Hi Everyone.
I did a test on a IFRZ44 Mosfet and measure a voltage of 1.5V across the Drain and Source when the Mosfet is supposed to be fully switched on and the amps thru the Load is 900mA.
Rds(on) is 10V and the Drain Source resistance should be 0.022 OHM but this is not what I found.
When I short the Drain and Source the Load now draws 1A on 12v .
What did I do wrong?
Thanks for replying.
This is my test circuit.
When the gate is at 0v is the load current at 0A ?
When the gate is at 12v is the load current at 1A ?
Circuit Simulator Applet (falstad.com)
$ 1 0.000005 10.20027730826997 50 5 50 5e-11
f 304 240 352 240 32 1.5 0.02
w 352 256 352 304 0
w 352 224 352 176 1
172 176 176 176 128 0 7 12 12 0 0 0.5 Drain Voltage
g 352 304 352 320 0 0
w 176 176 192 176 0
r 192 176 352 176 0 12
174 192 176 192 304 1 1000 0.16340000000000002 Resistance
w 304 240 208 240 0
g 192 304 192 320 0 0
p 304 240 272 272 1 0 0
o 0 64 0 4099 20 0.8 0 2 0 3
When the Gate is 0V the load current is 0A.
When the GATE is 12V the load current increase a little but when I short the D and S the load current is 1A so there is voltage lost over D and S junction.
Gate is 12V and voltage across D and S is 1.48V.
it seems to me that the mosfet is defective or fake
12V - 1.48V = 10.52V
Current should then be = 10.52 ÷ 12 Ω = 877mA
Is the MOSFET heatsinked?
Hopefully, you are not doing your tests on a breadboard.
Also your resistor is also an inductor.
@mrburnette
Why not 1A in simulation ? faulty Mosfet 
@b707
Mosfet is brand new still in plastic tube also not from some fake Chinese shop.
Got a hand full of them will do some more tests.
@LarrD
With my multimeter in series on the 20A Scale I get 880mA.
@ZX80
No breadboard.
Inductor on DC? When I short the D and S I get 1A , and the load is then not a inductor?
@anon57585045
So many replies I missed this one .
Yes BIG heatsink.
Show photo of the circuit. Either the MOSFET is damaged or your circuit does not match your schematic.
How about some photos? You managed one.. if you do that, so many replies won't be necessary. 
Can kill a mosfet.
12V on multimeter is the gate voltage -blue wire
Neg from psu to source - white wire
Drain to load - red wire
Hope this helps.
GMT +2 here , will continue tomorrow.
That's a strange looking pot. Did you confirm the pin outs?
Those thin crocodile wires may be the reason. Did you try to measure directly at MOSFET pins? I can imagine the wires drop 1.5 V carrying 1 A.
The pot is a multi turn 20k pot with a small pin drawing on the pot , so yes the pins is confirmed I can adjust from 0V up to the supply voltage.
I did measure direct on the Mosfet pins and I did mention that shorting out the D and S the amps go up to 1A.So the drop in the wires is not causing the lost ,the lost is1.5V direct over the mosfet D and S.
Update:
I replaced the IFRZ44 twice with new ones and get the same result.
Then I replace the IFRZ44 with IRF3205 and now I get 1A thru the load with only 10mV Loss over the D and S.
Maybe its just a typo but you refer to the IFRZ44
but I assume you mean this:
https://www.mouser.co.uk/datasheet/2/427/irfz44-1768892.pdf
Showing your test setup is working fine, but the "IFRZ44" MOSFETS are not working as they should.
So I cant see any answer other than they are faulty or they are another type that is incorrectly marked.
It appears you are using the POT to "sweep" the gate voltage from 0 to 12V. However doing this there will be a period the MOSFET is in its linear range. When it is this mode, the power dissipated in the MOSFET could be as much as 6V * 0.5A = 3 watts. This will likely make the MOSFET hot (and not happy) I can't say if it will damage the device but is is not a good condition.
You should use a switch to control the gate.
