# How do you calculate the power requirement for a circuit? (laser diodes)

I have 52 laser diodes, rated 5VDC, 5mW, wired in parallel and powered by a battery, rated 5V, 2A (I measure 2.7). I'm thinking some diodes burn out because as the voltage drops, the current becomes too great. The diodes manufacturer states the maximum operating current of each diode is 20mA.

I have two questions: 1) Is there a formula for calculating a safe power supply, and 2) would adding an MOV or thermal resistor help?

Mentor
I have 52 laser diodes, rated 5VDC, 5mW, wired in parallel and powered by a battery, rated 5V, 2A (I measure 2.7). I'm thinking some diodes burn out because as the voltage drops, the current becomes too great. The diodes manufacturer states the maximum operating current of each diode is 20mA.

I have two questions: 1) Is there a formula for calculating a safe power supply, and 2) would adding an MOV or thermal resistor help?
Welcome to the PF.

52 * 20mA = 1.04A. Why are you measuring 2.7A? Can you Upload a copy of the schematic or pictures of the setup?

What do you mean by "burn out"? If you provide 5Vdc to the laser diodes, they should just work. I'm assuming the laser diodes have internal series resistors that limit the current to 20mA, right?

What is the application?

Gold Member
I have 52 laser diodes, rated 5VDC, 5mW, wired in parallel and powered by a battery, rated 5V, 2A (I measure 2.7). I'm thinking some diodes burn out because as the voltage drops, the current becomes too great. The diodes manufacturer states the maximum operating current of each diode is 20mA.

I have two questions: 1) Is there a formula for calculating a safe power supply, and 2) would adding an MOV or thermal resistor help?
Do you have resistors in series with each diode?
Do you at least have a resistor between the voltage supply and any of the diodes (please tell me you are not just hooking up the power supply directly to the diodes).

berkeman
Electrical
Include the resistors with diodes since the current is excessive. As others said, post a schematic as well.

Gold Member
Do you have resistors in series with each diode?
Do you at least have a resistor between the voltage supply and any of the diodes (please tell me you are not just hooking up the power supply directly to the diodes).

Include the resistors with diodes since the current is excessive. As others said, post a schematic as well.

Laser diodes require much more than "just a resistor" that normal LED's do
Laser diodes require a specific driver circuit for current control

here's one basic example

there's many on the www

Dave

donpacino and berkeman
I think I get a reading of 2.7A because of the voltage drop. I assume the diodes burn out (COD) as they stop working. Here is the history:

I wired 52 diodes. 2-3 soon stopped working. I replaced the ones that stopped working then three more died (two were different). I stopped replacing dead diodes and sought help. I powered up the circuit for several hours many times for two weeks now, without any more diodes burning out. I fear replacing the present three dead diodes will cause more diodes to fail. The latter would suggest that inadequate power supply. But maybe I just had some bad diodes. I am a doctor, with only basic knowledge of electronics. I'm not sure if an MOV would help, connected between the positive and negative wires, between the Li-ion battery and the circuit.

Inspecting one of the diodes, I see no resistor therein as I recognize one to be (color banded). No data sheet is available. Attached is my circuit. The diodes fit into a hat, used for stimulating mitochondria in hair follicle roots to promote growth (<< URL Removed by Mentor >>).

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Mentor
As already mentioned, you cannot wire up laser diodes to a DC power supply without some form of current limiting and control. Where did you get them from?

davenn
Gold Member
I wired 52 diodes. 2-3 soon stopped working. I replaced the ones that stopped working then three more died (two were different). I stopped replacing dead diodes and sought help.

well I'm not surprised they burnt out, you had no current limiting drivers for them

EACH laser diode needs it's own driver

Dave

berkeman
Mentor
The diodes fit into a hat, used for stimulating mitochondria in hair follicle roots to promote growth (<< URL Removed by Mentor >>).
I've removed the URL of that "website under construction". Are you associated with that company?

Mentor
Are you associated with that company?
Which is kind of scary in the context of your questions in this thread, BTW...

Nothing scary about working with such low voltage.

Yes, that is my website under construction. I added the link in response to the question about what I am building. I am not selling anything...just seeking a little knowledge.

I am a doctor; and after reading the kind responses here to my question as well as the interesting references, I realize that indeed I am over my head. I think I may need to find an engineer to replace the one that I had to let go after learning he was defrauding me. My hope was to construct the device myself.

The prototype I built--with three failed diodes--has been working perfectly for over two weeks. The warmth of the diodes is not perceptible, even by a glabrous scalp. So perhaps the project can be kept simple.

Someone said I need at least one resistor between the power supply and the first diode. I figure 5kOhms, but surely my calculation is incorrect (R = V[SU]2[/SUP]/P = 52/0.005A = 5,000 Ohms). I guess I'm oversimplifying the matter, thinking someone can point me to a formula I can use.

So hooking up an MOV as in the attached pic would not solve my problem?

Oops! I forgot to mention that I got the diodes from Amazon. And Dave, not doubting your expertise, but if each diode needs a current limiting driver, why are the rest not burning out? They are not even warm after powered for two hours. Just asking. Thanks for your patience.

Gold Member
but if each diode needs a current limiting driver, why are the rest not burning out

If there is a sufficient load to bring the operating voltage withing acceptable limits than you could just be lucky.Your connections (plus the internal construction) are just enough resistance that the diodes don't blow immediately. You are certainly over-driving them.

BoB

davenn
Mentor
@davenn I wonder: does OP really need individual driver for each diode? Can't he connect them in series with a single driver powering several diodes at once? This way the current through each one is effectively under control. Yes, that means higher voltage is required, so may be unacceptable for this particular application.

In my RC model I run up to three 350 mA LEDs from a 3s LiPo (12.6 V, never discharged much below 11 V) through a single PT4115 based driver. I wouldn't be able to power fourth LED as the voltage is too low for that. But these are just CREE LEDs, perhaps laser diodes are somehow different.

jim hardy
Mentor
@davenn I wonder: does OP really need individual driver for each diode? Can't he connect them in series with a single driver powering several diodes at once? This way the current through each one is effectively under control.
If LEDs are well matched (same manufacturer, same lot, designed to be series-connected), then you can generally get away with connecting a string of them in series with one current control circuit. With non-matched LEDs, the brighness varies too much with small changes in current, so series connecting them is not usually done.

There may be additional current control issues with laser diodes. i seem to remember that being brought up in the other thread that I linked to.

EDIT / ADD -- Then again, it is a very unusual configuration to connect many laser diodes up in DC operation all at the same time, so maybe since the power levels are low, one resistor per diode may work okay. @davenn should be able to help more on this.

Gold Member
Dearly Missed
Someone said I need at least one resistor between the power supply and the first diode. I figure 5kOhms, but surely my calculation is incorrect (R = V[SU]2[/SUP]/P = 52/0.005A = 5,000 Ohms). I guess I'm oversimplifying the matter, thinking someone can point me to a formula I can use.

I always assumed the published milliwatt number of a laser diode is its power output not input.

An active current regulator would be your best bet, one for each laser diode.
three terminal regulators cost around fifty cents and work nicely

Vin will have to be about three volts more than you need for the laser diode.
The regulator keeps a constant 1.25 volts (nominal - see datasheet) between pins Adj and Vout , so it'll pass current equal to 1.25/R1.
So 100 ohms would give 12.5 ma.
Let's say you had 9v in and the laser diode took 3 volts at that current
Volts at Vout pin would be 3 + 1.25 = 4.25
power lost in the regulator would be ΔV X I = (9-4.25) X .0125 = .059 watts . so the little regulator won't be working overly hard.
Datasheet says its thermal resistance to ambient is 160 deg/watt so it'll run only about .059 X 160 = 9.5 deg C above room temperature.

That's how you can design a pretty nice current regulator that's easy for amateur fabrication and prototyping, and would hide inside a hat...

There exist drivers made specifically for series strings of LED's but they tend to be packaged and a bit large to fit inside a hat.

old jim

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davenn
Gold Member
Dearly Missed
If there is a sufficient load to bring the operating voltage withing acceptable limits than you could just be lucky.Your connections (plus the internal construction) are just enough resistance that the diodes don't blow immediately. You are certainly over-driving them.

Do you suppose the first one to blow failed shorted and protected the rest ?
Maybe sequential failures as each failed one's internal wires melt , changing it to failed open making the next weakest link in the chain absorb the overvoltage...

This isn't a laser diode but it conveys the idea... an accidental mis-connection caused this .

Collector-base junction fails shorted from applying overvoltage with a couple amps of capacity,
when base wire melts it forward biases BE junction and high current melts its wire too.

old jim

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Gold Member
well I'm not surprised they burnt out, you had no current limiting drivers for them

EACH laser diode needs it's own driver

Dave
Thanks dave!

I am assuming that is because the laser intensity output is highly sensitive to differences in current?

Gold Member
Nothing scary about working with such low voltage.

Yes, that is my website under construction. I added the link in response to the question about what I am building. I am not selling anything...just seeking a little knowledge.

I am a doctor; and after reading the kind responses here to my question as well as the interesting references, I realize that indeed I am over my head. I think I may need to find an engineer to replace the one that I had to let go after learning he was defrauding me. My hope was to construct the device myself.

The prototype I built--with three failed diodes--has been working perfectly for over two weeks. The warmth of the diodes is not perceptible, even by a glabrous scalp. So perhaps the project can be kept simple.

Someone said I need at least one resistor between the power supply and the first diode. I figure 5kOhms, but surely my calculation is incorrect (R = V[SU]2[/SUP]/P = 52/0.005A = 5,000 Ohms). I guess I'm oversimplifying the matter, thinking someone can point me to a formula I can use.

So hooking up an MOV as in the attached pic would not solve my problem?
The purpose of a single resistor is to set the current.

By supplying just a voltage source, the current through the diode will vary quite a bit from diode to diode.

You want to use a current source to directly control the current. A "cheap" current source is a resistor in series with the diode. You know approximately what the voltage drop across the diode will be, so if your voltage supply is 2.9v V, and the voltage drop of the diode is typically 2.8 volts nominal. The voltage drop across the resistor is 0.1 V. So the current through the diode will be 0.1V / the resistor value. Note, as current changes, the voltage drop across the diode will change, so it can be tricky to choose the resistor value, which is one of the reasons why this is a "cheap" method.

If you use multiple diodes hooked up to this cheap current source, then the current through each one will vary, therefore the brightness across all the diodes will vary.

I would recommend at a minimum using the precision current source Jim posted. You follow that equation to set the current based on your desired intensity.
Like davenn said, it is recommended that you use an individual driver for each one, but I am curious as to why.

jim hardy
Gold Member
Inspecting one of the diodes, I see no resistor therein as I recognize one to be (color banded). No data sheet is available. Attached is my circuit. The diodes fit into a hat, used for stimulating mitochondria in hair follicle roots to promote growth (<< URL Removed by Mentor >>).

How much does laser intensity matter? can it vary or does it have to be precise for each laser?

Mentor
Yes, that is my website under construction. I added the link in response to the question about what I am building. I am not selling anything...just seeking a little knowledge.

I am a doctor; and after reading the kind responses here to my question as well as the interesting references, I realize that indeed I am over my head. I think I may need to find an engineer to replace the one that I had to let go after learning he was defrauding me. My hope was to construct the device myself.
What is the European equivalent of the United States FDA (Food and Drug Administration)? Do you have approval for this therapy or this device? What stage in the approval process are you currently? I'm not sure we can be helping you with this if you do not already have approval for the device or for the trials on human subjects...

Mentor

@Djard -- Please start a Private Conversation with me to discuss some of the legal details of your project (just click my username to start a conversation). We need to figure out a couple of things before this thread can continue. Thank you.

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Mentor