Annex H (informative) — Heat and mechanical load

The heat a row produces, how a case sheds it, and the forces a panel and its screws carry. EIS places no requirement here; the figures are planning aids built on the currents modules already publish (PWR-001).

H.1 Heat from a row

Every watt a module draws becomes heat inside the case. From its published currents, a module on the bus dissipates at most

P = 12 V × I(+12 V) + 12 V × I(−12 V) + 5 V × I(+5 V)

and a module at a DC entry adds 12 V × I(12 V). A row dissipates the sum over its modules; a published current is a maximum, so the sum is an upper bound. A supply inside the case adds its own loss, P × (1 / η − 1), where η is its efficiency at that load.

For example, one 84 HP row whose modules publish 1.2 A on +12 V, 0.40 A on −12 V, and 0.10 A on +5 V:

A supply mounted on the product it powers publishes its loss (SUP-008); 9.1 already counts it inside 12 V × its input current, and it is published so the installer knows where the heat sits. A linear regulator from 12 V to 5 V for a module's logic turns (12 V − 5 V) × I into heat behind the module: 2.1 W at 0.3 A. The row budget counts the mounted supply's input current on the interface it takes.

H.2 How warm the case gets

A closed case sheds heat by natural convection and radiation from its outer surfaces, in about equal parts, through its walls. A rough estimate of how far the air inside rises above the room is

ΔT ≈ P / (h × A)

where h is the overall coefficient through the wall, about 5 W/(m2·K) for a sheet-metal case and about 3 for wood or plastic, and A is the outer surface open to the air: a desktop case's base on a table does not count. A 3U, 84 HP case about 0.48 m wide, 0.15 m deep, and 0.15 m tall has about 0.33 square meters of surface, 0.26 without its base, so 23 W raises a sheet-metal case by about 18 °C and a wooden one by about 30 °C. The same row in an open-backed or vented case runs much cooler.

EIS recommends that an enclosure publish, for each row, the dissipation it carries for a 10 °C rise above room temperature. EIS recommends that a module whose dissipation is much higher than its current suggests, such as one with a linear regulator dropping 12 V to 3.3 V, say so in its manual.

H.3 Mechanical load

EIS recommends that a module whose mass would bend its panel in transport, such as one with a large transformer or a display, publish its mass and supply a support bracket where it needs one.