The current controlled circuit of a laser pointer power supply explained in the following post was requested by Mr. Steven Chiverton (, who himself is an intense electronic hobbyist and researcher.

Technical Specifications

Dear swagatam,

I'm emailing you to ask you for your expertise, and you being one of India's finest electronics engineers I thought you be the best man in the world to know, bear with me my friend its a bit of a long story.

I brought some 10 milliwats laser diodes from xxxx electronics here, the data on them isn't much but maybe enough to go by, they are 2.4 volts and the threshold current is 24 milliamps and the maximum current is 40 milliamps

now I've looked all over the net for a power supply circuit using the lm317 regulator for this diode but there's circuits for other diodes only but there voltage and currents are different

so trying to find a simple lm317 regulator circuit that will deliver 2.4 volts dc at up to or near 40 milliamps is hard . so I used an lm317 regulator calculator for voltage and

so I breadboarded it only to find out the voltage output was no where near the output of 2.4 volts I wanted , despite what the lm317 regulator calculator says

so I wanted 40 milliamps or to be safe just under it so I used the lm317 current regulator calculator and the resistor I entered got me 40 milliamps

but when I bread boarded it I got no where near it . so the best way to go is may be to modify an already existing laser power supply for a laser diode

I know nothing about to try get the 2.4 volts at near 40 milliamps so ill include one here can you modify the circuit to deliver 2.4 volts dc at near 40 milliamps for me and powered from a nine volts battery .

thank you swagatam I hope you can get it right where I've failed .

The Design

The required laser pointer driver circuit was actually very easy to design, thanks to the versatile 317 IC, you can do almost anything with this chip.

As shown in the figure, a single LM317 is used for acquiring the required precise 2.4V output at 24mA current.

It's a standard 317 variable power supply design. The preset P1 is used for setting up the 2.4V output.

Or alternatively P1 may be replaced with a fixed resistor of 110 Ohms, which would yield exactly 2.4 volts at the output

R3 is adjusted for getting the 24mA threshold current limit.

As per the formula, the current control resistor R3 may be calculated in the following manner:

R3 = 0.7/0.024 = 29 ohms.


thank you very much swagatam ill give that circuit a go to just have to round up the resistors I need out of a draw full of them and the 110 ohms isn't an easy one

but resistors are never the exact values these days that's why they have the gold tolerance bands they are either above or below there values ,

and also due to the various calibrations of digital meters they don't all read the same values so anyhow 110 ohms is close to 120 ohms is a try and electronic calculators and theory circuits don't calculate values using the gold tolerance bands

so the actual results are not known till the actual circuit build is done or the resistors are measured to the present calibration of the meter you use to test them with ,

thanks swagatam pal ill get back to you soon hopefully the red 10milliwats laser diodes hold up ok and at just over 6 dollars each I have 2 only so ill try them soon.

More Feedbacks from Mr. Steven

here's a copy of the modified laser driver circuit you once emailed my back can you modify it again to be adjustable up to 1.2 amps max and minimum of as low as you can get it , as I want to build another but with a higher adjustable current

DDL Laser Circuit

here's a new printed circuit version I made from a schematic from the laser pointer site this is for the ddl laser driver circuit , its a test load circuit for that so you can adjust the ddl laser diode driver and use the next circuit the test load circuit for that to tune this ddl laser diode driver I think its for 2.8 volts laser diode or near that




Improving the Laser Circuit Further

here's the latest swagatam,

I've made a printed circuit of another ddl laser diode driver from the laser pointer forum

so I've added a new feature to it to solve the laser diode damage problem caused by an undischarged electrolytic capacitor in the circuit near the output to the laser diode

even though I got the same thing when I blew my test laser diode when I forgot all about the 10 uf 16 volts electrolytic that caused it .

so here is my solution , look at the picture and next to the electrolytic capacitor is a plain dc input socket and I've used just 2 out of its 3 pins so it bridges the capacitor and forms a short to discharge it

so to unshort it just put any plug into it and it opens the short so the capacitor can charge during use of the driver and when you finish pull the plug out to shorten the capacitor again fail to do so would result in the charge left in the capacitor being dumped into the laser diode and thus over volting it and blowing it






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