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I have a circuit that runs on 1V up to 15V DC. I want to attach a low-power battery-level (1.5V) device to this supply rail, and have it work over the broadest input range possible (yes, it will cut off before 1V most likely, but I’m okay with that). The device cannot take an input of more than 5V, and preferably I’d like it to not go higher than 2V or so. It draws on the order of 1µA or less current.

It seems like one simple way to accomplish the goal of limiting voltage and operating as close to the low end as possible is to put a JFET (N-channel) between the supply and the device, with its gate tied to ground. If the JFET’s threshold is around 2-3V, then once the supply (drain side) goes high enough, the JFET’s resistance will go up, keeping the source side from going higher than about 2V above the gate (ground), and there will still be enough current to power the device. On the source side (i.e. where the device is) I can put a resistive load if the draw isn’t consistent enough.

What I’d like to know is if there is any obvious flaw in this plan, or reason that it might not work as intended. Of course this is highly dependent on the variable pinch-off voltage, but bear in mind I don’t need a precise level; just a way to make sure the voltage doesn’t go too high for the device. I’m only making a few of these, so I can likely tweak a too-high threshold with the load on the device side; I can easily spare up to a few 100 µA without it affecting the rest of the circuit.

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  • \$\begingroup\$ Why not use a zener diode, then? \$\endgroup\$
    – Hearth
    Commented May 6, 2020 at 20:33
  • \$\begingroup\$ 1) draw the schematic of what you're proposing. 2) I would not expect a JFET to be suitable for this but draw the schematic and we'll see. 3) If the JFET’s threshold is around 2-3V, JFETs usually have a negative pinch-off voltage, not a 2 - 3 V threshold voltage. My guess is that you actually want to use a MOSFET. \$\endgroup\$
    – Bimpelrekkie
    Commented May 6, 2020 at 20:34
  • \$\begingroup\$ The second circuit vtolentino posted shows exactly what I'm talking about, although my statement lacked R1 & R2 and the gate would be tied to ground. A zener would probably work just as well, though, as has been suggested. \$\endgroup\$
    – Michael Winterstein
    Commented May 6, 2020 at 22:03

1 Answer 1

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This same technique is used to pre-charge some offline controllers when its regulated voltage is not yet available. For example the depletion mosfet in the following diagram does exactly what you want, but only at start up. So, your approach should work alright if the load dependence does not play a significant role.

circuit

If you want to extend the voltage range that the JFET limits, you can add a voltage divider between source and gate as follows:

circuit_2

As already mentioned in the comments, another easier alternative would be to limit the voltage with a zener diode.

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