do this correctly most appreciated. the ability to dim the 3 channels for color correction of the white values and not overpowering smaller parties would be very useful.