Now, suppose I also want to protect against someone applying overvoltage or reverse voltage to the atmega pin -- similar to the ruggeduino. Those same pins also have an output tied to the gate of a MOSFET, similar to a motor controller or other high-wattage output.
To provide overcurrent protection on the bare (unbuffered) outputs, you'll use a PTC resettable fuse, with something between 200 and 500 ohms of resistance.
Thus, the end result is a circuit that connects digital pin out to gate of MOSFET (drain is "low-end switch out"), to cathode of TVS diode (other end is ground), to one end of the PTC fuse (other end is "raw pin in/out"), to connection header.
Now, if someone provides overvoltage, or reverse voltage, on the raw pin, then the PTC will limit current to a few dozen mA before it shuts off. But assuming this current threshold is not exceeded, and/or during the fuse trip time when current is flowing, would the TVS diodes be able to "take it"? Or are they really only intended for the ultra-short ESD events?
The closest I could come in the data sheet was a listing of IR (reverse current) of 1 mA max, but that seems to be showing the "trip point" for breakdown, not being a maximum rating. There is also a 200 mA forward rating at 1.25V forward voltage drop. So... forward, I seem to be good. But how do I figure whether it could take a few dozen mA backwards continuously?
(or, I guess, vice versa -- if I have a zener protecting against overvoltage, will that provide sufficient ESD protection?)
The Arduino already has a protection diode between each pin and ground, and between each pin and Vcc. So I'm not sure what adding TVS diodes would accomplish. The datasheet doesn't specify a current rating for the internal diodes, but an application note suggests that one or two mA is OK. If you want to handle higher currents in conjunction with a PTC fuse, you could connect Shottky diodes between each pin and VCC/GND. The lower forward voltage drop of the Shottly diodes means that they should conduct before the internal diodes do.
The Arduino already has a protection diode between each pin and ground, and between each pin and Vcc. So I'm not sure what adding TVS diodes would accomplish.
Would this built-in protection also protect my MOSFETs?
Especially if those MOSFETs are on the other side of a resistor? (Or perhaps a PTC, for current limiting)
Do the built-in diodes provide overvoltage protection?
The resistor between the mosfet gate and the Arduino should generally be quite a low value, about 100 to 200 ohms. So the protection diodes in the Arduino would provide a reasonable amount of ESD protection to the mosfet gate also.
The diodes provided protection against overvoltage on the inputs, provided that any source of such overvoltage has a sufficiently high source resistance to limit the current in the protection diodes to a milliamp or so. If overvoltage is likely, then series resistors should be used on the inputs.
I see. The question is whether I can limit in current sufficiently. See also: Ruggeduino.
It's good to know that I don't need a separate diodes for my TO220 mosfets, too; that saves on cost!
Would this built-in protection also protect my MOSFETs?
Once a mosfet is wired into the circuit there is little reason to fear ESD type damage to the gate. The more likely damage that can be done is when the device in unconnected to anything and one comes in contact with the gate terminal while carrying a static charge. I do generally like to place a 5K-10K resistor between gate and source terminals, more to ensure the device is turned off in the event that the arduino is powered off but there is still external voltage on the drain terminal of the mosfet. Such a resistor will all give a little additional ESD gate protection, but as I said that is normally not a problem once the mosfet is correctly wired into a circuit.
In this special case, the gate of the mosfet also has a pin open to the world, because it's a generic shield type project where the pin could be turned into an input. Would you still not worry about ESD?
In this special case, the gate of the mosfet also has a pin open to the world, because it's a generic shield type project where the pin could be turned into an input. Would you still not worry about ESD?
The protection diodes on the pin will protect the mosfet gate against ESD when the pin is configured as an input.