![]() Thus it’s best to check the drive circuits of dead MOSFETs. The overload will also affect any other paralleled MOSFET gates. This can put the drain voltage back onto the gate where it feeds (via the gate resistors) into the drive circuitry, possibly causing voltage and current levels exceeding the limits of components in that section. MOSFETS that fail often go short-circuit drain-to-gate. The gate will discharge and the meter reading should go high, indicating a non-conductive device. With the meter positive lead still connected to the drain, short the source and gate. The MOSFET’s internal capacitance on the gate has now been charged up by the meter and the device is ‘turned-on’. Now move the positive probe to the ‘Drain’. (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 terminals until needed.) Touch the meter positive lead onto the MOSFET Gate. ![]() Connect the Source of the MOSFET to the meter’s negative lead. where the device is non-conductive with 0 V applied to the gate, operating like a switch). First consider the procedure for testing an enhancement-mode MOSFET (i.e. The usual testing procedure is for use with a digital multimeter in the diode test-range with a minimum of 3.3 V over d.u.t. A common multimeter can quickly do in-circuit tests, which are not totally definitive but generally provide acceptable go/no-go information, using either the meter’s diode-check or ohms mode. ![]() The sensible alternative is to test the transistor. ![]() Also, it is not reliable because an outside defective component can instantly destroy the replacement with no visible evidence. One approach to troubleshooting is to substitute a known good component, but that is a costly way to go. A bad transistor can sometimes be detected by its partly burned or distorted appearance, but more often there is no visible indication. ![]()
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