Running a laser pointer?

For a school project, I am running a 4.5 volt, 0.03 amp laser pointer. I need to run it from a 24v power supple. I have tried 1/4 watt resistors, and they ended up smoking. I don't know how to determine what components I need between the + power and the +lead of the laser pointer. Any assistance would be appreciated.

sort by: active | newest | oldest
I think you should have a look at a regulator like an LM317 they are cheap and available at any good electronic hobby shop and you can adjust the voltage to whatever you want. The are also good to 1.5 amps.
have a look at the pdf.
Screen Shot 2012-11-24 at 4.18.01 PM.png
lm317.pdf279 KB
dkop1 (author)  liquidhandwash4 years ago
Thank you. I should have thought of this, as I am using (2)-317s in a separate project.
While the 317 is a very excellent fast analog voltage or current regulator it
must follow ohms law.

+1 (or 0.585)
frollard4 years ago
You need to try and find a lower power supply, or run more than one laser diode in series.

The laser drops 4.5 volts, leaving 20 volts of potential difference to reduce. that's 80% of your power burning up in a resistor.

Resistor: 30mA at 19.5v is 0.585 watts -- a quarter watt resistor will not be able to dissipate that much power.
dkop1 (author)  frollard4 years ago
This is being run from a nanoline plc controller, and will be sent to Germany in the spring. I am trying to avoid an alternate power source, as converting to their outlets is sketchy at best. I don't mind burning up power in resistors, though it'll be inefficient. Would higher wattage resistors be my solution?
frollard dkop14 years ago

As iceng says use a 1 watt resistor, if you absolutely must. It will get warm/hot.

A cheap dealextreme dc:dc buck converter will do the job for less than 5 dollars.

Feed it anything and it switch-mode regulates it down to what you need. 80-95% efficient
iceng4 years ago
frollard is very correct in everything ;
  • You are wasting over a half watt  o.585W  in a resistor.
  • ( this means a 1 Watt resistor should be used )
  • A lower source voltage is an obvious improvement to pursue.
  • Using an LM317 will still dissipate the same analog power.
  • Until a lower voltage source like 12 or 6 VDC are used even in Germany.
  • A current regulation 317 mode is the best method of driving a laser.

If you want a circuit to do that just ask :-)

dkop1 (author) 4 years ago
As well, Iceng; In using a 317, I should still use my 0.03 amps in my P=IE calculation? If I have a 4.5-4.6 volt output, *0.03 to get my wattage?
iceng dkop14 years ago
If you are still using 24VDC as the source there is still a 19.5 volt
drop to 4.5 volts at 30 ma heating something to over a half watt.

BTW current control is preferred to voltage control.
I would first run it through a voltage regulator to step it down to 5 volts. Then use an 18 ohm resistor to drop it to 4.5 volts, 30mA.

Voltage Regulator:

Voltage Regulator Circuit (product above + 2 capacitors for stabilizing):

LED resistor calculator (works same way for lasers):