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# Resistor Calculation?

#### JSP4th

##### New member
i want to run a green laser with a 9v battery but i don't know what resistor to get radio shack has no clue does anybody know a web site that calculates resistors buy in/out voltage? the laser needs 4.5v so i need to go from 9v-4.5v. is that a 1/4w 45ohm resistor or a 50ohm resistor? just want to make sure i don't fry my module.

#### danq

##### New member
JSP4th said:
i want to run a green laser with a 9v battery but i don't know what resistor to get radio shack has no clue does anybody know a web site that calculates resistors buy in/out voltage? the laser needs 4.5v so i need to go from 9v-4.5v. is that a 1/4w 45ohm resistor or a 50ohm resistor? just want to make sure i don't fry my module.
At the very least you'll need to know how much current your module requires.
IF it has a current-regulating driver attached, then you can use "Ohm's Law" to calculate things:

E = I x R or R = E / I or I = E / R

where E is Volts, I is Amps, R is ohms (those are the letters I learned it by; you can use "V=A x R" if that's easier)

For example, if your module draws 200mA current:

R = 4.5V / .2 A = 22.5 ohms

so you'd need a 22.5 ohm resistor to drop 4.5 volts at 200mA

BUT, that only works if your module always draws 200mA - that is, if it has a current-regulating driver attached!  if not - if it's a bare laser diode you're connecting to - it won't act like a simple resistive load and the whole picture changes.

If your module acts more like a bare diode, its effective resistance can go very low.
For example, what if the module acts like a bare diode, and wants its voltage drop to be 2 volts? That leaves 7 volts to the resistor, so plugging that in to the equation:
I = E / R
Amps = 7 / 22.5 = .311A = 311mA

that extra current would either make your laser very bright, or kill it. It will also make the resistor dissipate 2 Watts! it might get a little warm  :
Remember though that those figures are just for example; yours will be different.

DanQ

#### jwc

##### New member
A resistor establishes a certain current flow based on some input voltage; it doesn't magically reduce voltage.

If you did want to use resistors for regulating voltage, you would have to use a voltage divider (Google it).  This isn't really a good idea since it's a waste of energy.

Dropping from 9V to 4.5V is pretty big, and generally not recommended.  You could use an LM317 setup in a voltage regulation circuit to do so, but it would give off all the wasted energy as heat.  Again, not recommended.

9V batteries have a very low capacity.  Is there a specific reason why you would want to use one?

Edit: Darnit, DanQ! You beat me!

#### JSP4th

##### New member
well i just got done building my 1st BR using rkstr's driver plans and i used a project box from radio shack, and i like how it came out alot. so i went out and got the same box and switch and a 3aaa batt holder to try to repair my greenie that broke, the LD and driver still work but the pen style case that came with the setup i got isn't working and i can't figure it out so i took the whoe module out (LD, Focing optics, Driver) and wanted to put everything in the box, and everything works except the batt holder doesn't fit, but a 9v does. the problem is i want to be able to replace the batteries when they run out so i don't want to just solder the batteries together. the only other option is to use a 3v Li. batt but the greenie i got drops mW with 3v, and doesn't with 4.5v. i'm in the car getting ready to go into work...i'll post some pics of the build so far hopefully someone will have some suggestions on how i can fit 4.5v into my host
LASER GAZERS

#### danq

##### New member
quick note: rechargeable Li is 3.7 - 4V, might do the trick. Or, just measure the current it draws at 4.5V; then replace the driver board with one that will deliver that current from whatever battery you want. Personally, I favor 18650's - they pack a hefty charge ;-)

#### JSP4th

##### New member
static got my LD my project is dead, on a better note I just took apart a broken sled and if i can get the old LD out of the green mod. and put in the new IR LD then game on, and this time i can just use the same driver and a single Cr2 or 2x N or or A Cr2 and a "AAA" any of those combos fit in the box