# threshold voltage of a MOS transistor

Discussion in 'General Electronics Discussion' started by Emam, Jan 15, 2015.

1. ### Emam

63
4
Jul 7, 2014
Hello,
I have a basic question:
I would like to realize a switch (an interruptor) in a circuit, using a MOS transistor.
I need to change the state of the switch, with a very low voltage (for ex. 1 V, or even 0.5 V).

In other word:
if I have input signal =0 --> switch OFF
input signal =0.5 or 1 V --> switch ON.

Could you please tell me if this is possible.
Becuase I know that standard MOS are functionning with 3 V or 5 V as threshold.
Bests regards

5,165
1,087
Dec 18, 2013
It will depend on how much current you want to control. Tell us more about your project and what you want to do.

3. ### BobK

7,682
1,688
Jan 5, 2010
Be aware that the threshold voltage is where the MOSFET just begins to pass any current, typically rated as the voltage at which 250uA will flow. The MOSFET cannot be considered ON at the threshold voltage (unless you only want 250uA or less).

I think what you want is a comparator that can switch at voltages close to 0 probably driving a MOSFET.

Bob

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4. ### Emam

63
4
Jul 7, 2014

In fact, the input of the switch is connected to a microcontroller (so I guess with low values of currents such as few mA).
All I want is that when the microcontroller sends a pulsel of 0.5 or 1 V amplitude ( duration few miliseconds), the switch turns ON.
And when the microcontroller sends nothing, the switch turns OFF.
The output of the switch is connected to a load resistance of 5 ohm.

5. ### BobK

7,682
1,688
Jan 5, 2010
So it is connected to an analog output on the microcontroller?

When you say "sends nothing" do you mean the output is at 0V?

What happens when the output is at 1.5V, 2.7V or are these not possible?

Do you have control over the programming on the microcontroller? If so, is there an extra pin you can use?

Bob

6. ### Emam

63
4
Jul 7, 2014
in fact the switch is connected to a programmable voltage DC generator which can gives values of 0.5 V, 1 V or other values by programming the microcontroller, this is the logic:
microcontroller ---> programmable DC generator --> switch

Yes by "send nothing" I mean 0V.
Its possible to have output such as 1.5V or 2.7V, we can have every value we want by programming the microcontroller.
I dont have any other pins of the microcontroller. I dont have any controls on it.

5,165
1,087
Dec 18, 2013
So what value are you sending out? 0.5V, 1.5V, 2.7V or all of them. What do you want the circuit to do when it receives the voltage? What are trying to do?

8. ### BobK

7,682
1,688
Jan 5, 2010
Okay, so the DC output is the only signal you have access to, and it can vary between 0V and some limit you have not specified. You have only told us what you want to do on 3 of these voltages, 0V, 0.5V and 1.0V. What is supposed to happen on any other voltage?

And when you say 0.5V or 1.0V what do you really mean? It cannot mean exactly those voltages because it will always be off by some amount. If you really mean that you only want the switch on for those 2 voltages, you have to supply a tolerance, I.e how close to those voltages must it be to activate the switch. Is it activated at 0.49V or at 0.51V?

I expect that what you really mean is that it should be activated for any voltage above some lower limit, but that is not what you are saying.

Writing a specification is often more difficult than it seems.

We are not trying to be mean here. Engineers often (maybe always) get specifications that are not good enough to ensure we deliver what is wanted. Getting clarification is a major part of our job.

Bob

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