Electronics > Beginners

Mosfet as variable resistor?

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FriedMule:
I am trying to understand the Mosfet in general here, so no specific project.

I can see that a Mosfet has a open max resistance and a closet min resistance. Lets say 200 Ohm to 300M Ohm.
Since the gate is voltage dependent. Could I, If we ignore the non liniarity of the mosfet, adjust the resistance between 200 Ohm and 300M Ohm by changing the voltage to the gate?

If so, how fine would such a adjustment be, could it simply be done by turning a single turn pot by hand?

iMo:
Yes.
Yes.
Mind there are many types of mosfets available. Therefore different wirings exist in your case.

https://en.wikipedia.org/wiki/MOSFET

aheid:
Pretty much. This is the basis for many DC electronic loads.

If you look at the datasheet for a typical MOSFET, you'll see the graph over V_DS vs I_D for various V_GS voltages. If you consider a fixed gate-source voltage, you'll see that the drain current varies very linearly with drain-source voltage until it reaches saturation. That is what we'd expect from a fixed resistor.

You'll also find a graph over I_D vs V_GS for a given V_DS, which is also pretty linear. So you can change the resistance by changing the gate-source voltage.

As with BJTs, they're usually very sensitive (0.1V might result in >1A change in drain current) as well there being individual differences between devices in a batch. So you'll most likely want to use some form of active feedback to control the gate voltage, rather than directly driving it via a potentiometer.

In a basic electronic load, you measure the voltage across a shunt resistor and compare it to a reference voltage with an op-amp. The op-amp drives the gate to the voltage required to keep the two inputs equal (within its limits of course).


An important point though is that not all MOSFETs can handle such linear loads. That is, they're fine if they're fully off or fully on, but not partially conducting. The issue is that if a small part of the device heats up more than the surrounding parts, that part conducts more, and that causes it to heat up more, which makes it conduct more. This can cause a runaway situation (thermal runaway), leading to a catastrophic failure of the device.

This mostly has to do with their construction, AFAIK trench-type MOSFETs are generally not safe for linear loads, while planar MOSFETs are. You can get an indication from the SOA (safe operating area) graph in the datasheet, no DC line = don't use, but even those with DC lines shouldn't always be trusted.

MrAl:

--- Quote from: FriedMule on January 23, 2019, 12:17:09 pm ---I am trying to understand the Mosfet in general here, so no specific project.

I can see that a Mosfet has a open max resistance and a closet min resistance. Lets say 200 Ohm to 300M Ohm.
Since the gate is voltage dependent. Could I, If we ignore the non liniarity of the mosfet, adjust the resistance between 200 Ohm and 300M Ohm by changing the voltage to the gate?

If so, how fine would such a adjustment be, could it simply be done by turning a single turn pot by hand?

--- End quote ---

Hi,

Ideally you would want to measure the voltage across the drain-source and adjust the gate voltage as needed.

The problem with the MOSFET is it is not perfectly linear as you noted.  But it is also not perfectly stable either as to voltage at the gate vs current through the drain-source.  If you set the gate to say 5.1v and see 1 amp current through the drain-source, as it heats up the current could change quite a bit while the gate voltage stays stable at 5.1v and that makes it a little more difficult to control.

Almost all circuits like this take note of the variations in mosfet operating points so if the mosfet character changes a little bit the operating point of the entire circuit does not change that much, if in fact the circuit needs high stability.  The variations are always handled by using feedback, negative feedback which helps to keep the operating point stable and they usually do an excellent job because much has been written about this and ways found to do this well.  It would not be unusual to see 0.1 percent accuracy in the DC set point for example, and even that may not be the best example.

So dont expect to get too accurate of a control unless you use feedback.  Once you use feedback however, assuming you can get the circuit stable frequency wise, the accuracy will be hard to beat.

David Hess:
FETs in (JFETS, MOSFETs, whatever) general can be used as variable resistors.  One of the tricks to doing this over a wide range is to use a pair of FETs with one correcting for the non-linear Vgs versus Rds of the other.  A small amount of positive feedback from the drain to the gate may be used to improve nonlinearity.

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