Field-fit note – clearances and mounting
The 6AG1961-3BA21-4AX0: Snaps onto standard 35 mm DIN rail (EN 60715) — no tools, no backplate mods. The enclosure is 70 mm wide, 125 mm high, 120 mm deep. For natural convection cooling, allow 50 mm clearance above and below the unit. You can butt it left and right against adjacent units (0 mm clearance needed on those sides). Screw terminals on the input side accept 0.33 to 10 mm² conductors; output and ground use the same. Auxiliary relay contact terminals take 0.5 to 2.5 mm². That covers standard panel wiring without adapter blocks.
Ratings you'll lean on in the field
At 25 °C ambient it delivers the full 40 A at 24 V DC (typical output power 34 W). Above 60 °C, derate 3% per Kelvin up to 70 °C max. At high altitude (2000–6000 m above sea level), knock off 7.5% per 1000 m or drop ambient temperature 5 K per 1000 m — whichever fits your install site. Output regulation is tight: Vout = Vin – approx. 0.5 V, so keep input between 24 and 28.8 V DC. The relay contact (changeover, 8 A/240 V AC / 24 V DC) signals OK when both input voltages exceed the switching threshold (adjustable 20–25 V). Green LED for good, red for fault. Galvanic isolation is SELV per EN 60950-1. No-load power draw max 1.5 W, meaning it's not burning standby watts on your panel control circuit.
Compliance lock – what certs ride with it
CE marked, IP20 finger-safe enclosure, Class III operating resource. Emissions per EN 55022 Class B (suitable for residential and light industrial). Immunity per EN 61000-6-2. If your spec demands these, the part has them on the label.