Not a good idea to put diodes in parallel.
It's not possible to make identical diodes, so one of them will take more current than the other.
Then you could start to get into a thermal runaway situation.
As it gets warmer, its "resistance" decreases, so it takes more current, so its "resistance" decreases, so... until pop!
If you must put them in parallel, each diode needs its own small series resistor to swamp this effect,
which rather negates putting them in parallel to start with.
Edit - added this: with 2 diodes in series, 100 ohms, 9v supply:
Current: 66 mA, Power: 436 mW in resistor, 79 mW in each diode, being 158 mW total
With 1 diode, 100 ohms, 9v supply:
Current: 78 mA, Power: 608 mW in resistor, 93.6 mW in diode, being 93.6 mW total
And before you ask
- with 3 diodes, 100 ohms, 9v supply:
Current: 54 mA, Power: 292 mW in resistor, 64.8 mW in each diode, being 194.4 mW total
with 4 diodes, 100 ohms, 9v supply:
Current: 42 mA, Power: 176 mW in resistor, 50.4 mW in each diode, being 201.6 mW total
So I would expect to get more light out from 2 diodes.
Assuming it's proportional to the power:
2 diodes 158.4/93.6 = 1.7 brighter,
3 diodes 194.4/93.6 = 2.1 x brighter
4 diodes 201.6/93.6 = 2.2 x brighter
The datasheet doesn't give any curves for performance.
Cheers,
Peter
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