Looking for a mosfet(?) very low voltage.

Thread Starter

Chefslot

Joined Sep 24, 2018
19
I have a board that has two LED's on separate circuits, ran by the same 'intelligent' driver (it might not be intelligent). The LED's have resistors before them but I can not see what resistance they occupy.
The traces from the LED's appear to be going into a micro controller of sorts breaking the connection turning off the LED.

One Yellow LED, that reads after the LED (only place i can access) .25v (250mv).
One Red LED that reads after the LED 1.65v.

The Red one stays ON all the time, and ideally I only need it ON when the yellow one is ON, more for aesthetics.
I was thinking about using a transistor of some sorts to act as a switch, turning off the Red LED when the Yellow LED goes out.

So my understanding is that I need the gate to be around 250mv and ideally as low of a current draw as possible so it doesn't upset anything.
Is this even possible?

Regards
 

dl324

Joined Mar 30, 2015
9,602
So my understanding is that I need the gate to be around 250mv and ideally as low of a current draw as possible so it doesn't upset anything.
Your understanding of your problem and it's solution is wrong. I did a parametric search on Mouser for low threshold voltage MOSFETs and there was only one with a threshold voltage of 300mV or less. When I checked the datasheet, it was actually 0.3V-1V. You should also understand that threshold voltage is the voltage at which the device starts turning on and Vth isn't tightly controlled. For switches, you would operate at a higher voltage to have lower on resistance.
upload_2019-8-30_20-32-25.png

There are manufacturing processes that have lower threshold voltages, but not in discrete parts.

If you post the circuit in question vs. your suggested solution, someone might be able to help you.
 

crutschow

Joined Mar 14, 2008
24,109
A comparator, such as a LM339 or LM393, could be used to detect the low yellow LED voltage and turn the red LED off when the yellow goes off.
The LM339 typically draws only 0.8mA power.
It can be configured to change output states for an input voltage within a few mV of an arbitrary set reference voltage (in this case you would probably use about a 150mV-200mV reference).

You need to determine how much current the red LED draws.
If it's no more that 6mA, the comparator can drive the LED directly.

What is the available power supply voltage?
 
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