J

#### [email protected]

- Jan 1, 1970

- 0

have a circuit that uses a MOSFET that pretends it is a high wattage

fixed resistor. My application is a homemade RC relaxation oscillator

EDM machine. Since I am experimenting, I don't know what resistance

value I am ultimately going to need so I would like to make it

adjustable. I also wanted to try my hand at coming up with something

more elegant than using light bulbs as power resistors. I have a few

questions about circuit 1 that I copied from an internet source.

https://ilocker.bsu.edu/users/jalbers/WORLD_SHARED/ElectronicLoad.PDF

Is the math correct for circuit 1?

To behave like a fixed resistor I=V/R.

V+ = V * R1/(R1+R2) and V- = I * Rs

V+ = V-

V * R1/(R1+R2) = I * Rs

I = V / (Rs(R1+R2)/R1))

R = Rs(1+R2/R1)

Suppose that I want the circuit to behave like a fixed 20 ohm, 200W

power resistor for example. What would be logical choices for Rs, R1,

and R2? A source that I have been reading says to make R1>>Rs. What

does the >> mean?

I believe that Rs should be a very low value to keep the I^2*R power

dissipation down. Also it would have to be less than the total

resistance that the circuit is trying to mimic in the first place. Is

this true?

Why is the R1, R2 voltage necessary. Why wouldn't circuit 2 work?

Also there was some mention of adding an RC to the output of the op

amp to prevent oscillation. Could someone provide more details on

that?

Why wouldn't circuit 2 work?

Is the math correct for circuit 2?

To behave like a fixed resistor I=V/R.

V+ = V

V- = I * Rs

V+ = V-

V = I * Rs

I = V/Rs

R = Rs

NEVER MIND!

Rs would have to be the power resistor that I am trying to have the

circuit mimic in the first place.

Any help would be greatly appreciated. Thanks