LED Switching

col_implant said:

Yea please do... cheers

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I'm having second thoughts. I ran some simulations on a low-current LED and a high-current one, and got wildly different results.
The key to fast turn-off is shunt switching, which will rapidly discharge the LED capacitance.
High-current LEDs have high capacitance, so turning them on rapidly may require some peaking in the drive waveform. Getting symmetrical turn-on and turn-off may be difficult for the high-current devices. I would need to see spice models of the LEDs you intend to try, and even then, the results of the simulation are only as accurate as the model.

hi Ron,
This is the circuit I have used, 30Watt LD1, Range 5000mtr, off three prisms.

ericgibbs said:

hi Ron,
This is the circuit I have used, 30Watt LD1, Range 5000mtr, off three prisms.

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How do you control the laser current? I have never played with solid-state lasers, but I thought that the current had to be carefully controlled.

Roff said:

How do you control the laser current? I have never played with solid-state lasers, but I thought that the current had to be carefully controlled.

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hi,
As you may know the transistor can be driven into a avalanche conduction state by applying a Vce greater than the transistor rating and driving the transistor base with a heavy current. This discharges the 10nF, which is charged to 150V via the 10K.

The collector effectively grounds the +Vside of the cap putting -150V across the LD1. The value of +V and the Cap are chosen so as to keep within the operating envelope of the laser diode.
The clamp diode is to protect the LD1 from reverse voltages.

We pulsed at 250 or 400 pps