Constant current source for a laser diode

Thread Starter

electrophile

Joined Aug 30, 2013
126
I'm trying to replicate this circuit for a 2W laser diode engraver. I could understand most of the other components except the current source schematic (Pg. 5, Fig 4). I understand that the TLC271 is being operated in high-bias mode (cause the datasheet says that when the bias select pin is grounded its in high-bias mode), however, I do not understand how the combination of RV2, R16 and R18 works and what values need to be set. Any help would be appreciated. Thanks.
 

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ronv

Joined Nov 12, 2008
3,770
I'm trying to replicate this circuit for a 2W laser diode engraver. I could understand most of the other components except the current source schematic (Pg. 5, Fig 4). I understand that the TLC271 is being operated in high-bias mode (cause the datasheet says that when the bias select pin is grounded its in high-bias mode), however, I do not understand how the combination of RV2, R16 and R18 works and what values need to be set. Any help would be appreciated. Thanks.
They are used to cancel the offset of the op amp. I'm not sure why they are worried about it as they adjust the current anyway. I'm not crazy about that op amp. It also seems like there might be easier ways to pwm it.
 

sailorjoe

Joined Jun 4, 2013
363
I believe the schematic in Figure 4 is labeled incorrectly. On the TLC271, pin 8 should be Bias Select, and pin 1 should be Offset In 1, and pin 5 should be Offset In 2. Perhaps there is a different package not in the datasheet. In any case, the circuit seems to be wired correctly for proper use.
Ronv's explanation is correct.
 

Thread Starter

electrophile

Joined Aug 30, 2013
126
This is what I could source from the web (attached). Also, I was trying to design for ST's ST1CC40 using their eDesign Studio software on their website. I was obviously trying to create a constant current source for LEDs and one of the design inputs was for something called "Rd (ohms)". I assume its the resistance of the laser diode which unfortunately is not listed in the datasheet. I calculated this value using the voltage (~4.5V) and current (~1.2A) required to turn on the laser. Is that right?
 

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ronv

Joined Nov 12, 2008
3,770
This is what I could source from the web (attached). Also, I was trying to design for ST's ST1CC40 using their eDesign Studio software on their website. I was obviously trying to create a constant current source for LEDs and one of the design inputs was for something called "Rd (ohms)". I assume its the resistance of the laser diode which unfortunately is not listed in the datasheet. I calculated this value using the voltage (~4.5V) and current (~1.2A) required to turn on the laser. Is that right?
I'm not sure what they mean by Rd. I'll try to look tomorrow.
But for now, here is what I would recommend.
Buy a good quality switch mode power supply (wall wort) rated at 3 amps or so rather than trying to build one.
Then I think their constant current source can be made with more common devices. I can draw one up if you like.
I haven't convinced myself that their fancy reference supply is needed - need to look closer at it.
 

Thread Starter

electrophile

Joined Aug 30, 2013
126
I'm not sure what they mean by Rd. I'll try to look tomorrow.
But for now, here is what I would recommend.
Buy a good quality switch mode power supply (wall wort) rated at 3 amps or so rather than trying to build one.
Then I think their constant current source can be made with more common devices.
Yep I have a 12V, 10A SMPS power supply that I'll be using for powering the whole setup including the stepper motors and build a 5V supply off it to power my Arduino and the motor drivers.

I can draw one up if you like.
I haven't convinced myself that their fancy reference supply is needed - need to look closer at it.
I'd really appreciate that. Thanks.
 

Thread Starter

electrophile

Joined Aug 30, 2013
126
Yes, but these aren't plug-in replacements, you'll have to redesign the circuit for these chips. Fortunately, the datasheets are very complete and give you lots of information to use.
Got it. Redesigning with these would be more simpler since they seem to be using common components and it would eliminate some of the other circuits that are in the original design.
 

Colin55

Joined Aug 27, 2015
519
You just need a 2 -3 amp DC power supply, a transistor, signal diode and a few resistors.
The DC supply should be low voltage. As low as 5v
You can get one on eBay for $2.00
 

ronv

Joined Nov 12, 2008
3,770
Take a look at this one.
It does not have isolation between the mico (PWM) and has just a simple adjustment for the power.
 

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Thread Starter

electrophile

Joined Aug 30, 2013
126
A simple two transistor constant current circuit will do like these...

https://hackadaycom.files.wordpress.com/2012/03/led_driving_and_controlling_methods_14.png?w=800

Use the IRF540 mosfet one,( right) make Rg 10k, and Rs 0.5 ohms 3W, that will give you a current of 1.4 amps use a 5V supply.
Thanks @Dodgydave. This helps. I'm gonna try this since I think I have most of the components at home.

Take a look at this one.
It does not have isolation between the mico (PWM) and has just a simple adjustment for the power.
Thanks @ronv. The help is much appreciated. I will try this one out too. For now I do not need PWM but a simple turn on and off and I think I can manage that with a simple optocoupler such as the 4N25.
 

ScottWang

Joined Aug 23, 2012
6,909
@ronv, Could you please attach the libraries and symbols for the potentiometer and the OPA743 op-amp?
I was used the normal op amp to replaced it, but I still can't find the potentiometer, it's weird, I used the pot before, but I can't find it after I was updated a couple times later.
The op amp is at the\lib\sym\opamp\opamp.asy
 
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