Hello everyone, I have been testing a mosfet which has been desoldered out of circuit and the readings are generally okay and in line with expectations. But I wasnt able to get the expected reading for the Drain-Source On-State Resistance. The steps that I have followed are as follows (which are based on the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17 ):-
(i) Trigger the MOSFET by setting the multimeter at diode mode then fix the meter black (negative) probe to the drain and touch the red one momentarily to the gate. This should trigger the gate (as shown in figure below). The MOSFET must turns ON by this.
(ii) Take an ohmmeter and check the drain to source resistance, Rds(on). This time the reading should be very low (zero or approximately zero) than the previous Rds(off) reading. This will confirm that the MOSFET is in good condition. Refer to the datasheet of the MOSFET to check the value of drain to source resistance in on condition, Rds(on) and compare it with the observed value. If the observed value has much variation with that specified by the datasheet, the MOSFET is faulty.
I have tested this many times and the Rds(on) is measuring several 10s of thousands ohms instead of the 0.47 ohms stated in the datasheet. I am sure that the MOSFET has been turned on by step (i) because when I check under diode mode there is voltage difference in both directions between the gate and drain which only happens when the MOSFET is turned on. Once I discharged the gate pin, the MOSFET is turned off and diode test will only show voltage difference in one direction only. So, I am sure the MOSFET is working in terms of turning on and off. But despite that, I dont know why I am still measuring 10s of thousands of ohms resistance between drain and source while it is in an "on" state.
Is there something wrong with the MOSFET or the tests does not work with every MOSFET? The full set of steps are in the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17/
Thanks!
(i) Trigger the MOSFET by setting the multimeter at diode mode then fix the meter black (negative) probe to the drain and touch the red one momentarily to the gate. This should trigger the gate (as shown in figure below). The MOSFET must turns ON by this.
(ii) Take an ohmmeter and check the drain to source resistance, Rds(on). This time the reading should be very low (zero or approximately zero) than the previous Rds(off) reading. This will confirm that the MOSFET is in good condition. Refer to the datasheet of the MOSFET to check the value of drain to source resistance in on condition, Rds(on) and compare it with the observed value. If the observed value has much variation with that specified by the datasheet, the MOSFET is faulty.
I have tested this many times and the Rds(on) is measuring several 10s of thousands ohms instead of the 0.47 ohms stated in the datasheet. I am sure that the MOSFET has been turned on by step (i) because when I check under diode mode there is voltage difference in both directions between the gate and drain which only happens when the MOSFET is turned on. Once I discharged the gate pin, the MOSFET is turned off and diode test will only show voltage difference in one direction only. So, I am sure the MOSFET is working in terms of turning on and off. But despite that, I dont know why I am still measuring 10s of thousands of ohms resistance between drain and source while it is in an "on" state.
Is there something wrong with the MOSFET or the tests does not work with every MOSFET? The full set of steps are in the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17/
Thanks!