Any jellybean low current (<10mA) opto-mosfet or opto-ssr for signal routing ? - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: February 24, 2024, 10:11:59 pm
content: Are there any jellybean/cheap opto-mosfet or opto-ssr for isolated bi-directional signal routing?Ideally multi-part-packages, like 4 per IC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2024, 12:03:45 am
content: What signals?SSRs are generally terrible for signals as the capacitance is high, and on/off ratio not impressive.Without knowing what kind of signals, I would be inclined to answer "no", because they'll do poorly at line-level audio (poor isolation at upper frequencies), digital (something like SPI, the edges would bleed through too much, let alone anything "high speed"), high or low impedance (say, electrometer or thermocouple amp stuff?), etc. Bonus points off for capacitance nonlinearity (more distortion in upper audio frequencies). There may be some parts that do fine at some of these applications, but there does not exist (and cannot, not in Si anyway) a part that is universal for all of them.If it's within supply rails, analog switches are fine, do check that you have adequate isolation for the application of course, but there are parts available with impressive specs (like <1dB insertion loss, >80dB isolation, at low GHz), if you really need it. Even ye olde CD4066 etc. is fine for signals of modest impedance and bandwidth (100s to k ohms, low MHz), again, give or take how much performance you really need.Tim

author: Psi
author_rank: Super Contributor
timestamp: February 25, 2024, 01:27:57 am
content: Under 3kHz square wave.Voltage will be above the 5V VCC. but no higher than 50V due to TVS clamp.Switching speed is not an issue, this is just to configure some routing at startup based on what mode it should be in

author: MasterT
author_rank: Frequent Contributor
timestamp: February 25, 2024, 02:02:31 am
content: https://www.arrow.com/en/products/smp-2a30-8dt/standex-meder-electronics

author: Psi
author_rank: Super Contributor
timestamp: February 25, 2024, 02:10:55 am
content: Quote from: MasterT on February 25, 2024, 02:02:31 amThanks,Strange there's such a large price difference for that, $3 on digikey vs $0.3 on Arrow.

author: MasterT
author_rank: Frequent Contributor
timestamp: February 25, 2024, 03:07:17 am
content: That's what I like in arrow, they drop a price if not many parts left. Something never seen on mouser.Thinking to get 100 mosfet's for a $1, not sure what I 'd use for.https://www.arrow.com/en/products/sihf8n50d-e3/vishay

author: BrianHG
author_rank: Super Contributor
timestamp: February 25, 2024, 03:43:35 am
content: Cheapest at digikey:https://www.digikey.com/en/products/detail/ixys-integrated-circuits-division/CPC1017N/555063And stupid digikey doesn't even have these listed in the optoisolator section...57cents at arrow., 74cents at digikey for 100.

author: BrianHG
author_rank: Super Contributor
timestamp: February 25, 2024, 03:48:33 am
content: https://www.mouser.com/ProductDetail/Toshiba/TLP175ATPLE?qs=HVbQlW5zcXXp9ILK7hvmMw%3D%3D

author: Peabody
author_rank: Super Contributor
timestamp: February 25, 2024, 04:09:55 pm
content: Quote from: BrianHG on February 25, 2024, 03:43:35 amNo, they are solid state relays. :-)I have a hand-held oscilloscope that uses these to switch between AC and DC input. So presumably they do pretty well, even at higher frequencies.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2024, 09:19:46 pm
content: The switching between AC and DC coupling has the switch essentially in parallel to the coupling capacitor. So the switch does not have to work with high high frequency. It only need a small and constant capacitance between the control and switch side.The photomos switches are still impressive - so far I have not seen much thermal EMF from the switch, though I was afraid that there could be some.For the leakage specs one really has to separate between the tested limits and the typical leakage. Testing to strict limits can be costly and is thus usually not found with the cheaper parts. The typical leakage can still be quite good.

author: Psi
author_rank: Super Contributor
timestamp: February 26, 2024, 08:29:05 am
content: Quote from: BrianHG on February 25, 2024, 03:48:33 amYep. i've been looking at the TLP175A. Seems pretty good value.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2024, 11:21:10 am
content: Does it matter if it's normally on?It won't be the cheapest, but J-FETs typically have a lower capacitance and leakage, compared to MOSFETs, so use a photovoltaic opto-coupler and a J-FET, or two back-to-back, if it has to block in both directions.

author: BrianHG
author_rank: Super Contributor
timestamp: February 26, 2024, 12:12:48 pm
content: There is always FET output optocouplers. Since only 10ma is needed, plus you also get a linear transfer of output resistance tracking LED current.Also, using 2x regular transistor output optos with the transistors wired back-to-back offers you the same AC switch like a mosfet output.This means a jellybean 4 channel transistor out opto can be wired as if it were a 2 channel AC mosfet out opto.Though, you need to make sure your IR-led current is goo and strong.If an 8 transistor opto exists, then that will give you the equiv of a 4 channel mosfet out unit.

author: BrianHG
author_rank: Super Contributor
timestamp: February 26, 2024, 12:19:26 pm
content: Here, take a look at this guy:https://www.mouser.com/ProductDetail/Lite-On/LTV-846S?qs=sGAEpiMZZMteimceiIVCB8%252BEnxlUsvj2XjK3CA4Ts7M%3D4 channel = 46cents for 100pcx. This means 23cents per AC channel, and you get 2 per device.The even have them in DIP16 package for 45centx for 100pcx.https://www.mouser.com/ProductDetail/Lite-On/LTV-847?qs=sGAEpiMZZMteimceiIVCB4BEdbJlF99qN8g%2Fz4u5H%2FM%3D

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2024, 01:16:04 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2024, 05:15:08 pm
content: Quote from: BrianHG on February 26, 2024, 12:12:48 pmNot 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.pdfFig.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

author: BrianHG
author_rank: Super Contributor
timestamp: February 26, 2024, 07:56:19 pm
content: Quote from: Kleinstein on February 26, 2024, 01:16:04 pmNormally 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2024, 10:02:59 pm
content: Quote from: BrianHG on February 26, 2024, 07:56:19 pmHmm, 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=904220GE 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 testTim