 ### Network # Need help with a MOSFET electronic load

J

#### [email protected]

Jan 1, 1970
0
I have been looking at some electronic load circuits. I would like to
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.

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

D

#### default

Jan 1, 1970
0
I have been looking at some electronic load circuits. I would like to
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.

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

Look at gain versus temperature and you'll see the light. Self
heating changes gain and in turn, changes current.

P

#### Phil Allison

Jan 1, 1970
0
I have been looking at some electronic load circuits. I would like to
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.

** I hope you realise that all power MOSFETs have inbuilt diodes in
parallel.

So can never behave like a physical resistor.

..... Phil

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