Power over Tether Considerations
Powering the Manifold 5S over a tether is more complicated than one would suspect at first blush. A tether is essentially a long cable, and it uses wires that are as small as practical to save weight and cost. The combination of these two factors means that the tether will dissipate power and lose voltage between the tether's home base and the Manifold 5x .
There are two primary factors to look at to see if a power-over-tether (POT) system will work for you:
The total power consumed at the Manifold 5x end of the cable
The voltage available at the output of the tether interface
The loss in the tether depends entirely on the current through the tether. It is a bit counter-intuitive, but the lower the voltage at the Manifold 5x end, the more power will be lost in the tether. This is because the power supplies in modern electronics are quite efficient, and use less input current if the input voltage is higher. If the power supplies were 100% efficient, the current needed would be the device's power consumption in watts divided by the supply voltage. Of course, the efficiencies are less than 100%, but still quite high so the principle is directionally correct.
A typical Blue Robotics 1-twisted-pair thin tether 100 meters long has a resistance of about 30 ohms. At 300 meters, the resistance is about 90 ohms.
The Manifold 5S with no hotel load and a brain installed consumes about 7.5 watts, which is about 500 mA at V-in of 15 volts. This would require you to supply the head end of the tether with 30 volts, and the tether itself would dissipate 7.5 watts. Using 7.5 watts in the tether gets close to the point where if the tether is on a spool, it can get warm or hot.
(Don't do this) If you are using a 300-meter tether, to achieve a V-in of 15 volts you would need to supply the head end with 60 volts, of which 22.5 watts is dissipated as heat in the tether, and 45 volts lost. This is fine, until the Manifold 5S load drops, then V-in will rise accordingly. For example, if you remove the Brain and apply 60 volts to the head end you might get 45 or 50 volts at V-in. This will fry the onboard power supply and probably the base electronics like the switch.
You can mitigate the tether power loss by either shortening the tether, or decreasing the resistance of the tether, or both. Decreasing the resistance can be done by using a tether with larger wires or by using more wires in the tether.
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