Inverter Board IPM Transistor Resistance Audit
Before condemning an expensive compressor, you must prove the IPM is sound. Every IGBT has a “free-wheeling” diode wired in parallel with it. By setting your multimeter to Diode Check Mode, you can measure the voltage drop across these six internal diodes without powering the board. A healthy diode drops around 0.400V to 0.600V. A shorted transistor will read 0.000V.
The 6-Step Diode Bench Test
Ensure the board is completely disconnected from main power and the DC capacitors are discharged. Set your meter to Diode Mode (the symbol looks like an arrow with a line across the point).
| Testing Sector | Red Lead (Positive) | Black Lead (Negative) | Expected Result |
|---|---|---|---|
| High Side U, V, W | Place on U, then V, then W | Place on P (DC+) | 0.350V to 0.700V |
| Low Side U, V, W | Place on N (DC-) | Place on U, then V, then W | 0.350V to 0.700V |
| Dead Short Result | Any Position | Any Position | 0.000V (Continuous Beep) |
Diagnostic Gateway Challenge
You are working on a Daikin VRV that instantly blows the 30-Amp main board fuse the second the contactor pulls in. You pull the board and test the IPM. Placing the red lead on U and the black lead on P results in a reading of 0.002V and a solid tone from your meter. What has happened?