Testing a MosFet

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Testing a MosFet

This testing procedure is for use with a digital multimeter in the diode test-range with a minimum of 3.3 volt over d.u.t. (diode-under-test)

testing a mosfet

Connect the 'Source' of the MosFet to the meter's negative (-) lead.

1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

2) First, touch the meter positive lead onto the MosFet's 'Gate'.

3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.

Such a simple test is not 100% -- but is useful and usually adequate.

When MOSFETS fail they often go short-circuit drain-to-gate. This can put the drain voltage back onto the gate where of course it feeds (via the gate resistors) into the drive circuitry, possibly blowing that section. It will also get to any other paralleled MosFet gates, blowing them also.
So, if the MosFets are deceased, check the drivers as well! This fact is probably the best reason for adding a source-gate zener diode; zeners fail short circuit and a properly connected zener can limit the damage in a failure! You can also add subminiature gate resistors -- which tend to fail open-circuit (like a fuse) under this overload, disconnecting the dud MosFet's gate.

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Menguji Mosfet

FET bentuk fisiknya seperti transistor. Fungsinya adalah untuk menaikkan tegangan atau menurunkan tegangan.

FET memiliki tiga kaki juga yaitu :

• GATE (G) adalah kaki input
• DRAIN (D) adalah kaki output
• SOURCE (S) adalah kaki sumber

Fungsinya biasanya digunakan pada rangkaian power supply jenis switching untuk menghasilkan tegangan tinggi untuk menggerakkan trafo.

Kakinya biasanya sudah pasti yaitu bila kita hadapkan FET ke arah kita maka urutan kakinya dari kiri ke kanan adalah GATE, DRAIN, SOURCE.

• Contoh FET penaik tegangan : K 793, K 1117, K 1214, IRF 630, IRF 730, IRF 620, dll.
• Contoh FET penurun tegangan : IRF 9610, IRF 9630, dll (biasanya 4 angka u/ IRF)