Voltage change to signal a close in circuit

Thread Starter

michaeldalton

Joined Oct 17, 2022
2
Hello
I am hoping you can help.

I have a switch that has a different Resistance on each output one 900kOhm and the other 459KOhm
I would like to have a simple circuit that would close when the 900kOhm is selected,
The supply voltage is 5v but i have a 24v supply that can be accessed as well

My thinking was to use a Comparator to activate the circuit when the 900kOhm resister was selected, this voltage would then go into a Mosfet to then close the 5v circuit?

Is this logical?
 

WBahn

Joined Mar 31, 2012
27,875
Seems like a reasonable approach.

With resistances in the megaohm range, just be careful about using a comparator that has very low input bias currents.

Conceivably you could use a single resistor in series with your switch to create a voltage divider that drove your MOSFET gate directly. You would want the two voltage levels to straddle the threshold voltage of the FET. But this approach has some dragons involved. For one thing, the threshold voltage of off-the-shelf FETs are usually only spec'ed over a range of a couple volts. Second, it might be hard to get hard-on and hard-off gate voltages this way. Third, there's that high-impedance needing very low leakage currents on your FET (that is probably not a huge hurdle).
 

Thread Starter

michaeldalton

Joined Oct 17, 2022
2
Hello

So i finally got around to getting my FET, i went for the TN2106N3-G as it had a gate threshold of 0.6 Volts
As from the 5V source which i would like to switch the 2 Voltages from my resistors are 0.459 and 0.859
Unfortuneatly i cant get it to do what i want, below is my circuit.

1668175504481.png
 

WBahn

Joined Mar 31, 2012
27,875
How are you coming up with 0.459 V and 0.859 V from that circuit?

You need to use a voltage divider arrangement.

Where are you getting your 0.6 V gate-source threshold voltage value from? The data sheet shows that it can be anywhere from 0.6 V to 2.0 V (when diode-connected at a drain current of 1 mA).

How are you limiting the drain current?
 
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