Universal voltage input using depletion mode N-FET - Advice

Question

In an attempt to idiot proof a device I am designing that needs both 24V DC and 120V AC inputs for logic, I set out to create an input system that could handle either voltage going to a simple 3.3V micro controller. Below is the schematic of what I came up with. Simulation shows that using a Depletion mode FET combined with a sensing resistor, I should be able to limit the input current regardless of the input voltage. This with an opto-isolator and rectifier before hand should effectively allow me to sense 24 and 120 volts interchangeably.

I'm new to depletion mode Mosfets and did not see much about this type of application other then an app note from Infineon here.

Below is the basic circuit I am describing, the 180V input can be exchanged with a 24V dc input. this is basically the input pin to the system.

Edit:

The simulation only has -0.6V Vgs to limit the current, that does not match the expected -2.1 - -1.0 in the datasheet, but that is for a mere 56uA. Turns out that the data sheet later on lists the variation of Vgs and my model falls within that range.

The one issue I found in the data sheet so far with this plan is that it appears that N-Mos Depletion Fets suffer from thermal runaway when used in this fashion... At least that is what I gather from the image below.

Can anyone give me any advice on if this is a good path to take?

@ThreePhaseEel Turns out I was not looking at the plot on the data sheet that shows the Vgs(th) vs load, it is correct. — MadHatter, May 31 '16 at 03:35
Your LED will starve at 3V3 .R1 helps defend thermal runaway . — Autistic, May 31 '16 at 03:49
@Autistic Please note the LED shown is an opto-isolator that is attached to a 120V or 24V line... and R1 is required for Q1 to regulate current, in this case to 2.25mA approximately. — MadHatter, May 31 '16 at 03:50

score 1 · Answer 1 · answered May 31 '16 at 10:17

Firstly, this is "like" the type of circuit used in constant current diodes except they may use JFETs. It is a practical constant current circuit but does have a certain amount of flakiness. For instance, you are relying on Vgs(th) to dictate the current flow through the 400 ohm resistor and Vgs(th) can vary quite a bit. The graph in the question indicates that at 20degC the threshold is about -1.43 volts but this could be ranged between -1.03 volts and -2.13 volts.

At 5x higher drain currents these thresholds are likely to be lower by (say) 10% and at one end of the scale would produce a drain current of 2.3mA and at the other end it would be 4.8 mA. Factor in temperature and you are probably going to be OK but I would consider constant current diodes as a good option. the trouble is finding one that can handle voltages above 100 volts.

Universal voltage input using depletion mode N-FET - Advice

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