The 6 pin PCIE cables use 3 x 12V lines, providing 6.25A and a a maximum wattage of 75W. That should allow 2 devices to be powered per cable using a splitter.
Also, how many amps does a K16 use?
You should expect around 40W (3.3A) . It probably will use less. Using the max ratings for regulators, under a condition of over clocking, you could probably draw up to 55W (4.6A) beyond which it would be dicey and shut down should occur if the regs are doing their job. Off hand, I didn't go check at what rating the shutdown kicks in. This is allowing an approximate 75% for regulator inefficiency but actually hope we get better.
Does this mean the overclocking capability is limited by hardware or what does this mean? Is it a protection of some kind?
Yes and Yes.
The regulators are designed with a maximum current rating (two 16A in the K-16 design). They have built-in protection (Over Voltage, Over Current and Over Temp) to prevent the regulator from frying itself and possibly other components on the board.
This calculation is just on the 16 chips, it does not include the other items on the board. The two regulators are split in the design (each powering 8 chips) but I'll combine the two here for easier math. Also this does not compensate for inefficiency.
Voltage (E) * Current (I) = Power (P)
1.2volts * (2amps * 16chips) =
38.4watts 1.2volts Avalon Typical Core Voltage, 2amps/chip Avalon Maximum Operating Current
Now let's bump the voltage up to 1.3volts to try for a faster overclock.
Voltage (E)^2 / Power (P) = Resistance (R)
1.2volts^2 / 38.4watts =
0.0375ohmsVoltage (E)^2 / Resistance (R) = Power (P)
1.3volts^2 / 0.0375ohms =
45.1watts 1.32volts Avalon Maximum Core Voltage
Power (P) / Voltage (E) = Current (I)
45watts / 1.3volts =
34.7ampsTranslation: Increasing the voltage too possibly gain a higher hashrate will result in a fungible/ballpark increased current demand of ~2.7amps (~1.35amps per) on the regulators.
There are three different protection mechanisms for the two 16A regulators, though here are the protection types to focus on:
THERMAL SHUTDOWNTemperature sensing is provided inside IR3895. The trip
threshold is typically set to 145C. When trip threshold is
exceeded, thermal shutdown turns off both MOSFETs and
resets the internal soft start.
Automatic restart is initiated when the sensed
temperature drops within the operating range. There is
a 20C hysteresis in the thermal shutdown threshold.
OVER CURRENT PROTECTIONThe over current (OC) protection is performed by sensing
current through the RDS(on) of the Synchronous Mosfet.
The current limit is pre�-set internally and is compensated
according to the IC temperature. So at different ambient
temperatures, the over�-current trip threshold remains
almost constant.
The over current implementation details are complex. See the IR3895 datasheet for the details.