mosFETs and Transient Voltage Protection (schematic Uploaded)

[Falcon69]

I don't think I got my answer clearly in previous post i did on a different subject. (Although, I changed the design and went with a OptoMOS as suggested instead).

But in another circuit I am working on, I will be using n-channel mosFETs.  Now, in some chips I see dual zener (or maybe TVS) diodes connected between Source and Gate (see the second chip in the picture).  The ESD protection is 2kV for HBM and 200V for MM according to the data sheet on a particular chip I am looking at.  Now is that enough, or do I need to add a TVS Diode?  Basically, the gate would be on a connector that gets plugged in and out all the time with a 10k resistor in series (or would 47k be better?. The mosFET would be sinking approximately 80mA to ground.

On mosFETs that DON'T have them built it, I understand adding a TVS diode is a must (see the first chip in the picture).

FYI, the voltage source is 5.2volts.

Also, in the same circuit, I have several logic gates.  Many only have a connection to ground, or to supply, but never both at same time on a single input of a 2-input logic gate. Now, if that input is ONLY connected to ground or supply, and nothing else, and the supply voltage is less than 5.5v, is there any reason to put a resistor in series between the ground/supply and the input? Would it be good practice to put a 100k or so resistor there just in case to help lower any current into the chip?

Thanks in advance.
Jason

[T3sl4co1l]

What's the rest of the circuit?

And, are you expecting ESD to be applied to the relevant nodes?

Also, in the same circuit, I have several logic gates.  Many only have a connection to ground, or to supply, but never both at same time on a single input of a 2-input logic gate. Now, if that input is ONLY connected to ground or supply, and nothing else, and the supply voltage is less than 5.5v, is there any reason to put a resistor in series between the ground/supply and the input? Would it be good practice to put a 100k or so resistor there just in case to help lower any current into the chip?

Certain families may require input resistors or special voltages (ECL for example).  The common stuff (CMOS, TTL...) can't care less.

You might have external reason to do so; it's good practice to put a small e.g. 100R-10k resistor at each input or output, so you can perform functional testing by forcing test points high or low.  Or even just 0R resistors, so you have the option of reconfiguring them, with some pain (oh noes, the blue wirewrap wire!).  More of a prototype / production thing though.

Tim

[Falcon69]

i dont think any esd on the drain/source.  just on the gate, as it gets plugged/unplugged.  The drain and source are stay connected on the circuit board to sink the LEDs to ground.
Yes, I will be using CMOS.  I'll go ahead and take them out then, Seems like extra $ to keep them there if not needed.

Thank you Teslacoil.

[poorchava]

gate oxide as such is very thin, and in small mosfets the gate has very small capacitance, so any voltage spike during handling or operation is a killer. This is even more true for low threshold mosfets, which sometimes have Vgsmax as low as 7V.

In bigger mosfets, that have typical 20V of Vgsmax and like a nanofarad or more of gate capacitance - this is not much of a big deal unless you really expect the gate to be exposed to external ESD events.

[Falcon69]

Okay, I understand, but original question still lingers.  This mosFET, http://www.mouser.com/ds/2/308/NTJD5121N-D-119599.pdf, for example has the protection diodes, but only protects up to 2kV HBM and 200V MM for ESD.  Would I need an external TVS diode as well with this chip?  Only the gate has a possibility of transient voltages, as the gate would be plugged/unplugged multiple times.

What ESD for HBM and MM is acceptable protection for normal operating conditions?

[T3sl4co1l]

Okay, I understand, but original question still lingers.
...
What ESD for HBM and MM is acceptable protection for normal operating conditions?

The question still lingers: What's the rest of the circuit? And, are you expecting ESD to be applied to the relevant nodes?

Or rather I should now say, *how much* ESD are you expecting, and where?

What a product needs for ratings varies by market and intent.  Automotive is different from consumer, which is different from industrial.  If you need reliability in general purpose or commercial / industrial settings, it would be a good idea to pursue something like IEC 61000-4-2, 8kV contact / 15kV air discharge.  The air discharge figure is very roughly equivalent to an HBM rating, IIRC, so a device rated for 2kV HBM might be 1kV with the contact method and test setup of this standard.

Needless to say, the zeners in those FETs are woefully inadequate for serious ESD -- they're mainly a handling precaution, and usually fall somewhere between pointless and detrimental in actual use (anything facing the outside world needs real protection anyway; a case where they're detrimental might be a low-bias amplifier where gate leakage is critical).

Tim

[Falcon69]

okay, that's what I needed to know.

I'm confused by what you mean nodes.

I will add a TVS diode to the circuit.  My circuit is used for Hobby CNC machines, and the plugging in and unplugging of switches I'm worried about.

[T3sl4co1l]

Nodes or nets, are sets of component pins which are connected together.  So, wires, lines, connections, traces, etc..

Tim

[Falcon69]

oh, no.  Not unless they are touched directly.  Mainly, I have switches wired to USB connectors.  The connectors will most likely be heatshrink tubed or using plastic casing around the USB.  I was just concerned with the plugging and unplugging of those connections. 1 pin on those supplies the power to the gate of the mosFET to turn it on.  If no switch plugged in, mosFet does not turn on, thus disabling that part of the circuit on the board.

[T3sl4co1l]

The procedure of an ESD test might not catch that (discharge is applied to any exposed metal surface -- a mated, shielded connector has no risk of getting struck), but a lower level test would still be helpful.

For example, suppose you plug in a cable that's sitting around, open circuit, that somehow accumulated a static charge: now you have charged metal wires plugged directly into your circuit.

Possible causes could be careless handling (I suppose it's possible someone could shuffle their feet over the carpet, then touch an inner wire or pin, maliciously or accidentally), ambient ionization, piezoelectricity (voltage from strain) or triboelectricity (voltage from friction) due to outside handling and movement, or strain and movement of the wires within the cable itself.

You might also consider something that's naturally more robust.  A 2N4401 might help -- I don't think I've seen ESD ratings on BJTs before, but as a PN junction device, they at least exhibit controlled (if not necessarily non-destructive) breakdown at high voltages.  With the MOS gate, voltage goes too high and, pfft, it's done, single shot, no warning, end of story.  Yes the internal protection thing helps with that, but obviously, not much.  Whereas a BJT might actually survive a few strikes.

Just as a matter of practice, I put ESD protection and EMC filtering on all connections to the outside world.

I have no idea what kind of requirements you have, for this specific connection or for the overall system, so I can't really be any more helpful..

Tim

[Falcon69]

So here's 1/8 of the circuit I am working on, meaning, there's 8 total switches.

The circuits in the blue box are the switch.  The rest is the main circuit (some of the circuit is shared by the other 7 switches).

As you can see, the stuff in the blue box has 5 connections that get plugged/unplugged from the main circuit. Voltage In, Ground, Signal to Comparator, Signal from Test Switch (which turns on all the LED's to test), and Signal to the mosFET in the Red Box.  That signal wire let's the main circuit know if a switch is plugged in or not, thus turns on or off the led's on the main circuit board based on whether or not a switch is plugged in.

The other mosFET is actually an optoMOS. From my understanding, I do not need any ESD protection on that.

The circuit is 5v-5.2v (i will be using 12v going into a LDO that puts out 5.2v.  5.2v to compensate for voltage drops of wire length to the switches.

So, My question is, this circuit, should I have a TVS diode for each connection to the switch?  Or just on that mosFET at the bottom right, in the red box?

If it is just the mosFET that needs it, seeing how the source goes to ground, would this TVS Diode Array work for it (and the other 7 switches as well)? http://www.mouser.com/ds/2/308/ESD7008-D-241517.pdf

If I need ESD Protection for EACH connection of the switch, then would this TVS Diode work for it? http://www.mouser.com/ds/2/115/D5V0L4B5V-218334.pdf ,and would I place it on the switch or the main circuit board?

Thanks in Advance

[Falcon69]

nobody has input on those TVS diodes?  Or suggest a better method?

[T3sl4co1l]

As you can see, the stuff in the blue box has 5 connections that get plugged/unplugged from the main circuit. Voltage In, Ground, Signal to Comparator, Signal from Test Switch (which turns on all the LED's to test), and Signal to the mosFET in the Red Box.  That signal wire let's the main circuit know if a switch is plugged in or not, thus turns on or off the led's on the main circuit board based on whether or not a switch is plugged in.

Ok, I think I understand what you've got here.

And, are you really actually using (/ trying ) Falstead for drafting?!  That's at least moderately insane to me, but, I give you credit for trying, I guess...

Anyway, a couple things happen when you plug that in:
- As soon as +5V and GND make connection, C1 starts at 0V but there's 5V somewhere else in the circuit.  The inductance of the lines (cable and connections) begins to charge, and current rises.  As current rises, C1 voltage starts to rise.  By the time C1 reaches 5V, current is at the maximum (I ~= 5V / sqrt(L/C) ), and C1 voltage continues to rise.  If C1 remains constant (e.g., a 0.1uF 50V 0805 X7R ceramic, or C0G, or film), it reaches no more than 10V peak, then rings down.  However, a low voltage / small size ceramic will experience a reduction in capacitance at higher voltages, leading to a peak higher than 10V.

Neither case is probably very good for the Hall effect switch (if it's a switch, do you really need a comparator after it by the way?) or logic gates (whatever they are; they're unmarked).

So, a 5V TVS across the supply would be helpful.  You can also add series resistance anywhere along the line (at the 'base' connector or on the attached thingy) to dampen the LC resonance and make it charge more smoothly.  Probably 10 ohms would be enough not to notice the additional drop, while damping most of this junk.  Also, if you put it on the base end, it provides short circuit or fusible protection in case some idiot starts shoving paperclips into your connector.

- The logic input is only pulled low with 100k, so it will be very noise sensitive, and the inputs are CMOS logic inputs (I guess), so they won't be the most robust.  You could get away with this as-is, but I'd rather have some input clamp diodes (e.g., a BAT54S from GND to input to +5V).  The hazard is against the chips' own input protection diodes (which are small), so although you could use a TVS here, it won't do much good against a spike if the 5V hasn't come up yet (which is likely to happen; during insertion, pins connect and disconnect and reconnect multiple times over a 1-10ms time span, probably in random order -- so, expect anything).  In that situation, the fact that C1 is still around 0V helps you, since there's nowhere for the input voltage to go except through one diode or another into ~0V!

- The sense / gate output thingy doesn't need to do anything here, but the same logic will apply to that line as well; in that case, the spike will be transmitted down the cable, then back up for a round trip.  You could dampen it with a 1k resistor or something like that.

- So, the FET in the red box will see all the ugliness that's around the wires.  It needs filtering and protection, I'd say.

- Also, any time your comparator is near threshold, it's exposed to, and amplifying, whatever noise is on the cable.

If it is just the mosFET that needs it, seeing how the source goes to ground, would this TVS Diode Array work for it (and the other 7 switches as well)? http://www.mouser.com/ds/2/308/ESD7008-D-241517.pdf

Well.. you could... but do you really want to deal with a grain-of-sand DFN / LGA thing?

You certainly don't need low capacitance here, and you probably don't want it.  You could use bog standard 1N4733 / 1N5232 zeners (1 / 0.5W, 5.1V), which aren't really specified for ESD, but it's a start.  Or you can use a proper device, like, I don't know any really tiny parts offhand, but the big ones start in e.g. the SMAJxxA series (equivalent to a through-hole P6KExx).

If you add, say, 1k series resistance to each logic input, followed by an optional capacitor of maybe 1nF or more to ground (I'm guessing you don't need any of these LEDs blinking in the MHz), and either a TVS to ground, or clamping diodes to the nearest (locally bypassed) supply, you'll have a good handle on both ESD and EMC (you won't be picking up stray radio waves).  This goes for logic outputs as well, though you can't necessarily tolerate the series resistance (sometimes even 10 ohms is too much), in which case a ferrite bead or (if it's a high current signal or supply) inductor is more useful.

Tim

[Falcon69]

Here is the schematic I am working on. It's BIG. Very Big.  But hopefully you can see what I am trying to achieve.

I have it working, I am just not sure how to go about protecting it from ESD or EMF.  Tim pointed out some good tips, but to put a series resistor and cap on each input of each logic gate is going to be a HUGE PCB footprint. 

As you can see, I already have a decoupling cap on each Logic IC, and I've taken measures to protect the jumpers for the normally open/normally closed input (sinking the gates to ground to perform that operation by plugging in the jumper). 

There are 8 Circuit Boards for the Switches, 1 Circuit Board for the Rear Panel, 1 Circuit Board for the front Panel LED Indication, and 1 Circuit Board for the Mainboard.....so a total of 11 separate circuit boards for this project.

I need to protect the circuit when the switches are plugged in and unplugged. The switches are in the upper right corner.  As you can see, each has 5 connections to the rear panel board. I was hoping that all i needed was maybe that one chip I linked above that would protect all of the mosFETs.



EDIT: Added Ref Numbering to the components.

[Falcon69]

I found another circuit. I redrew it though in a way I think I understand.

But, If this is what is needed on each input of each logic gate and mosfet, it's going to be expensive. As you can see from the schematic of my circuit, i have over 50 IC's, and several of them have 6 inverters/quad logic gates.

Maybe I am over thinking this or just don't understand this ESD/EMF/Transient Voltage Suppression.  I just want the circuit protected from damage from these things if it happens.

I've spent all day trying to understand what Tim said.

The circuit below shows MOV's (Metal Oxide Varistor), TBU's (Transient Blocking Unit), Resistors, and TVS (Transient Voltage Suppression) Diodes.

The Cap is a 0.1uF Ceramic type 16volt.  I'm not sure on the values of the rest of the stuff.

[Falcon69]

What if I put a Uni-Directional TVS Diode on each of the V-In, V-Out, S-Out, and EN pins of the switches (upper right corner of schematic)?  Would that be enough to protect the circuit from plugging and unplugging of the switches? I could use this one maybe to do that?  http://www.mouser.com/ds/2/115/DRTR5V0U4S-271613.pdf

The other connections will probably never be plugged in and unplugged once they are set-up. (i.e. 28pin, 20pin, and 8 pin connectors in the center of schematic).

[Falcon69]

Any Thoughts on that?  The chip is already getting pretty expensive. I can't afford to put the recommendation that Tim said on EACH and Every Input of the logic gates. Would the idea in the aabove me sufficient enough?

[T3sl4co1l]

Yeah, that looks nice: the zener protects +5V/GND, and the clamp diodes take care of the rest.  Plus, the loop between any given input clamped to power/ground, or to a TVS, occurs within the device itself, so there should be little overshoot.

I would still recommend a series resistor on the supply rail, or a larger zener/TVS (1N47xx or SMAJ range).  And then you'll be pretty much set.

What's expensive?  BAT54S is like pennies in quantity.

Regarding the circuit you found/drew: the input MOV stuff is unnecessary.  Just have some sort of protection at the input pin itself (clamp diodes or TVS), followed by one series resistor, going to all the logic pins.

That way, any overshoot or voltage drop in the clamp/TVS is dropped across the resistor into the logic pin(s), limiting the peak current by orders of magnitude relative to a naked input, and still by many times if they were just connected straight (a low-ohm resistor, or none at all).

Tim

[Falcon69]

Okay, so If I use that DRTR..... chip on each of the connection pins of the switch circuit board...ON THE MAINBOARD (not on the switch board, or should I put one on both?)....and all my input connections to the logic gates (i.e. A and B inputs) put a resistor in series, that should do it (or does a TVS need to be on each input as well, in addition to the series resistor)? What size resistor? 10k? 4K7? I remember you mentioned 10ohm.

And put the resistors on the logic gates on the other circuit boards as well?  Or just a single TVS Diode across the input of supply and ground just after the fuse?

By doing this, will it mess with the decoupling cap I have on each IC?

FYI, Just got done reading this. http://www.onsemi.com/pub_link/Collateral/TND335-D.PDF Most of it made no sense to me, but some does.  They have used a USB 2.0 device as an example, and in my case, My circuit uses the same voltage.  So, According to that, If I understood it correctly, I should choose a TVS Diode that is no more then 7V Vrwm and 7V Vbr.  The Logic Chips have a max operating voltage of 7V.  Since my circuit is about 5.2v, choosing a TVS Diode with an operating voltage of 5.5V and a Reverse Breakdown Voltage of 6V should be exactly what I need.  I am not sure on the Amperage rating though, I am confused on that one. My circuit, when the test led is switched and all LEDs are on, will only see about 2A, just alittle less then that.

You said a larger TVS on the supply line.  Does that go right before the fuse (or after) on the mainboard? I'm guessing if it does, a Bi-Directional one?  Same 5.5V Vrwm and 6V Vbr? What Amperage rating you think would be good?

What's Expensive?  If I went with that circuit I drew with the TBUs and MOVs plus the TVS....to put that on EVERY input of each logic gate would be VERY expensive.  It all adds up.

[Falcon69]

Tim,

You mentioned earlier about the comparator,  I have it there because I am concerned with the signal getting to close to the threshold and the gate following it may or may not switch to a 0 or 1 when it should.  I thought that adding the comparator may solve that problem if the wiring from the mainboard, along the machines cable tracks, to the switch point is too long.  If It is not needed, I will take it out.  The max distance the wire could be is 50 feet.  This is dependent on the size of the machine.  The machine I am building currently is a 4x8 table, but in the future, I;d like to build a 5x12, and in that case, some wiring will be 50 feet. So I wanted to plan for a voltage drop.  Also, I was told that a comparator helps take out any noise in the wiring, which would help with switching.

If You think it is unnecessary, I will take it out, that would save a lot of board space if it is redundant.

[T3sl4co1l]

Okay, so If I use that DRTR..... chip on each of the connection pins of the switch circuit board...ON THE MAINBOARD (not on the switch board, or should I put one on both?)..

No, on any input, regardless of side.  And probably outputs too.

Your circuit showing the switchy bits being right up next to the mainboardy bits belies the fact that there's one to a hundred meters (dunno, you didn't specify) of cable picking up nasty environmental signals and static between the two.  Two ends of a given wire are NOT the same connection!

..and all my input connections to the logic gates (i.e. A and B inputs) put a resistor in series, that should do it (or does a TVS need to be on each input as well, in addition to the series resistor)? What size resistor? 10k? 4K7? I remember you mentioned 10ohm.

Yeah, I mentioned 10 ohm... for damping the 5V output to the switchy bit.

100 ohms might be typical for a logic output pin.  1k-100k might be fine for a logic input.  Depends, obviously, but your application doesn't look too critical.  This is in line with what's on your comparator, for instance (10-100k resistors).

And put the resistors on the logic gates on the other circuit boards as well?  Or just a single TVS Diode across the input of supply and ground just after the fuse?

A resistor per gate is not necessary.  The goal is to filter / absorb all the crap that happens at the connector.  Once it's on the board and protected and filtered, it can be considered 'sanitized' for use.

Like filtering database inputs so this doesn't happen.  But electrically.

By doing this, will it mess with the decoupling cap I have on each IC?

No, in fact the closer they all are together (TVS array, bypass cap and IC), the better it will work.

FYI, Just got done reading this. http://www.onsemi.com/pub_link/Collateral/TND335-D.PDF Most of it made no sense to me, but some does.  They have used a USB 2.0 device as an example, and in my case, My circuit uses the same voltage.

No, your circuit uses the same supply voltage, not the same signaling.  USB2.0 is a low voltage differential, high bandwidth signalling method and cannot tolerate filtering for ESD or EMC purposes.  Your application has full 5V CMOS logic levels and, as far as I know, couldn't care less about bandwidth.

So, According to that, If I understood it correctly, I should choose a TVS Diode that is no more then 7V Vrwm and 7V Vbr.  The Logic Chips have a max operating voltage of 7V.  Since my circuit is about 5.2v, choosing a TVS Diode with an operating voltage of 5.5V and a Reverse Breakdown Voltage of 6V should be exactly what I need.

This logic is still correct, however.  This is in regards to the TVS diode on the supply rail.  If you are using diode clamps (or the kind like that array device, which contains clamp diodes), this is where ESD will be shunted to, so you need a zener / TVS / breakdown device to prevent the voltage from going crazy.  The array device contains a small one, but it wouldn't hurt to add e.g. an SMAJ5.0A somewhere as well.  The location isn't very critical, but should be near the location of greatest stress (connectors, power input, who knows?).

I am not sure on the Amperage rating though, I am confused on that one. My circuit, when the test led is switched and all LEDs are on, will only see about 2A, just alittle less then that.

Transient amperage is unrelated to power consumption, it's an external source.

ESD typically applies peaks up to 16A, but only for a ~50ns pulse.  The charge is small, so it's easily clamped by sufficiently large capacitors (on the order of 1uF or so), and whatever's left can be absorbed by a TVS.  The downside is, it has a very fast risetime, so you can easily get pulses over 200V (for just a few nanoseconds) sneaking past even a rather burly TVS directly at the input.  You generally want to bypass the input clamp diodes (+5V to GND) at point-of-use for best results.

You said a larger TVS on the supply line.  Does that go right before the fuse (or after) on the mainboard? I'm guessing if it does, a Bi-Directional one?  Same 5.5V Vrwm and 6V Vbr? What Amperage rating you think would be good?

Fuse, as in power input?  Or was there a fuse going out to the switchy bits too..?

You always want a TVS across the circuit, and on the load side of a fuse, so that 1. surges are able to blow the fuse, 2. when the fuse opens up, no transients get into your circuit, and 3. your circuit is clamped and all voltages well-defined, all by itself, whether there are cables into and out of your circuit or not, whether the fuse is blown or not.

What's Expensive?  If I went with that circuit I drew with the TBUs and MOVs plus the TVS....to put that on EVERY input of each logic gate would be VERY expensive.  It all adds up.

If you're working down to every last cent, you'll have to have your own introspective discussion regarding reliability versus cost.  I can tell you the best way to build something, whether you want to follow is up to you...

Tim

[Falcon69]

Okay,

The schematic I know is alittle confusing at the switch points.  Basically, the 5 conductor wire will be soldered to the little circuit boards for the switch, then on the other end (could be up to 50 feet of cable) soldered to a Mini-USB 5 pin plug.  That will get plugged into the Rear Panel Circuit Board.  Then, from there, via a 20 pin and a 8 pin connector, via ribbon cable, the wiring goes to the mainboard via the same type of connectors.

The input for the supply and ground (bottom left of schematic) will be done via a 2pin Screw Terminal.  The other Screw terminals (8 of them) are for the switch outputs, via the OptoMOS.

So the switches themselves only have one connector that is plugged/unplugged.
SO, Even if the output of one logic gate goes to the input of another logic gate, still need to put a TVS/Series resistor between them?  Each Input?  Or only need to go for inputs that aren't a direct connect to another gate/inverter?

So, for instance, the 1a-Y Xor Gate to the 1b-A XOR Gate, there's a 360 resistor going to the OptoMOS, and a 100k pull-down.  That circuit/trace would need a TVS AND a series resistor between the two Gates?  But, from 1i-Y to 1a-A, no need, because it's a direct connection with nothing between those two logic devices?

Still kinda confused on what needs.

If you say EVERY input needs 100k in series with a TVS, and EVERY output needs a 100ohm and TVS, regardless of connection, then I understand and will draw it up that way.  However, for the Gates that have the output going to a resistor and LED, would I need another series resistor AND the TVS as well??

[Falcon69]

And put the resistors on the logic gates on the other circuit boards as well?  Or just a single TVS Diode across the input of supply and ground just after the fuse?

A resistor per gate is not necessary.  The goal is to filter / absorb all the crap that happens at the connector.  Once it's on the board and protected and filtered, it can be considered 'sanitized' for use.

Think I missed that part. SO only the items that are actually connected to anything that is plugged/unplugged needs to be protected.  But what about the connectors? the 8 pin, 20 pin, and 28pin?  I don't think those will EVER be plugged/unplugged.  Just the initial plug in when setting them into the control box.  Once they are in there, they won't be unplugged.  The 2pin screw terminals may be plugged, unplugged, but the optoMOS protects those, correct?

[T3sl4co1l]

SO, Even if the output of one logic gate goes to the input of another logic gate, still need to put a TVS/Series resistor between them?

Only where "circuit stuff" connects to "cable stuff".

So, for instance, the 1a-Y Xor Gate to the 1b-A XOR Gate, there's a 360 resistor going to the OptoMOS, and a 100k pull-down.  That circuit/trace would need a TVS AND a series resistor between the two Gates?  But, from 1i-Y to 1a-A, no need, because it's a direct connection with nothing between those two logic devices?

Still kinda confused on what needs.

When "circuit stuff" connects to "circuit stuff" you don't need to worry (unless you're expecting dirty static-charged beasts to lick your board in normal operation).

If you say EVERY input needs 100k in series with a TVS, and EVERY output needs a 100ohm and TVS, regardless of connection, then I understand and will draw it up that way.  However, for the Gates that have the output going to a resistor and LED, would I need another series resistor AND the TVS as well??

If the LED is on a cable, perhaps.

If the LED is mounted near a metal enclosure, doubtful.

The operational case would be, if the LED is close enough to the surface that a spark can jump to one of its leads, it might be a hazard.  But most holders are deep enough that that's extremely unlikely.  (The IEC 61000-4-5 air discharge test waves up to 15kV static over the surface, and zaps whatever it can find.  If it's all good solid plastic, no jumps means you're probably not going to even notice it going on.)

As for the full schematic... that's... 20 logic chips?  And a, what the hell, like 50 inch sheet of paper?  My browser viewer doesn't even want to zoom out far enough to see the whole thing!    This is in serious need of some logic reduction.  I think I was seeing you could dump about half of them with some simple rearrangements..

At the power input, J9: you have a somewhat vulnerable semiconductor, Q1.  A bidirectional TVS from its drain to ground (i.e., just after the fuse), rated for at most 80% of Q1 Vdss, would help it survive.  TVSs are usually rated to fail shorted, so although you're still not guaranteed anything (maybe it'll physically blow out anyway?), it's at least more likely that it fails shorted (reducing the voltage on the circuit even further) and takes the fuse out, making for an easy repair.  U4 is rated 35V, so it would make sense to use Vdss >= 35V for Q1, and, say, a 24V TVS.  Like an SMAJ24CA (the -CA parts are bidirectional).

The OptoMOS has isolation so you don't need to worry about anything on the LED (driving) side.  You will have to be weary that anything connected to J2-J9 is limited to 60V (transient or otherwise) and 100mA.  Resistive loads up to 48V are fine, or even moderately inductive loads.  I wouldn't recommend relay or solenoid coils, at least at full voltage.  Maybe derating to 24V or less would be better.  You can address that by adding a 48V bidirectional TVS across each switch output.  However, protecting them against current overload would be much more difficult.

The isolation barrier is only 1500V or so, which isn't much against say 15kV of ESD (if they can be exposed to such, and need to withstand it).  One simple way to address that: use a 1nF "Y1" style capacitor to sink the ESD charge.  This is how Ethernet maintains an isolated communication channel, yet passes ESD with only a 1kV barrier -- part of the reason it's so popular, reliable and powerful, despite its otherwise difficulty to implement and use.

Also, what good is the 2.2k pullup to +5 if the switch contacts are otherwise floating?  Where's the ground reference?  And if one side is intended to be grounded (I don't know), why use SSRs at all, why not an "open collector" (or open drain) type transistor output?

Connectors:
- Board to board connectors are generally considered OK.  J20, J21, J23, J24, J26, J28: no need to screw with.  (I will note: it's probably better to put the series resistors for the LEDs near the source pins, rather than on the display board.  Transmission line stuff.)
- Series 10 ohm resistors: J32, J36, etc. V_IN pin.
- All the TVS stuff you have a choice on: probably best to put it on the J32, J36, etc. inputs (and outputs where applicable), as well as J29, J33, etc. (same idea).  Or instead of J23, J36, etc., you can push it up to the main board, on J19 / J20.1.

I've got to admit, as schematics go, I've seen some bad ones, but as far as human readability, I think this one takes the cake -- it's quite sufficiently horrible to try to follow any single line, there are no ground or supply symbols, and despite that, everything is neatly laid out!  Seems like it was autogenerated from a netlist or something.  Weird.

Tim

[Falcon69]

great! you've been a great help Tim Thank You.  I understand now.

And yes, the schematic is big. I need to work on it.  It was all hand done, no netlist or anything.  I did it this way with no ground or voltage reference so that I can "convert to PCB' and it all goes over without a problem.


I will have a look and see if I can reduce some of those logic gates.  I think you are right and I can.