I want to make a 300W capable electronic load but in every schematic I saw a current sense resitor connected to an op amp. That means I need a 300W resistor .The 300W resitors are not very cheap. I searched and then found this schematic. What are the advantages and disadvantages of using an op amp and not using a current sensor.
There is very little energy dissipated in the current sense resistor, it is not the load, so as long as the current sense module is rated for the max current you will be sinking, it should be okay.
With the ACS712 you don't need a "current sense" resistor. (You might want a "current limiting" resistor, depending on your application.)
I assume "electronic load" means the MOSFET serves a variable load resistance... The MOSFET will have to dissipate ALL of the power and 300W seems like a lot. You might need more than 1 MOSFET.
If the MOSFET only needs to switch the load on/off, then you CAN use a load resistor and the load resistor can double as the current sensing resistor. When the MOSFET is off, almost no current flows. When its fully-on the resistance is low so it doesn't have to dissipate the full 300W, but you probably still need a heatsink.
...If you do go with a current sense resistor, 10A through 1-Ohm is too much voltage for the Arduino. If you are using an Arduino you can use the optional 1.1V reference you can get resolution of about 1mV which means you can get-by with a much lower resistor value.
Does you current need to be steady through the load or can it be pulsed?
If pulsed you can use PWM to control the FET and have minimal FET dissipation.
If the load is a heater, pulsing should be OK.
To 'sense' the average power you will need a voltage divider across the load into a RC filter and then the Arduino A/D.
By the way, do you access to both sides of the load. i.e. both terminals.
That will also depend on the accuracy required on the ohms value of the load and its stability over temperature (temp coefficient) . The heater proposed for example is likely to exhibit lower ohmic resistance when connected to power which will then decrease at it heats up.
Some years ago I built an electronic load which needed a stable sense resistor, 0.1Ω if my memory is correct.
It was cheaper to use 10 resistors of 1Ω /2W 1% each in parallel than buy a single 1Ω 20W resistor. This way you also get better total tolerance. (Probably 1% /10 ) , better heat dissipation (distributed) and easier to handle package. I was able to use SMD reistors