PS 405 for S7-400: Backplane Power Delivery
The Siemens 6ES7405-0DA02-0AA0 is the PS 405 power supply for the SIMATIC S7-400 rack. It accepts 24, 48, or 60 V DC input and delivers regulated 5 V DC at up to 4 A to the backplane bus, with no base load required. It also supplies up to 0.5 A at 24 V DC to the backplane, idling-proof — meaning the 24 V bus stays up even if the 5 V rail is unloaded. The unit occupies one slot in the S7-400 rack. Typical power loss is 16 W, with active power input of 48 W. Inrush current peaks at 18 A with a 20 ms half-width pulse — factor that into your 24 V DC breaker sizing if you're hot-plugging a populated rack.
Hold-Up Time and Buffering
On a mains/voltage dip, the PS 405 holds up the backplane for 20 ms — enough to ride through a brief sag or to let the CPU execute a safe-shutdown OB. It also supports mains buffering per NAMUR recommendation, which is the standard for process-industry brownout ride-through. An optional backup battery (2x lithium AA, 3.6 V / 2.2 Ah) can keep the RAM and real-time clock alive when the main supply is off entirely.