Interesting find with optocoupler - Page 1
Forum section: electronics / beginners

author: unitedatoms
author_rank: Frequent Contributor
timestamp: November 24, 2019, 01:14:03 am
content: 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-circuitReplace 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

author: StillTrying
author_rank: Super Contributor
timestamp: November 24, 2019, 02:27:57 am
content: "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 usingjustthe photo diode part of an opto to get it to switch much faster, but it didn't switch any faster.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: November 24, 2019, 02:43:52 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2019, 06:15:04 am
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2019, 09:16:52 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2019, 10:30:13 pm
content: A related riddle: how do you get negative voltage out of a 2N3055?Not special to 2N3055 of course, it's merely one of the best candidates for abuse.The trick is, E-B breakdown creates light, a very small amount of it. You can see this if you cut one open. It's hardly visible under room lighting, but can be seen clearly in a dark room.Or actually, the avalanche itself probably generates enough free carriers, not needing light to transmit a few microns further. I wonder how you could tell the difference...In any case, the C-B junction gathers up some of these freed charge carriers, developing a small negative bias, just as a solar cell does.Tim

author: StillTrying
author_rank: Super Contributor
timestamp: November 26, 2019, 12:01:17 pm
content: Quote from: T3sl4co1l on November 24, 2019, 06:15:04 amAfter lots of experiments I kept finding the ~300kHz analogue BW limit was in the emitters, with the only way around it to increase the emitter current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2019, 12:46:02 pm
content: Ah, reproduction?How about emitter capacitance, does a smaller series resistor help? Or parallel resistance (dropping the Thevenin equivalent)?Hm, forget what I was testing with, if there was much resistance after the signal generator or if it was 50 ohms straight in.IR LED should be pretty fast, up to 10MHz or so. I'm not aware of limitations this low...Tim

author: StillTrying
author_rank: Super Contributor
timestamp: November 26, 2019, 11:52:34 pm
content: Quote from: T3sl4co1l on November 26, 2019, 12:46:02 pmIf you're digitally switching around 20mA, the physical 10-30pF LED capacitance doesn't seem have much effect in practice or theory. With 4 74AC14 gates in parallel I must have had the total resistance down to 10R or less, but 10R or 56R seemed to make no difference to the light switching time at low currents ~8mA. In an early experiment with an old red LED I had up to 8V across the LED for 10s of ns to get it switch on faster, but it didn't! the light and current still just took ~150ns to rise.There's a fewanalogueLED experiments here, I didn't get around to using a much faster and higher current op amp.www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1858251/#msg1858251I'm not sure I believe this 30-70MHzanalogueone.https://www.eevblog.com/forum/testgear/cheap-diy-fiber-optic-isolated-voltage-probe-with-bandwidth-up-to-70-mhz/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2019, 01:04:16 am
content: Should be able to get confident step response with a small photodiode, with reverse bias; the kind of thing that's used for 100s MHz (or even GHz) in fiber. Not sure where to find such a thing (other than ordering one directly, possibly $$?).Could compare step response with, like... atmospheric spark discharge, maybe? Xenon photoflash is too slow, some microseconds, but air sparks can be nanoseconds. Requires HV supply and a small amount of capacitance, of course. Keep good distance, partly just to attenuate the intensity of the light, but also to keep EMI away so you aren't measuring the spark itself, but its photo response.I remember doing some experiments with this and getting good results with a generic IR photodiode, but I didn't have an IR LED at the time; I did demonstrate a xenon light curve (but again, at some us, that's not very impressive), and I think I did an air discharge but I don't seem to have a shot recording what it was.Tim

author: StillTrying
author_rank: Super Contributor
timestamp: November 28, 2019, 09:27:05 am
content: Quote from: T3sl4co1l on November 27, 2019, 01:04:16 amI've looked at lots of xenon flashes over the years, and even the total light shape of multiple slave flashes. I don't think even air sparks can get close to the clean ~15ns rise of a small 5mm LED.200mA through Super Bright green and blue LEDs were the fastest rise and fall times and lowest propagation times I got here. The light->current shapeshouldstill be quite linear at ~X9 the LED nominal current.https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg2411274/#msg2411274Despite all the theories of the light spreading into the wrong areas ofthe photo diodeand causing the curves and long tails, I still think most of that is in the emitter,because I could improve the curves and long tails by increasing the LED current, and increasing the LED->PD distance so that the light level at the PD was still the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 03:37:22 am
content: Same thing affects PDs, apparently some have so much internal resistance that it trashes the bandwidth and noise floor, much as Rbb' in BJTs does the same. I'm not entirely sure if that's a sheer bandwidth thing (if it can be overcome with current, as in your example), or if it's a noise optimization problem (which was where I heard this from, Phil Hobbs).Tim