How Do You Know If a MOSFET Is Blown?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is likely blown if it exhibits a short circuit, an open circuit, or fails to switch correctly, which can often be determined through visual inspection and specific multimeter tests. Identifying a damaged MOSFET is crucial for diagnosing and repairing electronic circuits.

Visual Inspection: First Clues

Before even touching a multimeter, a visual inspection can often reveal a blown MOSFET. Look for the following signs:

Physical Damage: Cracks, bulges, or chips on the MOSFET's casing.
Discoloration: Burn marks, charring, or excessive darkening around the component or its pins, indicating severe overheating.
Burnt Smell: A distinct burning odor emanating from the circuit board, often associated with components that have failed due to overcurrent or overheating.

Multimeter Testing: The Definitive Check

The most reliable way to confirm if a MOSFET is blown is by testing it with a digital multimeter (DMM). It's generally best to remove the MOSFET from the circuit board for accurate testing, as other components can interfere with readings.

1. Diode Test Mode

This is one of the most effective methods for quickly assessing a MOSFET's health, particularly its internal body diode.

N-Channel MOSFET Test:
1. Switch your multimeter to diode test mode (often indicated by a diode symbol).
2. Place the red (positive) probe on the Source (S) terminal.
3. Place the black (negative) probe on the Drain (D) terminal.
4. Healthy MOSFET Indication: A healthy N-channel MOSFET will typically display a voltage reading between 0.45VDC and 0.65VDC. For example, a reading of 0.569V indicates the MOSFET is likely good. This reading represents the forward voltage drop of the internal body diode.
5. Blown MOSFET Indication: If the reading is close to 0V, it strongly indicates that the MOSFET is damaged, likely short-circuited between the Source and Drain. An "OL" (open loop) or very high reading might suggest an open circuit.
6. Reverse the probes (red to Drain, black to Source); the multimeter should show "OL" or a very high reading, indicating no conduction.
P-Channel MOSFET Test:
1. In diode test mode, place the red (positive) probe on the Drain (D) terminal.
2. Place the black (negative) probe on the Source (S) terminal.
3. Healthy MOSFET Indication: A healthy P-channel MOSFET will also show a voltage reading typically between 0.45VDC and 0.65VDC.
4. Blown MOSFET Indication: A reading close to 0V or "OL" suggests damage.
5. Reverse the probes; it should show "OL".

2. Resistance Test Mode

Using the resistance mode can help identify shorts or opens across the MOSFET's terminals.

Gate-Source (G-S) Test:
1. Switch your multimeter to resistance mode (Ω).
2. Place one probe on the Gate (G) and the other on the Source (S).
3. Healthy MOSFET Indication: You should see a very high resistance reading (often "OL" or in the megaohms), as the gate is isolated by an oxide layer.
4. Blown MOSFET Indication: A low resistance reading (close to 0 ohms) indicates a short circuit between the gate and source, meaning the MOSFET is likely damaged.
Gate-Drain (G-D) Test:
1. Repeat the resistance test between the Gate (G) and Drain (D).
2. Healthy MOSFET Indication: Similar to G-S, you should see a very high resistance reading.
3. Blown MOSFET Indication: A low resistance indicates a short.
Drain-Source (D-S) Test:
1. Repeat the resistance test between the Drain (D) and Source (S).
2. Healthy MOSFET Indication: With no voltage applied to the gate, the MOSFET should be off, so you should see a very high resistance reading ("OL").
3. Blown MOSFET Indication: A low resistance reading (close to 0 ohms) indicates a permanent short circuit, which is a common failure mode. An "OL" might indicate an open circuit.

3. Continuity Test Mode

The continuity test is a quick way to check for shorts.

Switch your multimeter to continuity mode (beeper symbol).
Touch the probes across any two terminals (G-S, G-D, D-S).
Healthy MOSFET Indication: The multimeter should not beep, indicating no direct short circuit.
Blown MOSFET Indication: A continuous beep between any two terminals signifies a short, confirming the MOSFET is damaged.

Summary of Multimeter Test Results

Test Type	Probe Connection	Healthy MOSFET (N-Channel)	Blown MOSFET (N-Channel)
Diode Mode	Red to Source, Black to Drain	0.45V - 0.65V (e.g., 0.569V)	Close to 0V (short) or OL (open)
	Red to Drain, Black to Source	OL (Open Loop)	Close to 0V (short) or OL (open)
Resistance	Gate to Source (G-S)	Very High (OL/Megaohms)	Low resistance (short)
	Gate to Drain (G-D)	Very High (OL/Megaohms)	Low resistance (short)
	Drain to Source (D-S)	Very High (OL/Megaohms)	Low resistance (short)
Continuity	Any two terminals	No Beep	Beep (short circuit)

Note: For P-channel MOSFETs in diode mode, reverse the main test (red to Drain, black to Source).

4. Gate Threshold Voltage Test (Functionality Test)

This test confirms if the MOSFET can turn on and off properly. It's more involved but useful for confirming functionality beyond just shorts/opens.

Connect the multimeter in resistance mode (e.g., 200Ω range) across the Drain and Source terminals. You should see "OL" initially.
Briefly touch the positive (red) probe to the Gate and the negative (black) probe to the Source to "charge" the gate and turn the MOSFET on.
Now, reconnect the probes to the Drain and Source. The resistance should drop significantly (to a few ohms or less, representing R_DS(on)).
To turn it off, briefly touch a finger or a metal wire between the Gate and Source to discharge the gate.
Re-measure Drain to Source. The resistance should return to "OL" or very high.
Blown MOSFET Indication: If the resistance does not drop when the gate is charged, or does not return to high resistance when discharged, the MOSFET is likely faulty.

In-Circuit Testing Limitations

While possible, testing a MOSFET while it's still in the circuit can be misleading due to parallel components or power supply paths. Other components can provide alternative paths for current, skewing multimeter readings and making it appear as if the MOSFET is fine or shorted when it's not. For definitive results, always remove the MOSFET from the circuit board for testing.

Symptoms of a Blown MOSFET in a Live Circuit

Beyond direct testing, a blown MOSFET can manifest in various ways within an active circuit:

Complete Circuit Failure: The device might not power on at all.
Malfunction or Reduced Performance: The circuit might operate erratically, inefficiently, or only partially.
Overheating: Other components, or the entire board, may get unusually hot due to a shorted MOSFET drawing excessive current or preventing proper regulation.
Smoke or Arcing: In severe cases, a blown MOSFET can lead to visible smoke or electrical arcing.
Tripped Circuit Breaker/Blown Fuse: A shorted MOSFET will often draw a very high current, causing protective devices to activate.

By combining visual inspection with comprehensive multimeter testing, you can accurately determine if a MOSFET is blown and needs replacement.