Ho do you calculate the power requirement for a circuit?

Thread Starter

Djard

Joined Oct 20, 2017
5
I have 52 laser diodes, rated 5VDC, 5mW, wired in parallel and powered by a battery, rated 5V, 2A (I measure 2.7A). 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. A Attached is my beginner-to-electronics schematic.

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?
 

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philba

Joined Aug 17, 2017
960
how are you limiting the current to each diode? Your schematic says you aren't.

Typically a resistor is used to do that. Then you can parallel the resistor/diode pairs with no problems. You need to know the voltage drop of the diode (Vf from the data sheet) and then use ohms law to determine the size of resistor to limit the remaining voltage to 20 mA.

And if you hooked up 52 in parallel to your battery like you wanted to, they would all burn very bright for a short time and then probably burn out because they will each pull around 50 mA. Though I'm curious where you got a 5V battery.
 
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dl324

Joined Mar 30, 2015
13,144
Welcome to AAC!
I have 52 laser diodes, rated 5VDC, 5mW, wired in parallel and powered by a battery, rated 5V, 2A (I measure 2.7A).
Battery capacity is generally given as amp hours (AH) and a rating of 2AH is for a 10 or 20 hour discharge, at 10% or 5% of the rated AH. If it was 10%, a 2AH battery would give 0.2A for 10 hours before being considered discharged.

In general batteries can be discharged at high currents. So high in fact that they can explode.
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. A Attached is my beginner-to-electronics schematic.
It is unclear whether the diodes include any current limiting. If they don't, connecting diodes in parallel can result in some burning out, but not because the voltage is dropping.

What happens is that without current limiting one diode will hog current. It may do that to the point where it destroys itself. When that happens, another diode will hog current and may also destroy itself.
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?
No and no.

A MOV stand off voltage has to be higher than the working voltage and they aren't fast acting or precise. Thermal resistors (PTC fuses) will only help in your case if each diode has it's own.
 

BR-549

Joined Sep 22, 2013
4,938
Have you ever built electronic circuits before? What is your electronics level........so that we may answer you.
 

Thread Starter

Djard

Joined Oct 20, 2017
5
I bought both the diodes and lithium-ion batteries from vendors on Amazon. The little "TalentCell" 5V, 2A bettery is impressive, able to power a TV for over an hour.

My knowledge of electronics is beginner level, though I have done a lot of wiring and maintenance on musical equipment when I was an musician, also converted all the lighting in my home to LED. I'm now a doctor who is trying to build a medical device, after letting go my engineer for defrauding me. I have not found another to replace him.

My prototype device--with three fried diodes--has been working perfectly for over two weeks now. After hours of being powered, the diodes do not get perceptibly warm, even by a glabrous scalp.

Would a regular 5kOhm resistor between the power supply and first diode help? I calculate R = V square/P = 5x5/0.005 = 5,000 Ohms. That seems awfully high. Thanks to all for your patience.
 

Alec_t

Joined Sep 17, 2013
12,215
I'm now a doctor who is trying to build a medical device
As a doctor you are no doubt aware that even one 5mW laser can cause blindness and other body damage. A device with 52 of them sounds extremely dangerous :eek:.
Presumably the 5mW rating is the light output, not the laser power consumption? (5mW consumption would imply a current of only 1mA @ 5V).
 
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philba

Joined Aug 17, 2017
960
Ah, that talent cell is a battery backed charger/power supply. The actual battery is probably a 3.7V LI Ion with a boost converter and charger.

The diodes probably have built in resistors if they've run that long. Your diagram didn't show the resistor so it was an easy assumption to make. Paralleling is probably ok.

For what it's worth, the standard way to power large numbers of LEDs is to put them in series and uses a constant current boost power supply. This keeps the current a lot lower. Your diodes with their built in resistors won't work that way. Your 52 diode set up will need 1Amp when wired in parallel.

Take a look at this page - it gives some background on laser diodes and how to drive them.

Finally, as an MD, I'm sure you are aware of this but the "battery" is not approved for medical use. At least it didn't look like it from the ebay pages I saw.
 
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