Discrete LM3909: Blue LED Waveforms

The circuitry and instrumentation is essentially the same discrete LM3909 as before:

LM3909 - blue - test setup
LM3909 – blue – test setup

With a few minor tweaks:

  • Blue LED, forward voltage 2.56 to 2.97 V
  • 24 Ω R1
  • One Q2 current mirror transistor driving Q3

With a pair of fresh AA alkaline cells producing 3.1 V (not the NiMH Duracells you see in the picture), the blue LED blinks brightly.

The 610 mV peak voltage across R1 shows the LED starts at 25.4 mA:

LM3909 blue - 3.1 V - R1 24 ohm
LM3909 blue – 3.1 V – R1 24 ohm

The capacitor reaches 1 V, then goes about 150 mV into reverse charge during the flash (note the different horizontal scales):

LM3909 blue - 3.1 V - C1 V
LM3909 blue – 3.1 V – C1 V

The Darlington version of Q1 seems to do a decent job of keeping the cap out of reverse charge. A Shottky diode would add a few hundred mV, but I doubt there’s anything nasty going on inside the cap as it stands.

The blue LED has a forward drop of 2.97 V at 20 mA, so I’m surprised the voltage across it hits 3.1 V at 25 mA:

LM3909 blue - 3.1 V - LED V
LM3909 blue – 3.1 V – LED V

Very little of the voltage appears across Q3, the driver transistor:

LM3909 blue - 3.1 V - Q3 coll
LM3909 blue – 3.1 V – Q3 coll

With a pair of nearly dead alkaline cells for a 2.0 V supply, the LED current peak drops to 4.6 mA:

LM3909 blue - 2.0 V - R1 24 ohm
LM3909 blue – 2.0 V – R1 24 ohm

The LED lights brightly, then fades away exactly like you’d expect from that waveform.

The cap still charges to about 1 V and stays well above 0 V during the (much longer) flash:

LM3909 blue - 2.0 V - C1 voltage
LM3909 blue – 2.0 V – C1 voltage

The voltage across the LED now reaches only 2.7 V, which is substantially higher than the 2.0 V battery supply and exactly why the LM3909 existed:

LM3909 blue - 2.0 V - LED voltage
LM3909 blue – 2.0 V – LED voltage

Q3 continues to saturate, although you can see the effect of the decreased base drive during the flash:

LM3909 blue - 2.0 V - Q3 coll
LM3909 blue – 2.0 V – Q3 coll

The blue LED won’t light at 1.3 V, but still gives out a weak flash at 1.7 V, so I’d say the tweaked LM3909 circuitry works reasonably well.

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