# Why is my Arduino frying my lasers?

I've gone through 3 now. True they are cheap Dollar Tree pointers, but their own (tiny watch type) batteries measure 5 volts. They work great for abut 10 minutes on 5v then gradually dim out. I assume they must be drawing too much current from the 5v Arduino supply. I'm no authority on Ohm's law, but if it is already the correct voltage, how do I limit the current. I know motors kill themselves when they don't get enough current. Should I be using PWM to limit what it gets? I'm down to my last laser.

Battery powered toy LED lights rely on the internal resistance of the battery (battery voltage collapses if too much current is drawn). I suppose it's the same for your laser.
Try a 220ohm current limiting resistor between 5volt (or output pin) and the laser(+).
Leo..

jeanpaulsc:
I assume they must be drawing too much current from the 5v Arduino supply.

In that case… measure the current for conditions that this laser operates at… ie…whatever battery it normally uses. That will give you an idea of how much current should be used.

I would be very surprised if the pointer runs on 5V - there is not "standard" chemistry that will give you 5V.

Are you sure it's not running at 4.5V from three alkaline buttons cells in series?

// Per.

Laser diodes are VERY current critical. From a TI article on driving a Laser Diode:

"Laser diodes, because of their nature, need to be powered by a current source.
While having a negative temperature coefficient, laser diode resistance drops with
temperature rise. This means the longer the diode is used continuously the smaller
its resistance becomes due to the heat generated by wasted power in a diode.
With voltage source supply it simply means that the current will rise along with
temperature and at some point destroy the diode due to thermal shock. That is why
semiconductor laser diodes need proper current source driver, preferably with
temperature feedback to stop the diode from overheating."

As `Southpark` suggested, measure the current to the Laser Pointer while it is being driven by the batteries [that it is usually powered by]. Then use some sort of current limiter circuit to force that current in all conditions. Also, consider that a Laser Pointer is, really, not designed for long periods of continuous operation. Their might not be adequate heat sinking, especially on a cheap unit.

And, if the batteries internal resistance is being relied on for current limiting, as `Wawa` suggested [very likely -- most cheap "keychain" coin cell LED lights do this very thing], then the voltage across the Laser Diode is probably less than the full battery voltage. But, as long as the current is limited, and the current regulator can supply enough voltage to energize the Laser, the operating voltage will find it's correct level. This voltage is, actually, the Forward Drop of the Laser Diode [unless there is some sort of driver within the pointer, which is doubtful].

A simple and cheap current regulator can be made from an LM317 (Google: "LM317 current regulator circuit"). Because a Laser Pointer uses minimal power, the inefficiency of a LM317 current limiter can probably be overlooked, but if efficiency is an issue, then consider a switch mode current limiter.

Typically the nominal current for one of these Laser Pointers [and I'm assuming "Red" because of the description of it as a cheap, Dollar Store pointer], is 20ma. If this is the case for your pointer, then there is an IC [looks like a TO92 transistor] designed to drive LEDs at precisely 20ma.

The CL520N3-G!

54¢ at Mouser.

There's also one for 25ma: CL525N3-G

It requires a minimum of 4.75 volts and has a "dropout" voltage of 1V [i.e. it will go as low as 1V when trying to keep the current at 20ma]. Thus, the Max Laser Diode Forward Voltage + 1V = Minimum Power Supply Voltage OR 4.75V -- whichever is larger.

This is a "Current Sink" device, so you will need to insert it at the "negative" side of the Pointer circuit. Check the datasheet for an example of how to use it.

Zapro:
I would be very surprised if the pointer runs on 5V - there is not "standard" chemistry that will give you 5V.

Are you sure it's not running at 4.5V from three alkaline buttons cells in series?

// Per.

On fresh alkaline batteries, you get more than 1.5v out open circuit - my guess is he took a brand new one and cracked it open and measured the open circuit voltage.

It's really common to rely on the internal resistance of coin cell batteries for applications like this (those little keychain flashlights usually just have an LED and coin cell inside, with one lead against one side, and the plastic switch pushing the other lead against the other side of battery when turned on). Just add a series resistor. 220 ohm would be a good starting point. Maybe 330 for good measure.

I don't know how these cheap pointers work but if there is not a driver circuit, I'd also guess it relies on internal battery resistance to make a crude constant-current source.

I've got LED flashlights that use battery-resistance for current regulation.

Lasers don't work properly from a constant-current source (like an LED) but it might be "good enough" for a laser pointer as long as the current is low-enough that it doesn't fry the laser.

Normally laser diodes are powered by a controlled-current source in feedback loop. There are 3 pins on a laser diode, and the 3rd connection is for monitoring light-output for the feedback loop to maintain constant/controlled light output.

I'm no authority on Ohm's law, but if it is already the correct voltage, how do I limit the current. I know motors kill themselves when they don't get enough current. Should I be using PWM to limit what it gets?

PWM won't help... PWM switches full-power on & off (faster than your eye can see or faster than a motor can react). This lowers the average power & current, but you can still fry stuff!

You can't use Ohm's Law directly on the laser because diodes (including LEDs & lasers) are non-linear. That means the resistance changes, drastically, as the voltage changes.

With LEDs, we use Ohm's Law to calculate the required resistor from the voltage drop across the resistor and the current through it. With the LED & resistor in series, the voltage divides between the two components (i.e. 3V across the resistor and 2V across the LED), and the same current flows through both. With the correct resistance, the LED voltage magically "falls into place". (It works because of the negative correlation between the resistance and voltage.)

...Since we know the current-through and the voltage-across the LED, we can calculate the resistance of the LED under these specific conditions. Ohm's Law is a law of nature and it's still true but it's not that useful because the resistance will change if the voltage (or temperature) changes.

You can also calculate the effective internal battery resistance by measuring the no-load voltage, the voltage under load, and the current. (The difference between those two voltages is the voltage dropped across the battery-resistance.)

Of course, there is not a resistor inside the battery but there is a characteristic that makes the voltage drop when current increases, similar to a resistor in series with the battery.

Normally laser diodes are powered by a controlled-current source in feedback loop. There are 3 pins on a laser diode, and the 3rd connection is for monitoring light-output for the feedback loop to maintain constant/controlled light output.

DVDdoug is positively correct about that!! Jogging my memory you are! But, these cheap, low-wattage Red laser pointers run fairly well on a constant current, as long as the current is set to a point between the functional extremes.

jeanpaulsc:
they are cheap Dollar Tree pointers,

And there is probably the reason, how long do you run a pointer laser for?
Just long enough to point something out, not continuously.
Can you feel how hot they get?
Tom...

LASERS work perfectly fine from, and actually require, constant current drivers. You don't need feedback from the photodiode the alter current if you have adequate heat sinking and/or run fairly sort duration. If you are running long duration, like a DVD player or LASER projector, then you may need the use the feedback. But you might still get away with adequate cooling. I build multi-Watt LD pointers and only use heatsinking.

If you want maximum life/reliability from a high performance laser diode, you need to pay attention to temperature and current control (typically using feedback from a light-output sensor). The current and
photon densities for lasing are very high and the device is under a high stress compared to most semiconductor devices.

For a laser pointer diode the replacement cost isn't scary so you can afford to be less careful.

No, not really, as stated above. nobody who builds hobby, multi-Watt (optical power), bothers to use feedback current limiting. we just rely on accurate constant current drivers and massive heat sinking, or, active heat remediation. even the hacks at Wicked Lasers didn't bother with feedback limiting.

Replacement cost of multi-Watt LASER Diodes IS SCARY to me, at \$60-100 USD ea.

But, for the cheap little 5mW and below pointers, no big deal. rip one open and the only current limiting you are likely to find is a resistor. applied voltage isn't that big a deal either, as long as you are above the LASing threshold Vf (that is what the LD drops, not an absolute max.).