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Any jellybean low current (<10mA) opto-mosfet or opto-ssr for signal routing ?
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T3sl4co1l:

--- Quote from: BrianHG on February 26, 2024, 12:12:48 pm ---There is always FET output optocouplers.  Since only 10ma is needed, plus you also get a linear transfer of output resistance tracking LED current.

--- End quote ---

Not really; yes, but over an extremely narrow range of voltage, and that voltage varies level.  They were never substitutes for Vactrols, and I imagine many have been burned trying to do so.

FETs don't make great variable resistors, at the best of times; variable over a mild range, 2:1 for example, at up to modest signal levels (100s mV, maybe a V or two?).  An OTA does much more, if at low input signal levels (10s mV) -- of course, we don't know if an OTA is at all suitable here (but, diode-gate and BJT mixer style circuits might well be applicable, or with some changes, who knows).

It's kind of funny I guess, why no one ever made a silicon LDR?  It should be absolutely trivial, intrinsic Si in a sinuous path between same-doped pads or something like that; same thing CdS does.  Maybe they did and the world already moved on from Vactrols etc.

But anyway, for switching purpose, only the large-signal response matters (probably?), and you do get a response similar to the photoMOS types; just at very low current (~mA max?).  Still not clear if that's even enough...

Edit: hmm, let me see, it's been a while since I reviewed a datasheet anyway:
https://www.onsemi.com/pdf/datasheet/h11f3m-d.pdf
Fig.2 shows flatter voltage range than I remember, within 50mV perhaps, across a wide impedance range.  I'd be tempted to test it and make sure that's what's happening, since JFETs don't do that.  I don't have any particular reason to doubt the curve, but it's always possible they fitted or smoothed points.  The Idss even at max If is quite pitiful though, 100s µA.  The 30V rating I guess wouldn't be suitable here anyway, though I forget if they made higher rated versions..?

Tim
BrianHG:

--- Quote from: Kleinstein on February 26, 2024, 01:16:04 pm ---Normal transistors don't work well in the reverse direction. The breake down voltage is usually quite low. In forward direction there is a little saturation voltage of some 10 mV, even at low current. So 2 transistor optocouplers can not really replace a fet output optocoupler.

--- End quote ---
Normally I would have said no, but looking at the attached optocoupler data sheet's chart:



You see right at 0v, the current does move up in a straight line.  It's like the LED is directly injecting a neutral charge to the surface of the entire transistor.  It is like T3sl4co1l's double take on my opto-fet recommendation, then looking up the data sheet.  When turned on, even in the reverse bias, each transistor should hold a predictable current curve in either polarity so long you don't exceed around 0.5v, IE significantly more than 10ma.  When off, whichever transistor is in the correct direction to be open will leave too little current for the opposing transistor to even exceed the 0.7v point where that 'off' transistor begins to act like a diode and begins to conduct.

Minute linearity distortion aside, it may be too low distortion for Psi's application to even notice.  IE: line level audio muxing, where now with a little clever PWM during transition on the led controls, you can achieve a soft contact switch instead of a harsh pop just like the design intended use of the opto-fets.

If Psi needs that cheap 23cents per switch with 2 per package, I would just recommend buying a dip version of my recommended 4 channel opto, breadboard it, and take a test measurements.  2 and 1 channel equivs may be lying around for a cheap test.
T3sl4co1l:

--- Quote from: BrianHG on February 26, 2024, 07:56:19 pm ---You see right at 0v, the current does move up in a straight line.  It's like the LED is directly injecting a neutral charge to the surface of the entire transistor.  It is like T3sl4co1l's double take on my opto-fet recommendation, then looking up the data sheet.  When turned on, even in the reverse bias, each transistor should hold a predictable current curve in either polarity so long you don't exceed around 0.5v, IE significantly more than 10ma.  When off, whichever transistor is in the correct direction to be open will leave too little current for the opposing transistor to even exceed the 0.7v point where that 'off' transistor begins to act like a diode and begins to conduct.

--- End quote ---

Hmm, there might be some similarity involved; I don't know what structures are used for optoFETs, offhand.

Oh hey, a citation just from next door: https://www.eevblog.com/forum/metrology/measurements-of-leakage-current-and-offset-voltage-on-some-optofets-and-relays/
with the attached PDF being quite relevant,
https://www.eevblog.com/forum/metrology/measurements-of-leakage-current-and-offset-voltage-on-some-optofets-and-relays/?action=dlattach;attach=904220
GE original part; oh hey, that explains the letter-numbers-letter-number schema?
They don't really discuss construction at all though.  The interdigitated pattern makes sense, but I wonder what the substrate and contact dopings, patternings or connections are.  Or, conversely, do CdS photocells ever saturate [FET current saturation]?  It must be at quite high voltages given their long channel length, and have been used at mains voltage (not that they're necessarily still ohmic in those applications).

Hmm, not finding any likely patent references. Keywords too generic.  Will need more searching.

The effect of the phototransistor is easy to understand with normal BJT theory at least -- the photocurrent is deposited in the B-E junction (at the surface), and simply forward-biases it.  The C-E or E-C path therefore conducts as any transistor in saturation does; Vce(sat) at low currents can be quite small voltages (10s mV).  Which isn't resolved on the above plot (which also appears to have smoothing added, look at that "overshoot" for 5mA heh) but will be visible under zoom.  And is roughly symmetrical around zero, give or take some offset, and some kink as the Rce(on) may not be linear.

Probably the inverted hFE is awful, as is usually the case for planar BJTs, maybe worse due to the optical optimizations used, unsure; I could test a 4N35 to hand and see.  Could also be better.

Might also be worse for another mechanical reason, that E-B is being reverse-biased, meanwhile less charge diffuses into the C-B junction where it would cause multiplication; it may be that the curve simply ends at low currents, basically the photocurrent is shunted to the emitter without hFE multiplication taking place.  Put another way, it's operated as a photodiode.

In any case, easy to test :)

Tim
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