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I'm planning to use a circuit as a laser diode driver shown in this datasheet. It is not for production, just for lab test purposes, but I would like to obtain fair accuracy and be sure the circuit won't exceed its max. limits.

The laser diode specs are follows (link is not kept alive by the vendor so I share the screenshots):

enter image description here

enter image description here

Instead of SMD components, I want to use another op-amp (LT1490) and transistor (BD139) as follows:

enter image description here

Above, D1 and D2 are there to mimic the laser diode.

I want to control the laser current between 0 to 500 mA by the 10 kΩ Rpot. I will set the laser current and leave it, so I don't any switching or modulation.

My question are:

  1. Can the transistor (BD139) handle a continuous current of 500 mA?

  2. I also saw drivers using a Howland current source as follows:

    enter image description here

    What is the difference between the circuit in question and this one in terms of accuracy or any advantages?

  3. LTspice simulation shows the following plot for 0 < Rpot < 10 kΩ:

    enter image description here

    For a larger input the current goes beyond 1A. How can I guarantee that the laser diode would not get damaged? Should I limit the voltage across it or just be sure the current does not exceed 500 mA?

The diode metal case is tied to the anode so as an alternative circuit:

enter image description here

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    \$\begingroup\$ Nevertheless you should provide some link to the laser diode. \$\endgroup\$
    – Andy aka
    Commented Jan 26, 2023 at 21:20
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    \$\begingroup\$ You're doing way more work than necessary (unless this is an academic exercise and you're trying to learn something in which case more power to ya!!!) All you need is any current limited DC bench supply (you probably already have one). You set the output for, say 5V (so it's above the voltage the diode needs), then short together the PS's output and adjust the current limit until it puts out exactly 400mA. Now when you connect your diode to the PS, it'll be the constant-current supply you're trying to design except it only took you 2 minutes instead of 2 weeks to setup. \$\endgroup\$
    – Kyle B
    Commented Jan 26, 2023 at 21:24
  • \$\begingroup\$ If the supply doesn't have a current meter built in, you can short it with any handheld multimeter set to 'DC A' scale and accomplish the same thing. (probably with more accuracy even). Just start with the PS current limit set to almost zero so you don't blow fuses in the multimeter. \$\endgroup\$
    – Kyle B
    Commented Jan 26, 2023 at 21:26
  • \$\begingroup\$ @KyleB No Im required to make a current source cannot use the bench supply. Also Using voltage source with series resistor is not what I want. Radiation is dictated by current. \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 21:39
  • \$\begingroup\$ @Andyaka thorlabs.com/drawings/… and it uses thorlabs.com/drawings/… They expire the link so hope you see it. \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 21:41

1 Answer 1

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I want to use another opamp (LT1490) and transistor(BD139) as follows

If Vcc is 5 volts then you can limit current to 0.5 amps like this: -

enter image description here

Can the transistor(BD139) handle continuous current 500mA current?

The data sheet says it can in absolute maximum ratings but, prepare to use a heatsink.

I also saw drivers using Howland current source as follows

If you must have the body of the laser diode grounded then that is one option. The other option is a high-side current source (similar to the LTSpice circuit but referenced to the 5 volt rail).

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  • \$\begingroup\$ Thanks a lot! Finally someone answered. And you mean high-side current source like this? codrey.com/wp-content/uploads/2020/04/… If I connect like high-side do I need to tie the body of the laser diode to Vcc and keep it floating? Or if I keep it like in question and do not ground the laser diode body, what would be an issue? I also saw some add parallel RC to laser diode twovolt.com/wp-content/uploads/2016/07/LASER-DIODE-DRIVER-3.jpg Is that to prevent voltage spike at power up? \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 22:15
  • \$\begingroup\$ The first circuit is a more classic version of the current source you did in LTSpice so, I'd use it (or yours) and tie the body of the laser via a 100 nF capacitor to GND but, don't connect it to a power rail. A high side source is like this: content.instructables.com/FSQ/H0K2/HEXS4N45/… but, it's a bit tricker to find an op-amp that can manage it so, stick with either of the low side circuits @cm64 \$\endgroup\$
    – Andy aka
    Commented Jan 26, 2023 at 22:39
  • \$\begingroup\$ Sorry for last stupid question. I will tie body to GND via 100nF X7R but the diode's body is PIn1 correct?thorlabs.com/drawings/… \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 22:48
  • \$\begingroup\$ Or the cathode is the body? thorlabs.com/drawings/… I never done this and the diode is expensive that's why Im asking to be sure \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 22:55
  • \$\begingroup\$ OMG I just noticed this diode is E type thorlabs.com/images/TabImages/Pin-Codes-A-G-V21-dwg764.gif So it is anode ground. Do I need a negative power supply to drive it? Or if I keep it floating I can use it as in circuit in question? \$\endgroup\$
    – cm64
    Commented Jan 26, 2023 at 23:52

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