No, I’m afraid not.
I’ve been busy with Proffieboard V3, Color display support and various real-life things.
would it be possible to buy one, even if it’s just in prototype stage, to test out?
Buy one, no.
Make one yourself. Maybe.
I wish I had some time to actually finish and test the prototype I made, it would make me a lot happier about releasing the plans.
So I’ve been working on testing my TeensySaber V4. However, I’ve encountered some problems.
The biggest problem is that Teensy V4 is not 5v tolerant. That means that things that connect directly battery power (like neopixels) should ideally not be connected directly to the board.
That in itself can be fixed fairly easily by adding a logic gate in between the board and the neopixels. However, if the gate is always on, then blade ID becomes impossible, so I would also need a pin to enable/disable the gate.
This takes up two pins on the side of the board, and also adds another output pad somewhere, which the current design doesn’t really have a good spot for.
So I have two possible solutions:
- Add some spring-loaded pogo pins to the TeensySaber v4 board, these would connect with pads on the bottom of the Teensy and provide more signals.
- Instead of targeting a Teensy 4.0, I could make a board for a Teensy 4.1. The Teensy 4.1 is significantly bigger, but it has the SD-card slot already onboard, which would make the teensysaber board simpler (and cheaper).
I’m not very happy with either of these solutions though.
One makes the board very large, which makes installation in a saber somewhat iffy. The other makes the board more expensive, harder to test and harder to work with. 
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I think maybe I have a solution.
I think all I would need is a diode that shorts to 3.3v, that way, if the incoming voltage is higher than 3.3v+diode dropout, the diode will form a short, and protect the chip. A small resistor would further protect the diode so that it doesn’t get overloaded when a short occurs.
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What you need is a TVS / ESD suppression diode with a clamping voltage of 3.3V. In many automotive devices all external pins have these diodes with a small resistor. It protects the device form static electricity and can allow the input to handle higher voltages.
Interesting. Searching on mouser only shows me four such devices, which makes me worry that if I use them in my design, they could suddenly just not be available.
Also, I can’t seem to find any data on how much current these things can handle continuously. I only need a few mA, which I suspect is fine, but I don’t know since it doesn’t say…
It was a long time ago that I worked with these devices, so I am a little rusty on finding the parts! There should at least be a 100 to pick form. You are already using this type of device to protect the USB input on the Proffieboard. We should be able to 10 to 20 part that are interchangeable.
I usually use mouser, but I am not sure how to find the part in this case. I get three parts also.
Try this on Digikey, Voltage - Reverse Standoff (Typ) = 3.3V :
https://www.digikey.com/en/products/filter/transient-voltage-suppressors-tvs/tvs-diodes/144?s=N4IgjCBcoOwGwWiAxlAZgQwDYGcCmANCAPZQDa4MALGGAKwgC6RADgC5QgDKbATgJYA7AOYgAvhKA
Ignore the current for now, since almost any diode should be able to handle the current you need.
Maybe you seen this before, since you did it for the USB:
https://www.st.com/content/ccc/resource/sales_and_marketing/presentation/product_presentation/group1/24/f1/62/7c/68/5f/4b/3c/EMC_system_immunity_against_electrostatic_discharges_webinar_presentation/files/EMC_System_immunity_against_Electrostatic_Discharges_Nov20.pdf/jcr:content/translations/en.EMC_System_immunity_against_Electrostatic_Discharges_Nov20.pdf
It has a good explanation and makes some good recommendations.
So I looked at a few of the diodes with a reverse standoff of 3.3V.
A lot of them don’t have very detailed data, but those who do don’t seem to kick in at low enough voltages to actually protect the board. Basically, it seems like the breakdown voltage is always ~1v higher than the “working voltage”, which isn’t good enough for most CPUs. Ideally, the “working” voltage should be 3.3v, and the clamping voltage should be less than 3.6 volts.
Unless I’m missing something, relying on the low forward voltage of a shotky diode hooked up between the pin and 3.3v would do a better job. The shotky would start conducting at around 3.5 volts, and resistance would go down sharply as the voltage goes up. WIth a 470 ohm resistor in front of it, it would take a fairly high voltage before you pass 3.6 volts I think.
I made a crude simulation using an MBR0540 diode and a 470 ohm resistor, and doesn’t go over 3.6 volts until you reach an input voltage of 25 volts.
Yes, you want clamping voltage around 3.6V. Now that I thought about it a little, the reverse standoff and working voltage can be ignored. A TVS diode is basically Zener diodes designed to handle ESD.
A schottky diode, as you described it, can be used for ESD, but it does not really work well to limit voltage on a pin. A regular zener with resistor would work better. These things are really tricky to simulate. It would be better to go with a design a simple proven design to limit voltage:
- TVS diode with resistor
- zener diode with resistor: https://www.rs-online.com/designspark/how-does-zener-diode-do-overvoltage-protection-in-circuit
These two approaches are the most common in automotive electronics. It don’t about other industries. The TVS diode is preferred because of the better ESD protection, but in this case it may not be as important.
The right zener diode maybe easier to find.
Why not?
I must admit that I don’t know enough about diodes to know what works well and what doesn’t work well. However, going by the data sheets, I’m having a tough time figuring out how to make TVS/Zeners work. TVS diodes don’t seem to document well what happens when you feed them DC voltage, and from the few zener diodes I’ve looked at so far, the MIN/MAX breakdown voltage has too large of a gap to reliably kick in between 3.3v and 3.6v range.
I looked at the link you suggested, and the first link there is exactly the same circuit that I’m using, but instead of a zener hooked up to ground, I have a shotky hooked up to 3.3v. It seems to me that utilizing the VDD rail reduces the requirement for a precise component, so wouldn’t that be better?
I’m wondering if maybe the CPU components used in cars are 5V tolerant to start with, and TVS/Zeners are used to make them also tolerate higher voltages, like 12-48V? The parts I’m finding seems like they would be very well suited for that.
To be honest, I don’t know all the details, so this is not going to be satisfactory answer! This is not my area of expertise. I was the two-way radio expert, when I last worked on this kind of issue. I have seen enough engineers design and build things that go up in smoke because they did not follow design guide lines. These guys are usually experts at what they do, but sometimes they forget they don’t know it all. I have also damage a few PCB, but luckily, it was never on the job. These PCB were things I built for myself. I don’t know all of this, so I just follow the many of the design rules I learned over the years.
A schottky hooked up to 3.3v works well for transient voltage spicks. The capacitor on the 3.3V side handles voltage spick. I don’t remember why this not a good way to limit voltage, and I have not be able to find good explanations in Google.
Yes for cars, in the past, the many of the parts use to be 5V but had to handle 8-20V or 12-48V. Today many are also 3.6v max, but the migration is slow. Also the automotive industry has access to parts we don’t see on Mouser or Digi-Key.
It usually takes time to find the right part, and in the end there is some compromise. For 3.3v and 3.6v, we need 3.45v ±4%. D3Z3V3BF is 3.43V 400 mW ±3%, 3.32 to 3.53. You may need to pick a slightly lower voltage, and it should not matter for a digital pin. It the case of an A/D input, a voltage divider would be better, assuming nobody applies more than something like 5.5V.
Unfortunately voltage divider will not work, because I also need to output data on the same pin.
I looked up D3Z3V3BF, which seems like it could work. However, the 3% rating is specifically for 5mA. The voltage-current curve shows that is starts conducting below 3.3v (which would give me a leak current of ~1mA @ 3.3V) and goes over 3.6 volts at maybe 8mA or so. This should be fine, but it still seems like I get a steeper curve with the shotky.
Not a easy problem to solve, but I guess you can go with a schottky diode, if you are will to go with some risk.
What are you using for your simulations? Try LTspice. It can models diodes and switching devices better than other free spice tools. You should model the 3.3V side as a non-ideal voltage supply, and the input as high impedance with a small amount of capacitance. It’s probably best to build and test a simple circuit once you get far enough along.
I will think about this some more, and let you know if I come up with something better.
I was just going to hook it up to a signal generator and an oscilloscope and measure the response. 
Now, I realized why we usually do not rely on schottky connected to Vcc to limit voltage: What happens when there is no power connected to the regulator? What when the battery is not connected on the regulator, but it is connected to what ever is feeding the pin? In most cases without the power to the regulator the voltage will not be limited. The diode will still suppress a surge but not limit constant voltage.
This a disconnect scenario that usually my must be handled in automotive electronics. There maybe a live signal on one side but no power to the device. It should not be damaged in this case. If this not a scenario, you are going worry about, then the schottky is fine.
The HSP061-2M6 you have on the USB connection has 4 schottky doides and one zener diode. I forgot exactly how it works, until this afternoon. It works with or without power, and the diodes don’t need as tight tolerances. The zener diode is important when there is no power on Vbus.
I think, you can use one zener diode with schottky per an input, or one schottky per an input with only one zener on the 3.3V side. This should allow the zener to have looser tolerances.
Good point.
I think the 3.3v regulator has some sort of discharge function that pulls the 3.3v rail down to ground when the power is off, but I’m not sure how that works, or if it would be helpful here.
The previous prototypes had some problems, so I’ve made some updates. Got the updated PCBs today:
I’m considering populating the PCB live on YT, but then I would definitely need to clean up a bit first…
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