ultra-low-power voltage trigger (~500nA max) - Page 1
Forum section: electronics / general

author: flimshaw
author_rank: Contributor
timestamp: July 11, 2019, 08:26:07 pm
content: I'm trying to build a circuit that can passively observe a charging capacitor and trigger a logic level high when it reaches a certain set voltage. I am using a calculator-style amorphous solar cell that will put out a respectable 3-5v even in super low light, but which puts out very low currents as well (~2ua in dim indoor light). There are some dedicated ICs on the market but they are fairly expensive and complicated, and in most cases require more current then I am able to produce, I wonder if there is a simpler way using jellybean components or chips to do it.For example, the quiescent current of a 74hc14 seems in the ballpark to achieve what I would like, but how can I trigger one of its inputs at a specific voltage (+/- 20% is fine) with such low currents? Zeners seem too leaky in the nanoamp range. Or is there a circuit I could build with discrete mosfets that would only leak in the ~500na range?Thanks!

author: Rerouter
author_rank: Super Contributor
timestamp: July 11, 2019, 08:42:23 pm
content: Take a look at comparitors. Parametric search for the lowest supply current.

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2019, 02:57:08 am
content: The quiescent current of the 74HC14 is only given for when the inputs are at ground or the positive supply so be very careful about relying on it when the input is somewhere in between.I would start by considering various micropower comparators. Linear Technology makes some micropower sampling comparators which would be particularly suitable.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 12, 2019, 04:18:29 am
content: If temperature stability is not a concern, a high impedance voltage divider connected to a Darlington NPN can probably do it. (Actually, if anything, the temperature dependence will help it track the solar panel's temperature dependence.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2019, 05:31:49 am
content: Perhaps the simplest possible circuit to generate a relatively high power trigger, with low leakage: a blocking oscillator may be able to do that. Mind, you'll rarely see a transistor guaranteed for <1uA leakage, but they typically are better than that, if you don't need it to be hard guaranteed.A blocking oscillator works by charging a capacitor, whose voltage is applied to a transistor's base/gate. As bias gradually increases, gain rises exponentially, so the feedback loop needs to be designed such that the transistor flips on at a point where the leakage is still modest.Example, I built a flashlight "joule thief" with a single AA cell, and a PBSS303NX. It has two power levels, selectable by series base resistor; if I bleed some charge over the switch (in the off position) with my finger (so, a few megohms equivalent resistance), it flashes at a few Hz. That'll be some uA, if probably not fractional uA. Not bad for a 3A transistor and dumping several watts into an LED some microseconds later!Otherwise, more conventionally -- yes, there are indeed comparators and voltage references that will do an excellent job here. You aren't going to be making them discrete, they require transistors much too small to handle.Besides the individual parts, there are also combined ref+comp parts, and don't leave out reset generators and voltage monitors like these,http://ww1.microchip.com/downloads/en/DeviceDoc/20001906D.pdfQuite cheap, easy to use, self contained -- if the logic and threshold are right for your use, why not, right?Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 12, 2019, 04:11:25 pm
content: Check voltage supervisors out.Eg:http://www.ti.com/product/TPS3831

author: flimshaw
author_rank: Contributor
timestamp: July 13, 2019, 01:41:47 am
content: All good ideas, thanks! I'll grab a few of these and try them out. In terms of a comparator circuit for detecting a voltage threshold with very little current, how does something like this look? It's a bit rough, was just playing around in circuitJS, which only has open collector comparators. I'd probably do some kind of latch circuit on the right to just drain the storage cap once the threshold voltage is hit.Thanks again, all great feedback.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: July 13, 2019, 02:22:04 am
content: I think you should just use LTspice and get over with it. at least you could import actual spice models for comparators

author: NeverDie
author_rank: Regular Contributor
timestamp: June 22, 2020, 05:28:16 pm
content: Did you ever find what you were looking for?500na is actually an easy target to meet. Torex has a plethora of voltage detectors, some with quiescent current of 88na, some with 44na, and even one that consumes just 10na.I'd like to find a voltage trigger that consumes less than 1na in quiescent mode at low voltage (max 3v, but the lower the better). If not that, then a low voltage timer which consumes less than 1na.Anyone know of a circuit which can do that?i'm aware that TI has timers that consumes as little as 30na (I have some of those already), but I'm specifically interested in 1na or less (or as close to that as it's possible to get with whatever currently exists).