Device with constant voltage drop in the mA to uA range
John Duffy wrote 09/02/2023 at 05:10 • -1 pointDoes anyone know of a part that will drop a constant voltage (shooting for about 2 or 3V) while passing ~3uA through ~5mA. So like a zener would work except that below a few hundred uA they tend to start to drop towards 0V drop. Trying to avoid using an opamp for this, but could use a fet or something.
Doesn't have to be super well controlled, so if it's like 2V at 5uA and 3V at 5mA that's fine, I just need a couple volts drop to have overhead for something.
Update: For this particular application, I ended up replacing this with a source follower mosfet, didn't do quite the same thing (it drops a variable amount and maintains a constant output voltage), but turned out cheaper and to work better in this application.
Details here: https://hackaday.io/project/176607-hydrameter/log/223270-schematic-and-operation
in the ohms section, referring to Q1.
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It's not the passing current (uA) or voltage drop (2-3V) that is your issue but what load will be taken from the voltage source. In electronics we like the "Bleed current" to be ~20 times the load current, so as a two resistor "Voltage Divider" or "Potentiomenter" i.e. two resistors in series across a stable voltage, is loaded, the voltage will become less stable as the load current is drawn from the joint between the two resistors changes. That is because the load current must come from one of the two resistors meaning that they then draw different current, and that difference causes the voltage drop across each to change, and the tapped off voltage will also change. So mA or uA, or nanoAmps in the voltage source cannot provide much current to the load before that voltage changes!
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It's not the passing current (uA) or voltage drop (2-3V) that is your issue but what load will be taken from the voltage source. In electronics we like the "Bleed current" to be ~20 times the load current, so as a two resistor "Voltage Divider" or "Potentiomenter" i.e. two resistors in series across a stable voltage, is loaded, the voltage will become less stable as the load current is drawn from the joint between the two resistors changes. That is because the load current must come from one of the two resistors meaning that they then draw different current, and that difference causes the voltage drop across each to change, and the tapped off voltage will also change. So mA or uA, or nanoAmps in the voltage source cannot provide much current to the load before that voltage changes!
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A device that maintains a constant voltage drop in the milliampere (mA) to microampere (uA) range.
That is exactly what I was looking for!
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You have the option of utilizing just one MAX6008. This component functions as a 2.5V shunt reference and operates with a mere 1uA current. Although the maximum specified operating current is 2mA, the datasheet illustrates a typical graph showcasing it running at 5mA. The absolute maximum operating current is 20mA, and it's reasonably priced, costing less than $1.
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You could use a single MAX6008. It is a 2.5V shunt reference with a 1uA operating current. Max specified operating current is 2mA, but datasheet shows typical graph with it operating at 5mA. Abs. Max operating current is 20mA. It's pretty cheap < $1.
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REF1112 could do it. Voltage is 1.25V, so you'd need 2 in series.
https://www.ti.com/product/REF1112
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Absolute legend, thanks Paul!
That is exactly what I was looking for!
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