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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I'm just testing BPV10NF photodiode (lighting it with his complementary 940nm led)
The led is powered by a 1.5KHz square wave and driven with 50mA of peak current, the led rise time is 800ns (1.6us measured). The distance is approximately 4 cm.
The phototransistor has 4pF (11pF at 0 V) capacitance and the load resistor is 47Kohm (ensuring more than 4,5V voltage swing at VDD = 5V) considering 4pF stray wire capacitance we get a time constant of 376ns. The rise time (10%-90%) should be 2.2 time constants = 826ns, taking into account the 800ns of diode fall time we should get 1.6us of photodiode rise time. Since i need 200ns resolution for a speed trap i don't think that the capacitance of the photodiodes and resistance of the load resistor will vary more than 20%, so the error should be less than 200ns.
BUT
measuring the rise time with a DSO, gave me an unpleasant surprise:
the fall time of the diode is 1.6us and the rise time of the photodiode is 18.7 us !!!! how can be an order of magnitude different from predicted? Stray capacitances cannot be 40pF!!
this would give too much error even with 5% component tolerances (1us!!)
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
It is a 10:1 Probe. The capacitance is 13pF, maybe is this the cause.. counting 2.2 time constants we get 1us. With this probe the rise time is 7us. Total capacitance must be: 7us must correspond to 2,2 time costants (rise time 10%-90%), so the time constant is 3,2 us. Since the load resistance is 47Kohm the total capacitance must be 67pF.
So:
15pF is the probe 4 pF is the phototransistor 3 pF are the wires (worst case estimation)
so the stray capacitance : 45pF, a lot!
but it is realistic, since reducing the load resistor doesn't help a lot (it reduces only the effect of the phototransistor).
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I reduced the load resistor to 22KOhm and added a LM393 with some hysteresys(10%) with the threshould slightly above VCC/2 , transforming a "slow" changing waveform in a sharp (with some delay though) transition from 0 to VCC. The delay between the LED turn off (1us fall time) and the action is about 750ns +- 100ns (faster when the beam is interrupted partially); so in an hipotetic speed trap the error would be only 200ns, only one count for my system, giving and error of 0,2% at 1Km/s with 10cm separation, quite reasonable. To investigate the effect of stray light i tested the system with a 40W fluorescent almost striking inside the sensor giving an envelope wide approx 250ns, making the error still reasonable.
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Incoerent photodiode behaviour
