Constant current load design problem

[McMonster]

Inspired by Dave's constant current dummy load video (#102) I've decided to build my own, but beefier and with some more features (actually something like the original battery capacity logger).

I started from the basic circuit with a pot, opamp, logic level MOSFET and resistor on a breadboard (MOSFET on a heatsink, connected to the rest of the circuit using alligator clips cables, no high currents through the breadboard), just like on the video and here's where the strange thing happens. I've hooked this up to my bench power supply and powered the opamp from batteries, but I could only get up to about 1 amp from my PSU (up to 1.4 A closer to 30 V). Checking the voltages revealed that since the opamp is LM358 it could only go up to just over 3 V at the output from this battery pack to drive the gate of my IRLZ44N pass transistor. But it doesn't make sense, since the datasheet says at 3 V gate voltage I should get much higher drain current, around 20 amps.

I'm obviously missing something important here, but can you tell me what is it?

[vtl]

What is the value of your shunt resistor? There will be a voltage drop between the gate and source so you'll need a lot more than 3V at the gate for higher currents.
If you want to have higher currents but cant have such a high gate voltage youll need to lower the value of your current shunt. If you used a 0.1ohm resistor then you'll be able to get 1A from just 0.1V across the resistor.

Once you've fixed that, the next step is to watch out for oscillations in the opamp, this circuit seems to be prone to it.

[McMonster]

Shunt is already a 0.1 Ohm 5 Watt resistor.

[vtl]

Luckily I had the simulation on hand. It shows that youll need to have a gate voltage of 4.8v to get 0.3A across the shunt (this is to get 3A).  I've attached the LTpsice simulation if you feel like playing with it. You'll need to have a higher supply voltage for the opamp. You can always power the opamp from the load you are testing.

[Rufus]

Luckily I had the simulation on hand. It shows that youll need to have a gate voltage of 4.8v to get 0.3A across the shunt (this is to get 3A).  I've attached the LTpsice simulation

Try it with a model from IR, the model in LTSpice is obviously junk.

For the OP the MOSFET might be honking (oscillating).

[McMonster]

Luckily I had the simulation on hand. It shows that youll need to have a gate voltage of 4.8v to get 0.3A across the shunt (this is to get 3A).  I've attached the LTpsice simulation if you feel like playing with it. You'll need to have a higher supply voltage for the opamp. You can always power the opamp from the load you are testing.

You're right, I was just probing everything referencing to ground, I forgot about the voltage across the shunt resistor. Vgs was 2 V, this was barely an amp drain current and it's consistent with datasheet. I'll power it from a higher voltage when finished, I just hooked it all up to check if it's working at all and test the heatsink.

Thanks for help.