simulate wire resistance with mosfet

[mazur50]

How can i simulate wire resistance with a mosfet. I want to use it so i can see voltage drop as if i was going to add wire to a project.

[Chippie]

Drain-Source resistance changes with applied gate voltage....effectively a voltage controlled resistor...

[JohnMuchow]

As Chippie mentioned, you can make a MOSFET into a resistor by changing the gate voltage (actually, keeping it within a certain range). Hook up a pot between the source voltage and drain voltage for the FET and have the tap go to the gate through a 100 ohm resistor. As you vary the voltage to the gate, via the pot, you'll see the current through the FET change.

Be sure you take into account the power being dissipated in the MOSFET!! If you drop the voltage by, lets say 2V, and there's 5A of current flowing through the FET, you need to dissipate that 10W of heat or your FET is toast. Operating a MOSFET in its "linear region", as you're doing (i.e., not switching it hard on/off), creates a ton of heat.

[jeffs555]

The resistance vs gate voltage is very temperature sensitive for most mosfets, and is also usually non-linear. You will have to measure the current through and the voltage drop across the mosfet and readjust the gate voltage as the mosfet heats up. If you are going to adjust it manually, you might be better off with a rheostat or just a length of bare wire and a slider or clip or fixed taps.

[JohnMuchow]

Good points, thanks!

[mazur50]

you mean like this.

[Acetronics]

Hi,

I think Elektor magazine has released a Constant current/ Constant Resistance High Power Load project, using a Mosfet as ballast ... some years ago.

Seen other similar projects on the Web.

But a Mosfet with a couple of resistors around is just a view of mind ...

Alain

[Chippie]

Like Alain says, there was a project for a variable load based on mosfets...There is a project on here if you search the electronics section...I've attached a cct..Hope it helps.

[jeffs555]

That circuit is for a constant current load which would not simulate a long wire.

It should however be simple to modify it into a constant resistance load. Just eliminate the range switch and everything above it, then connect the top of R6 to the drain of the mosfet. That will create a feedback loop that compares the voltage drop across the fixed 0.1 ohm resistor to the total voltage drop scaled down based on position of R6.