Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers



Is it ok to run a laser diode from a 3v to 5v step up converter with a resistor?

chris47368

New member
Joined
Feb 2, 2020
Messages
4
Points
1
Hi everyone,

I am getting back into the hobby again.

My main question is the title really - the diode i plan to operate is a 405nm 150mw diode(a SLD3236VF). The forward voltage ranges from 5v to 5.3v max - with a driving current of 140-150ma.

I plan to connect a 3.7V 18650 Li-ion battery to a 3v - 5v step up buck converter, then on the 5v output I plan to stick a 1W 36 ohm resistor along with the laser diode - to maintain a ~140ma current at 5v DC.

Will this be sufficient to operate my diode in a diode safe manner at/close to its rated output?

Or am I much better off using a laser driver board?

Thanks everyone!
 



Externet

New member
Joined
May 21, 2022
Messages
4
Points
3
If the laser current is respected under the maximum specified, the initial and final voltages are irrelevant.
 

chris47368

New member
Joined
Feb 2, 2020
Messages
4
Points
1
If the laser current is respected under the maximum specified, the initial and final voltages are irrelevant.
Thanks very much - I thought it might be ok but given I am new to this building hobby thought I should confirm! You set my fears at ease 😂
 

kecked

Well-known member
Joined
Jun 18, 2012
Messages
748
Points
63
It about current not voltage. So if the resistor guarantees the current can’t exceed the rating of the diode at a the supply voltage it could work. The diode may not be very stable however as it heats the draw will change so it may flicker. Use a constant current driver.
 

Giannis_TDM

Well-known member
Joined
Apr 27, 2019
Messages
738
Points
93
It about current not voltage. So if the resistor guarantees the current can’t exceed the rating of the diode at a the supply voltage it could work. The diode may not be very stable however as it heats the draw will change so it may flicker. Use a constant current driver.
It is two way actually, both the resistor and diode will drift, and not that marginally might I add since there is a fair amount of power running to heat the resistor up, not to mention the diode. However, he can just compensate by just driving under the specified max current by a couple of dozen mA(around 20-50 depending on how safe he wants to play it) since relatively speaking 140mA is not that much, if it was a higher power I could not guarantee anything since the resistor and diode drift would be way too much. Another way he could compensate is by using a low tempco resistor in the 100ppm/c neighbourhood resulting in less 'underdriving' needed.
 

paul1598419

Well-known member
Joined
Sep 20, 2013
Messages
19,397
Points
113
Of course that laser diode will try to draw more current as it heats up, but that 36 ohm resistor will keep the current at 5 volts below 140 mA since your resistor doesn't include the Vf of the diode. I wouldn't use this instead of a driver for a more expensive diode though. Good luck.
 

Giannis_TDM

Well-known member
Joined
Apr 27, 2019
Messages
738
Points
93
Of course that laser diode will try to draw more current as it heats up, but that 36 ohm resistor will keep the current at 5 volts below 140 mA since your resistor doesn't include the Vf of the diode. I wouldn't use this instead of a driver for a more expensive diode though. Good luck.
That is not true... Resistors do have a temperature coefficient and cheap ones a very high one at that, It is not a matter of will it fluctuate, but how much it will. also, your point does not really make sense in general, if the diode vf fluctuates assuming an ideal resistor if we want it to make any sense mathematically(aka no tempco) the value should change accordingly to the voltage change for it to keep the current constant.
 

paul1598419

Well-known member
Joined
Sep 20, 2013
Messages
19,397
Points
113
Of course resistors have a temperature coefficient, but the Vf of all laser diodes decreases as they heat up and assuming no temp. coeff. of the resistor even if the effective resistance of the diode was 0 the current at 5 volts would not get higher than 140 mA. I really don't understand your point at all.
 




Top