Electronics > Beginners
Interesting find with optocoupler
unitedatoms:
I found new use of leak collector current for NPN optocoupler, when I need for example control high side resistor of 555 timer by low side voltage.
If you connect base of NTE3040 to emitter, the optocoupler turns into current limiter with range of 2..60uA, when LED current is in range of 0.2...20mA.
This can be used in Arduino to control SCR or something. Like in this circuit.
https://www.edn.com/design/analog/4415447/555-timer-triggers-phase-control-circuit
Replace R3 with NTE3040 output with base shorted to emitter and control timer using LED current.
Here is experimental data for current transfer ratio. (attached picture)
Edit: The collector load was 15k resistor with 5v DC
StillTrying:
"I found new use of leak collector current for NPN optocoupler,"
base-collector sounds like the photo diode current, it should still be produced with no bias voltage, just a DVM directly connected across the base-collector. I've tried using just the photo diode part of an opto to get it to switch much faster, but it didn't switch any faster. :(
unitedatoms:
Right. Forgot about photodiodes. What if reverse emitter current is faster ? I also think if emitter reverse current is applicable for low voltage situation. May be it will be more sensitive to photons, if just by order of layers emitter is thinner and closer to surface or whatever other solid state physics reasons. Usually the emitter leak current is not specced in datasheets.
T3sl4co1l:
In a typical 4N35, the C-B diode is more sensitive IIRC. CTR in the 1% range. Actually, I measured it recently but didn't write it down... ::)
What's this about speed?
Photodiodes are notoriously slow at zero bias anyway, but you can reverse bias them for an improvement (about 2-3x), and you with phototransistors, you can also play tricks like this:
Gets about 300kHz bandwidth, with a mostly 1st-order response implying still more bandwidth can be had, just keep increasing loop gain.
Tim
David Hess:
The same trick can be used to generate a bias current allowing single supply outputs to sink current below ground or source current above the positive supply.
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