2N7000 Protection

[karbomusic]

I've not delved too deeply into the ^best method especially since I'm sure my design is the root cause. Below is probably the very first circuit I designed, and was mostly strictly from the two datasheets minus much knowledge at the time as to what the datasheets meant.  

Now, this is similar to a ROG ruby etc. It works phenomenally well except, occasionally when plugging unplugging stuff from the unit, I toast the 2N7000 in the buffer circuit. I'm just terribly versed in where to start. The area in the red square is of interest. I'd ignore any mistakes on the right half unless that side is part of the problem. The goal is really just to fix this sensitivity to blow the 2N7000 being used as a buffer. I had zero idea of what I was doing at the time (moreso than now LOL).

This should be the exact circuit in the box, I drew it on a dry erase board and is the only copy I have. Should be accurate though...

[R.G.]

Connect the anode of a 12V zener to the source of the MOSFET. Connect the cathode of the zener to the gate. Insert a resistor in series with the gate+zener that will prevent the input current from overheating the zener for all reasonable overvoltages/energy content.

[knutolai]

ROG suggests a red LED in a similar configuration:
http://www.runoffgroove.com/peppermill.html

I won't ever question R.G. knowledge, but a red LED might be a part you already have on hand.

[R.G.]

A red LED will probably work. I would use the base-emitter junction of an ordinary NPN silicon transistor though.

The protection results from the added part conducting from source to gate at a low voltage when the gate is pulled lower than the source, and then conducting from gate to source when the gate is pulled higher than the source. It's OK for the conduction from source to gate to be a diode-drop, as the MOSFET is turned off anyway. But for the gate going positive, you want to be sure the protection device does not conduct until the gate is more positive than the threshold voltage plus any voltage needed for the working current.

A red LED will conduct forward at about 1.2-1.4V, and break over in reverse at a few volts. That works, and protects the MOSFET, but LEDs don't really like to be reverse broken. Zeners are designed to break over sharply from non-conduction to conduction, which is good for a protection device, and the 12V "stock answer" zener works well for most MOSFETs, which usually have a +/-20V gate limit. In low current linear applications, you almost never need to raise the gate much higher than the threshold voltage, which is usually less than 5V, and for the 2N7000 it's less than about 3V. So you can use a lower-voltage zener.

The base-emitter of most modern NPNs has a zener breakdown voltage of about 5-8V, often about 5.2 to 5.8V. So it's handy. And cheap.

[karbomusic]

Thanks RG and others...

I'll get this mod in next opportunity as I'm tired of changing out 2N7000s   

[bool]

Small SMD zeners are cheap and you can add them to your build "after the fact", under the traces - usually with almost no modification required.

A 2N7000 will usually have 2-something volts on the gate, so you pick a zener that don't "zener" at this voltage even with miniscule microcurrents happening in buffers. Imho 8.2 - 12V is safe.