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### Topic: How to calculate the value for a current-limiting resistor for a laser diode. (Read 491 times)previous topic - next topic

#### BabyGeezer

##### May 16, 2019, 03:31 pmLast Edit: May 16, 2019, 03:40 pm by BabyGeezer
This is really something basic that i thought i should know by now, but i'm not sure how to proceed after the following;

i have a Laser pointer (diode?) which i want to switch on and off from an Arduino.
(no other specs other than "5V 650NM FOR ARDUINO")

1) i connected it directly to the 5V power rail and measured current draw with a DMM at 27mA.

2) so i thought, i should use a 185 Ohm resistor.

3) i used a 220 Ohm but now the measured current draw is only 9mA, as opposed to a calculated 22.7 mA.

where is the flaw in my above thinking, and what value resistor should i be using then ?

EDIT:
FWIW, it's the bare casing and two wires, without the PCB breakout in this picture.

also, does #3 above mean that the forward voltage of the Laser Diode is 3.02V ?
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#### wolframore

We are all bombarded with the idea of adding resistors to LED's... the laser pointer diode is designed for 5V per the spec.  Then usually rated at some wattage.  I have seen 5V laser pointer diodes that are designed for 40mA.  You are fine just giving it 5V and 27mA makes it nice and bright.

Your calculations are correct but my guess is that there is current limiting designed into that diode.

Bad Boys Rate Our Young Girls But Violet Goes willingly - Get Some Now - ELI the ICE man

#### BabyGeezer

Your calculations are correct but my guess is that there is current limiting designed into that diode.
ahh, pasting the picture made me just notice the SMD resistor on it with "[910]" = 91 Ohm ?

...
You are fine just giving it 5V and 27mA makes it nice and bright.
but, say if i wanted to bring it down "a tad" to 20mA - would i now add a 22 Ohm resistor (to that existing 91R) ?
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#### wolframore

#3
##### May 16, 2019, 03:56 pmLast Edit: May 16, 2019, 04:00 pm by wolframore
yep just use ohms law  figure for fwd voltage of the diode.

You can assume that 91 ohm is accurate and forward voltage would be about 2.5V.  So to get to 20mA you would need total of about 133 ohms round it to the one you can get.

Better yet get an SMD one and take the one off and replace with the value you want... for a soldering challenge!
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#### BabyGeezer

yep just use ohms law  figure for fwd voltage of the diode.
i see - so;
VF = 2.457V
91R * 27mA
or;
123R * 20mA

Better yet get an SMD one and take the one off and replace with the value you want... for a soldering challenge!
indeed - but then i'm buying "just one" 125R ?
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#### wolframore

#5
##### May 16, 2019, 06:05 pmLast Edit: May 16, 2019, 06:06 pm by wolframore
You'll pay more in postage
Bad Boys Rate Our Young Girls But Violet Goes willingly - Get Some Now - ELI the ICE man

#### BabyGeezer

You'll pay more in postage
it would be free from China though !

actually made me curious to see since i'd never browsed for SMD resistors before - just about a dollar for a minimum HUNDRED pieces of 130 Ohm !

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#### wolframore

You big spender you . If you don't want to wait and can afford a little more. Tayda electronics is a good source. They ship out of Colorado.
Bad Boys Rate Our Young Girls But Violet Goes willingly - Get Some Now - ELI the ICE man

#### BabyGeezer

i had a look - nice site, they should have given you some kind of "link reward" !

granted i can buy just ONE (at the same price of 1 cent !), but the price of the shipping is a dollar too !
(plus a minimum order of FIVE dollars !!)

might at least try a test order to get a stock of reliable passives, see what the delivery is like.
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#### ChrisTenone

Buying just one component. Wow - that's a new concept for me.
What, I need to say something else too?

#### BabyGeezer

Buying just one component. Wow - that's a new concept for me.
although, a n00b quickly learns to buy *two* ) to cover any silly error or bad soldering technique !
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#### ChrisTenone

although, a n00b quickly learns to buy *two* ) to cover any silly error or bad soldering technique !
I'm up to the 5 pack myself. But then, I'm clumsier and more cavalier than most.
What, I need to say something else too?

#### wolframore

I buy resistors 50-100 at a time. That way if those little buggers get lost. 0805 and smaller get lost very easily I don't mind and gets me to my minimum faster. They're hard to screw up soldering but still a challenge.
Bad Boys Rate Our Young Girls But Violet Goes willingly - Get Some Now - ELI the ICE man

#### BabyGeezer

can i ask what you mean by "and gets me to my minimum faster. " ?
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#### wolframore

some have minimum order qty... so 100 resistors at \$0.01 each is a dollar... quicker to get to my \$5 min.
Bad Boys Rate Our Young Girls But Violet Goes willingly - Get Some Now - ELI the ICE man

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