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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
While measuring the response time and waveforms with my scope i stood up on a strange phenomenon.
Experimental Setup: IR led (QEC113) drived by 1.5KHz square wave (58% duty) via 50 ohm resistor (50mA continuous current) Matched phototransistor (QSC113) with collector tied to VCC (5V) with 2,2Kohm load resistor Sensor distance 5 cm
The behaviour is much fairly tipical , with 10-15us rise time, and 90us fall time
BUT
BEfore the fall of the signal there is a positive peak of 50us duration (it happens only when the photodiode is driven by 50ohm resistor, at 100 ohm it disappears)
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
could be some sort of oscillation caused by the fast switching?
this affects power MOSFETs badly too. i have somewhat fixed this by adding a parallel 1K resistor (doesen't affect the performance but improves switching speed) on the driver LED, and this helped.
regards, -A
"Bother" said Pooh, as he measured 20 ohms G-C-E on his IGBT...
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I scouted for reflections and parasitics but after a bit of searching i found that it isn't caused by excessive optical density. The base of the phototransistor is floating (lack of base connection) .The problem seems the led (it is overdriven at 100mA instead of 50mA). Maybe a some kind of fluorescence...? I will try the 1K resistor thing. Driving the led with 25mA (50mA) peak, this problem seems to disappear but at the cost of higher phototransitor rise/fall times...
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Do the LED and detector circuits share a power supply? That could cause interaction -- for example, when the LED is turned off, the power supply voltage could go up until regulator has time to react.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
good point, i didn't though about that. Maybe the feedback circuit (LM317) isn't able to compensate the power supply change. To verify this i will scope the VCC :D. Thank you
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Strange phototransistor behaviour
