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Topic: How to TTL a laser? (Read 41535 times) previous topic - next topic


Oct 26, 2015, 10:38 pm Last Edit: Oct 27, 2015, 12:53 am by Shanjaq
Vo = 1 if V+ > V -
Vo = 0 if V + < V-

For H = 5V,
V+ = 3.4V (due to voltage divider)
If current through R = 0.140 A, Vbe = 0.7V, and V+ = 0
thus the comparator output is driving the led driver transistor between 0.5  and 0.7 V.
If the the feedback transistor turns on completely, V+ will be pull below V- , shutting down
down the output of the comparator.

At least that's how I see it. If I am wrong, please correct me.
That sounds about right.  So since a state in which the 5-ohm sense-resistor is passing 140mA would pull it higher than the Vbe reference voltage, this should cause the control transistor to turn on, which pulls V+ down, thus causing the op-amp to turn down/off the LED driver transistor.  Once the current across the sense-resistor drops to ~114mA, the control transistor should begin to turn off, which would cause the op-amp to begin turning the LED driver transistor on, thus it's a feedback-stabilized control loop.

I = Vbe / R

Since the op-amp is intended to be hooked up to the 5v logic supply, its output could potentially swing from 0 to 5v depending on the base currents required to pass the desired collector current through the LED driver transistor.  The LED power supply can be any voltage that won't dissipate too much power through the driver transistor, something like 9-24V seems reasonable.

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