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Old 07-15-2010, 07:41 PM   #1
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Default Series or Parallel for two diodes?

Ok, i"m a little embarrassed to ask this question cause I should know the answer. I did this as a subject in school...but that was a long time ago. Here it goes:

I have a Flexmod N2 laser driver and I need to power 2 laser diodes simtaneously @ 1 amp each. The Flexmod is good for 6A and a 5A regulated PSU is connected to it. So, do I connect the diodes in series or parallel? What current on the Flexmod?

Thanks in advanced
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Old 07-15-2010, 08:01 PM   #2
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Default Re: Series or Parallel for two diodes?

Parallell.
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Old 07-15-2010, 08:03 PM   #3
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Default Re: Series or Parallel for two diodes?

Quote:
Originally Posted by Grix View Post
Parallell.
amps on flexmod?
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Old 07-15-2010, 08:10 PM   #4
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Default Re: Series or Parallel for two diodes?

If you're feeding a constant current and the driver has enough voltage overhead to handle the additive voltage of boths diodes, I would put them in series, so they're both getting the same current and taking whatever voltage they need.

The danger in driving a constant current in parallel is that the 2 diodes will likely have different internal resistances and different voltage characteristics at the same current, and the internal resistance can change. So they'll be taking different proportions of the current, because the voltage will be equal and the diodes will be dividing the current. I've never tried it, and I'd have to think about it a bit more to come to a firm conclusion, but it seems as though in parallel, since the diodes will be taking different proportions of the total current relative to their individual I-V characteristics and internal resistances, you could end up in a thermal runaway situation where one diode continues to draw more and more of the current until it kills itself.

Not sure, haven't fully thought it out, but series seems like a safer bet for a constant-current driver that has enough voltage overhead.
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Old 07-15-2010, 08:14 PM   #5
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Default Re: Series or Parallel for two diodes?

hey! stop! never place two or more diodes in parallel!

pullbangdead is right... it might be possible that both diodes are slightly different and one of the diodes get the full current -----> *RIP*

diodes are not linear resistors....
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Old 07-15-2010, 08:30 PM   #6
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Default Re: Series or Parallel for two diodes?

I think(from what I've read on PL), the standard is series with a small balance resistor between them.
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Old 07-16-2010, 10:19 AM   #7
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Exclamation Re: Series or Parallel for two diodes?

Basical electric / electronic laws .....

Two diodes in parallel takes the same current ONLY IF THEY ARE ABSOLUTELY IDENTICALS, thing that is virtually impossible .....

For have the same current in both the diodes, also if they are not identical, you need to connect them in SERIE ..... no need of balance resistor .....

If you need to connect them in parallel, then you MUST use a decoupling resistor for each diode, for set their currents .....
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Old 07-16-2010, 10:25 AM   #8
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Default Re: Series or Parallel for two diodes?

If your driver has the voltage overhead then you can drive them in series and I would put a 100-150 ohm resistor across each of them to ballance the voltage a bit.

And yes if you want to run them in parallel you must put a resistor in series with each diode.
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