You can test an IGBT with a standard digital multimeter set to diode test mode. The process checks three things: the built-in freewheeling diode between the collector and emitter, the electrical isolation of the gate, and the basic switching function. A working IGBT will show predictable voltage drops across its terminals, while a failed one typically reads as a dead short or completely open.
Safety Before You Start
IGBTs are found in high-voltage circuits: variable frequency drives, inverters, welding equipment, and induction heaters. Before you touch anything, power down the equipment and follow lockout/tagout procedures. The critical step most people overlook is discharging the DC bus capacitors. These capacitors can hold a lethal charge for minutes or even hours after power is removed. Use a discharge resistor or grounding stick to bleed the stored energy, then verify zero voltage with a meter before making contact with any terminals. Wear voltage-rated gloves if you’re working within about a foot of capacitor terminals.
Once the circuit is confirmed dead, you can either test the IGBT in place or remove the module. Testing in-circuit is faster, but other components on the board can sometimes give misleading readings. If your results seem off, pull the module and retest it isolated.
Tools You Need
A digital multimeter with a diode test function is all you need for basic go/no-go testing. Most handheld meters sold today include this mode. For deeper diagnostics, like evaluating switching speed and waveform quality, you’d need an oscilloscope and a gate driver circuit, but the multimeter covers the vast majority of field troubleshooting.
Identify the Pins
Every IGBT has three terminals: Gate (G), Collector (C), and Emitter (E). On discrete packages, these are usually labeled on the component or in the datasheet. On larger modules (the kind found in industrial drives), the terminals are clearly marked on the module housing. Inside the device, the main switching element sits between the collector and emitter, and a freewheeling diode is wired in parallel across those same two terminals, with its forward direction pointing from emitter to collector.
Step 1: Test the Freewheeling Diode
Set your multimeter to diode test mode. This mode pushes a small current through the junction and displays the forward voltage drop.
Forward bias: Place the red (positive) probe on the Emitter and the black (negative) probe on the Collector. A healthy diode will display a forward voltage drop between 0.3V and 0.7V. The exact number varies by model, but it should fall within that range.
Reverse bias: Swap the probes, red on Collector and black on Emitter. The display should read “OL” (overload/open line), meaning no current flows in the reverse direction.
If you see “OL” in both directions, the diode is open, and the IGBT is likely failed. If you get a very low reading (near zero) in both directions, the collector-emitter path is shorted, which is the most common IGBT failure mode.
Step 2: Check Gate Isolation
The gate on an IGBT is electrically insulated from the collector and emitter by a thin oxide layer. In a healthy device, no current should flow between the gate and the other two terminals in either direction.
Still in diode test mode, place one probe on the Gate and the other on the Emitter. You should see “OL” regardless of probe polarity. Then repeat between the Gate and Collector. Again, “OL” in both directions.
Any voltage reading here means the gate oxide is damaged. This kind of failure sometimes shows up as unusual gate leakage current or a shift in the voltage needed to turn the device on. Once the gate insulation breaks down, the IGBT can no longer be reliably controlled and needs replacement.
Step 3: Verify Switching Function
This test confirms the IGBT can actually turn on and off. It works because your multimeter’s diode test mode supplies enough voltage to partially charge the gate.
Start by shorting the Gate and Emitter pins together momentarily (touch both with a piece of wire or a screwdriver). This ensures the gate has no residual charge, putting the IGBT in its off state. Now place the red probe on Collector and black on Emitter. You should see “OL,” confirming the device is off.
Next, charge the gate: touch the red probe to the Gate and the black probe to the Emitter for a second or two. This applies a small positive voltage to the gate. Now move the red probe back to the Collector (keep black on Emitter). On many IGBTs, you’ll see a voltage drop reading instead of “OL,” indicating the device has turned on. The reading may be higher than a normal diode drop because the multimeter’s test current is low.
Finally, short the Gate and Emitter together again to discharge the gate. Recheck collector to emitter. It should return to “OL.” If the device stays conducting after you discharge the gate, or if it never turns on at all, it has failed.
Not every IGBT responds cleanly to this test. Some high-voltage modules have gate threshold voltages that exceed what a standard multimeter can supply. A typical IGBT turns on with a gate voltage around 15V in normal operation, but the threshold where conduction begins is lower. If your meter can’t trigger switching, it doesn’t necessarily mean the device is bad. The diode and gate isolation tests are more reliable indicators.
What Failure Looks Like
Most IGBT failures fall into a few recognizable patterns when tested with a multimeter:
- Collector-emitter short: Near-zero reading in both directions between C and E. This is the most common failure, often caused by overvoltage events that punch through the semiconductor. It can also result from latch-up, a condition where the internal structure locks into permanent conduction and the gate loses control of current flow.
- Open circuit: “OL” in every direction on every pin combination, including where the diode should show a forward drop. The device has physically disconnected internally, often from a bond wire lifting off due to thermal cycling.
- Gate damage: Any measurable voltage drop between the Gate and either the Collector or Emitter. The insulating oxide layer has failed. This can happen gradually from electrical stress or suddenly from static discharge or voltage spikes.
If you’re testing a module with multiple IGBTs inside (common in three-phase inverter packs), test each IGBT section individually. It’s not unusual for one to fail while the others remain functional.
Oscilloscope Testing for Switching Health
A multimeter tells you whether an IGBT is dead or alive, but it can’t reveal degradation. For that, you need an oscilloscope to observe the switching waveforms while the device operates in a test circuit or during live operation.
The key waveform to watch is the gate-emitter voltage during turn-on. A healthy IGBT shows a distinct plateau in the middle of the voltage rise, called the Miller plateau. During this flat section, the gate voltage holds steady while the collector-emitter voltage drops rapidly. The collector-emitter voltage falls at a rate determined by the gate drive current and the internal capacitance between the gate and collector. If this plateau is missing, distorted, or shifted significantly from the datasheet profile, the IGBT’s internal characteristics have changed.
During turn-off, the collector-emitter voltage rises in a characteristic pattern: slow at first (while the internal capacitance is large at low voltage), then accelerating as voltage increases and the capacitance decreases. This “slow then fast” rise is normal. Abnormally slow switching, excessive ringing, or voltage spikes that approach the device’s rated maximum are signs of trouble, either in the IGBT itself or in the gate drive circuit.
For most repair and maintenance situations, the multimeter tests are sufficient. Oscilloscope analysis becomes worthwhile when you’re diagnosing intermittent faults, qualifying replacement parts, or monitoring equipment health in a preventive maintenance program.