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FrozenGate by Avery

Lasers in series?

Apocalypse

Apocalypse

0
Joined
Sep 11, 2011
Messages
249
Points
18
Hello all,

Is it safe to power 2 of the same lasers, say, 2 LPC 826's or the new 3.8mm diodes in series with an adjustable power supply? The current through both diodes should be the same and the voltage would just be doubled? Correct if I'm wrong please.
 





Correct, though make sure your power supply has current limiting and doesn't give large voltage spikes on turn on/off or adjusting.
 
Apocalypse said:
Hello all,

Is it safe to power 2 of the same lasers, say, 2 LPC 826's or the new 3.8mm diodes in series with an adjustable power supply? The current through both diodes should be the same and the voltage would just be doubled? Correct if I'm wrong please.
Click to expand...

The problem when you connect 2 diodes in series is that the 2 leakage reverse currents could be not the same and the blocking voltages of the 2 diodes could be very different. You can see this problem explained here.
If your driver or power supply has negative spikes you can destroy the diodes because these spikes could exceed the maximum reverse voltage allowed.
One solution is to use balancing resistors but, to be effective, you must choose their values so that the current across the resistor is at least 10 times the leakage reverse current of the diodes and you have some power loss.
Another solution is to place a fast schottky diode in antiparallel with each one laser diode. This limits the reverse voltage at a safer value of 0.4V.
 
laserluke said:
... If your driver or power supply has negative spikes you can destroy the diodes because these spikes could exceed the maximum reverse voltage allowed.
Click to expand...

tumblr_m2usf7QIGj1r22up4.png


This holds true for single diode setups, too. Don't use a pissy driver and there's nothing to worry about, sir.
 
Um, anyone know if the M140 diodes are more resistant to backvoltage than red diodes or the other way around? I've been using this powersupply on them and it works fine.
 
Cyparagon said:
This holds true for single diode setups, too. Don't use a pissy driver and there's nothing to worry about, sir.
Click to expand...

You should think that when you are using two diodes in series the driver's output voltage is double to maintain the same current and so the spikes are larger. Without a proper balancing, one diode could see a larger spike than the other, sir.
Moreover, when you deal with diodes, a little voltage noise becomes a large current noise because of the exponential I-V diode curve (when forward biased a small change in the voltage produces a large change in the current).
 
Last edited:
You seem to be forgetting... we use drivers that aren't a heap of imitation penile warts. There are no spikes to worry about. They're called constant current drivers because they keep the current constant, and they sure as hell don't reverse-bias the diodes.
 
Cyparagon said:
This holds true for single diode setups, too. Don't use a pissy driver and there's nothing to worry about, sir.
Click to expand...

Cyparagon said:
You seem to be forgetting... we use drivers that aren't a heap of imitation penile warts. There are no spikes to worry about. They're called constant current drivers because they keep the current constant, and they sure as hell don't reverse-bias the diodes.
Click to expand...

You don't deserve an answer but a good mouthwash.
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To answer the question, yes the current flow has to be the same through the diodes if they are in series.

The PSU has to provide enough voltage to overcome the combined forward voltage. The voltage drop across each will vary from diode to diode though.

Without getting involved in the above argument, yes if you have a lab PSU which spikes on startup it can be bad, but most of the good diode drivers don't have this issue.
 
Speaking of diodes in series, just finished these :)

DSCF4811.jpg


Be careful when dealing with red lasers though, they are case negative!
 
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