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# Logical problem - calculation of watts

#### ultimatum

##### New member
:wave: Hello, this is my first post on this forum and I'm starting with building my DIY lasers :bowdown:

I have a physics problem about lasers, perhaps something that I don't understand.
The driver of a laser regulates the electric flow (miliampere) that is given to the diode. And nobody speaks of how many Volts are given in the driver, but the volts also have an influence to the power (watts).

Volt * Ampere = Watts = how much Energy is given to the diode / time

So, does a diode driver also regulate the power (???)
Is it a difference when I take 6V or 12V for a 100mA driver?

Cheers
Pascal Studer
Swtzrlnd

:thanks:

#### ElektroFreak

##### New member
Yes. A constant-current PS does control power by controlling the current. The power (in watts) that the driver feeds to the laser diode is not the same as the light output of the laser diode, since some of the input power is lost as heat in the process of generating light. In a linear constant-current driver (like the common LM317 driver), if we control current we also control voltage and power. With simple linear drivers the current is tightly controlled and the way the circuitry works boils down to basically allowing the load to set the voltage. When working with laser diode drivers, current is the most important factor since LDs are current driven. This, combined with the way these CC circuits work, means you don't hear nearly as much about the voltage as current.

#### ultimatum

##### New member
Thank you very much!
I can't waiting for building my own laser!!
Besides, does an 75mw green or a 100mw violet beam appear brighter?
(I have a dragonlaser 75mw pointer)

peace

##### New member
Because the human eye is most sensitive to the green wavelength, the 75mW green will be much brighter.

#### Krutz

##### New member
welcome to the forum!

you are right, the voltage is a factor too. having two diodes, both with 100mA in and 100mW out, one with 2 volts and one with 3 volts (perhaps an IR and a red one?), one gets 0.2w electrical input, the other 0.3 watts. both having 100mW optical output, so the first is 50% efficient and generates 100mW heat, the other 33% efficient with 200mW heat. of course the first has longer battery live too!

the voltage seems pretty consistent with reds, noone speaks much about voltage with these.
with blu-rays, being brand new technology, the voltage varies a lot! which changes heat and efficiency, but can as well cause difficulties for driving them, when the driver can deliver just up to 6v, for example.

manuel