Hacked pulsed laser driver using Arduino

  • #26
OK thanks for your input.
I replaced the laser diode with a resistor, and the battery with a lab PSU set at 3V.
I am still getting a flatline of voltage across the resistor. The PSU reports no current flowing through it.
Think all my experimentation has killed the transistor?
 

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  • #27
BvU
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  • #28
Paul Colby
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Okay, one needs a battery. ##V_{BE}## should be about +0.7V for a silicon NPN for it to be saturated. ##V_{CE}## in this state would be zero should the battery be absent. Your scope trace shows a negative ##V_{BE}##. Signs of voltages and currents really matter here. An NPN is like a current control where a small current, ##I_B##, injected -into- the base results in an ##I_{E}## ##\approx \beta I_{B}## where ##\beta\approx 100##. I think the transistor (any NPN transistor) spec sheet would have all these things plotted and would be helpful for you.
 
  • #29
OK guys, got it working :smile:
Thanks for everyone's help.
The problem was the transistor - it was faulty.
Replacing it with another transistor got the circuit to work.
The circuit without the battery with a PSU instead is attached below.
 

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  • #30
BvU
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Looks as if it only works because the transistor is abused to short-circuit the "voltage" source (the psu)
 
  • #31
OK, how would you improve it?
 
  • #32
rbelli1
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How to use a transistor to drive an LED

Your circuit only works because the series impedance of the battery is high. If you put that on a strong enough battery you would instantly explode the transistor.

BoB
 
  • #33
Thanks for the article. It is very readable and explains basic concepts which I have forgotten from my mammoth hunting classes.
 
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