2.8 Test MOSFET Output

Check List · Step 8 of 12 · Measure each MOSFET terminal switching 0 V ↔ 24 V

🎯 Objective

The board has two MOSFET outputs, io.4-MOS-GND and io.5-MOS-GND. The test YAML maps GPIO.4 to mist coolant (M7) and GPIO.5 to flood coolant (M8); M9 turns both off. You'll confirm each terminal switches between 0 V and about 24 V with a multimeter while its indicator LED follows along. This is the first step that uses the multimeter — take it slowly.

🔌 How it works: each output is a low-side MOSFET. The pin name io.4-MOS-GND tells you it is the GND side that output io.4 switches. In normal use your load wires between Vmot-Main (+) and io.X-MOS-GND; turning the output ON connects io.X-MOS-GND to GND, so the full motor voltage (Vmot, ~24 V) appears across the load. With no load attached, the multimeter reads that voltage straight across the two terminal pins — about 0 V when off, ~24 V when on.

V5.88 Ultra board: the two orange MOSFET output terminals (io.4-MOS-GND / Vmot-Main and io.5-MOS-GND / Vmot-Main) and their GPIO.4 / GPIO.5 indicator LEDs

The two MOSFET terminals (right, orange) — each has an io.X-MOS-GND pin (top) and a Vmot-Main pin (bottom) — and their indicator LEDs (red circle, GPIO.4 / GPIO.5). (click to enlarge)

🔧 How to Measure

Set the multimeter to DC voltage, auto-range. On the orange io.4-MOS-GND terminal, put the probes across its two pins — top pin and bottom pin, exactly as in the photo:

RED (+) → Vmot-Main (bottom) BLACK (−) → io.4-MOS-GND (top)

With nothing switched on it reads about 0 V. (If the meter shows a minus sign, your probes are reversed — just swap them.)

Output 0 — io.4-MOS-GND terminal

M7 → 24.0 VGPIO.4 LED on

M9 → 0.0 VLED off

👈 Now move both probes to the io.5-MOS-GND terminal — same way: RED (+) on Vmot-Main (bottom), BLACK (−) on io.5-MOS-GND (top).

Output 1 — io.5-MOS-GND terminal

M8 → 24.0 VGPIO.5 LED on

M9 → 0.0 VLED off

✅ Pass: io.4 reads about 24 V with M7, io.5 reads about 24 V with M8, and each returns to about 0 V after M9. If both do, the MOSFET outputs are good.

⚠ Careful: Vmot-Main carries the full motor voltage (~24 V) and each output switches up to 500 mA maximum. Use an external relay or suitable power driver for heavier loads; overloading can burn the MOSFET. Don't let the two probe tips touch each other, and keep stray wire strands away from the terminal.

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