Dimming a laser diode via external voltage

Hello everyone, I've been lurking around here for a while looking for solutions, but decided to ask the people who have experience instead. Basically I need a driver with analog dimming - PWM won't cut it, beam needs to be continuous. The catch is that dimming needs to be done by applying an external voltage - I am planning to use a microcontroller + DAC to be able to dim it programmatically and most of the drivers with adjustable current seem to have trimpots.

It would be used for driving a ~5mW IR range diode. I'm guessing it would need 20ish milliamps at full power, so fine tuning of current needs to be available. Stability, high reliability and low ripple are also huge advantages. It will not be protable, so any input voltage range and size is fine. Does anyone have a solution to accomplishing such a task? Or maybe there are pre-made drivers with such capabilities available for purchase? I know they exist for high power LEDs.

I found this thread proposing a simple driver, at the beginning the author claims to use it for analog dimming, but all I see is PWM dimming, which makes me a little confused. Can it be used?

You can control it with external voltage, since you will limit the input voltage, at some time the driver will not regulate and the power should drop, I tried it with my red and my blue laser and worked, but you will only be able to regulate using a little tension, for example, from 2V (0mW) to 2,4V (5mW), this will depend on the driver and the diode used.

alessnilsen said:

You can control it with external voltage, since you will limit the input voltage, at some time the driver will not regulate and the power should drop, I tried it with my red and my blue laser and worked, but you will only be able to regulate using a little tension, for example, from 2V (0mW) to 2,4V (5mW), this will depend on the driver and the diode used.

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2-2,4V interval is a bit too small, I'm looking for something like 0-5V. Though I'm not sure what you're suggesting, if I'm using a constant current driver (which is mandatory for laser diodes, as I understand) then changing the input voltage does not yield much anyways?

if you want full control, like 0~5V, you will need a driver with analog modulation, I never seen one that works.

In my idea, you will have the control of the output by varying the voltage drop of the LD, because of this, the adjust interval is small. I cannot see any other way to control other than this or a full driver with analog modulation.

You'll need to design a driver for that, because none out there work that way by default, but itv shouldn't be hard. Use something like an ATTINY to turn the voltage measurement into control of a digital potentiometer, and use the digital potentiometer in place of the trimmer that would normally scale the FB voltage from a current sense monitor.

rhd said:

You'll need to design a driver for that, because none out there work that way by default, but itv shouldn't be hard. Use something like an ATTINY to turn the voltage measurement into control of a digital potentiometer, and use the digital potentiometer in place of the trimmer that would normally scale the FB voltage from a current sense monitor.

Click to expand...

I've thought about it, replacing the trimpot with a digipot. Though I'd prefer using a DAC for their higher resolution, if there is no alternatives then it will have to do. For the other part, what kind of drivers do you guys use for ultra weak <10mW diodes? I've seen drivers around here that handle over an amp of current, but what about those occasions when it's too much? Do you increase the resistor values or something along those lines?

This driver will accept a 0-5V signal for modulating the laser - should be able to set the current to whatever you need it to (Up to the limit of the regulator). Courtesy of Sam's Laser FAQ. See the attached text file for more detail.

There are of course off the shelf options available (Such as this: https://innolasers.com/shop/index.php?id_product=11&controller=product) - you should be able to find more information on those over on www.photonlexicon.com.

No reason you can't drive an analogue driver with PWM.

You can just use a transistor in its linear region to control the amount of current that is flowing to your diode. Here's a simple circuit for it. Note that you'll need to use a correction amp to get it in the input voltage range you need.

The above is pretty much like what Diachi's circuit above does, only it's using an LM317 instead of a MOSFET.

Note that both of the above methods will create a lot of heat for the regulating devices (the MOSFET and LM317) because they both drop voltage while regulating current. Make sure you heatsink them.

diachi said:

This driver will accept a 0-5V signal for modulating the laser - should be able to set the current to whatever you need it to (Up to the limit of the regulator). Courtesy of Sam's Laser FAQ. See the attached text file for more detail.

Click to expand...

That seems perfect! Though I find it a little complicated, since I'm still learning electronics and a lot of the information in the text file went over my head. As I understand the limit is 1.5A with the current resistors. I would need to cut the maximum current to <100mA if I want to have reasonable resolution using a 0-5V external signal. According to the file the maximum current is determined by the regulator's fixed resistor. Is that R11? How would one calculate the required resistor for lower currents? Or maybe the trimpot and modulation can be used at the same time, with the trimpot reducing the maximum current at 5V?

diachi said:

There are of course off the shelf options available (Such as this: https://innolasers.com/shop/index.php?id_product=11&controller=product) - you should be able to find more information on those over on Photonlexicon.com.

Click to expand...

I've looked at this one before, I believe it's quite popular around here. What drove me away is that the datasheet (if you can call it that) states that the minimum output voltage is 3.5V. I need to drive a single IR diode, it's operating voltage is 1.7-2.5V.

diachi said:

No reason you can't drive an analogue driver with PWM.

Click to expand...

But as I understand if I drive it with PWM then the output will be pulsed as well, right? Or does your proposed analog driver produce a continuous beam even when PWM modulated?

Bionic-Badger said:

You can just use a transistor in its linear region to control the amount of current that is flowing to your diode. Here's a simple circuit for it. Note that you'll need to use a correction amp to get it in the input voltage range you need.

The above is pretty much like what Diachi's circuit above does, only it's using an LM317 instead of a MOSFET.

Note that both of the above methods will create a lot of heat for the regulating devices (the MOSFET and LM317) because they both drop voltage while regulating current. Make sure you heatsink them.

Click to expand...

That's the same driver I was asking about in my first post. How do you use it with analog input, exactly? I'm not really sure of what the correction amp does here.

A correction amp is often used for laser show applications, matching the output signal from a usb sound card to the input of a laser driver and such. You have no need for one if you can supply a voltage in the 0-5 volt range from zero to full power.

As for your application, you could try my circuit here: Merghart.com - TTL/Analog controlled opamp based current source

It basically gives you a zero-to-full response with 0 to 5 input voltage, though there is no correction built in for threshold current (you can do that in software if you need).

The design scales down to low power diodes just fine - follow the calculations for the shunt resistor R4 to meet your needs. The design is fairly universal and will happily power a 5 mW diode to a 1W+ one as long as you get the value for R4 right.

IHaveNoIdeaWhatImDoing said:

That seems perfect! Though I find it a little complicated, since I'm still learning electronics and a lot of the information in the text file went over my head. As I understand the limit is 1.5A with the current resistors. I would need to cut the maximum current to <100mA if I want to have reasonable resolution using a 0-5V external signal. According to the file the maximum current is determined by the regulator's fixed resistor. Is that R11? How would one calculate the required resistor for lower currents? Or maybe the trimpot and modulation can be used at the same time, with the trimpot reducing the maximum current at 5V?

I've looked at this one before, I believe it's quite popular around here. What drove me away is that the datasheet (if you can call it that) states that the minimum output voltage is 3.5V. I need to drive a single IR diode, it's operating voltage is 1.7-2.5V.

But as I understand if I drive it with PWM then the output will be pulsed as well, right? Or does your proposed analog driver produce a continuous beam even when PWM modulated?

That's the same driver I was asking about in my first post. How do you use it with analog input, exactly? I'm not really sure of what the correction amp does here.

Click to expand...

You are correct, the maximum current is determined by R11 - So at 5V it will be at "full current". VR1 adjusts the minimum current, or the output current when the signal voltage is 0V.

The value for R11 is calculated by 1.25/A, if memory serves me correctly, so for 100mA you would need a 12.5 Ohm 1/4 Watt resistor.

The LM317 will go down to 1.2V, so no issue there!

Yes, if you drive it with a PWM signal the output will be pulsed accordingly, seeing as this driver is at least theoretically capable of a modulation rate of up to 1MHz - faster than your typical PWM signal. One solution to that would be to add a low pass filter to the signal input, that would convert your PWM signal to analogue.