SK-PW7 Power Bus
From Skookum Robotics Wiki
Why does the SK720 use a power bus while other flybarless systems don't?
It is a design consideration for larger helicopters. High power servos draw a lot of current. For example, a set of four 8717 servos will often be pulling 12 amps!
The SK-PW7 Power Bus was designed for up to 20 Amps of continuous current. It allows you to connect your high power 8.4 V servos directly to a 2S LiPo battery, without
having to draw large currents through your helicopter's other on-board electronics. Also, six heavy servo cables will transfer a lot of vibration to the SK720's acceleration
sensors. High vibration will degrade the quality of the self-levelling and flight log playback. And the soon-to-be-released advanced power bus will also include a soft switch!
The PWR-HV side is connected directly to the battery while the PWR-LV side is not. If you connect a servo to the PWR-LV side you must supply power through a separate
BEC or use a jumper from PWR-HV to PWR-LV.
Is there a voltage regulator in the power bus?
No. The power bus is just makes electrical connections. The soon-to-be-released advanced version of the power bus will contain a soft switch.
Do I need the power bus?
You should use a Power Bus if your heli has a main rotor diameter larger than 1100mm (43 in), otherwise you do not need one. The power bus allows you to connect your heavy
duty servos (which can draw up to 4 amps each) directly to a 2S LiPo battery with the built-in 18-Gauge silicone wires. It will also reduce the vibration felt by the SK720
because there are fewer and thinner wires going to the SK720.
Are the supplied wires heavy enough to carry current for my heavy duty servos?
The servo jumper wires supplied with the SK720 are rated to 3 amps.
I get a voltage error during a flight, why?
This is caused when the voltage from your BEC drops below 3.3V. This can happen if you have an underpowered BEC or battery. Even on a mid sized heli, servos can draw
up to 4 amps for a short period. Make sure your BEC can supply enough current for the number of servos that you are using.
How do I hook up the power bus? Do you have a schematic?
Yes. See the sub-sections below for how to connect a traditional receiver and how to connect a satellite receiver.
Connecting a Traditional Receiver and Power Bus
In the above figure, the power bus is hooked up to a 2S LiPo battery. The battery powers the swash servos and the external BEC. The output of the BEC is connected to the PWR-LV side of the power bus to supply 5 volts to the tail servo.
In the example shown, voltage from the LiPo battery goes through the "BUS-A" port to the SK-720. The external receiver is also supplied with 8.4V over the cables connected to IO-A(T) and IO-B.
With a traditional receiver, the throttle servo or ESC will by-pass the SK-720 unit. The SK-720 will not know how much throttle you are giving your heli.
Connecting a Satellite Receiver and Power Bus
In the above figure, the power bus is hooked up to a 2S LiPo battery. The battery powers the swash servos and the external BEC. The output of the BEC is connected to the PWR-LV side of the power bus. This supplies 5 volts to the throttle servo.
In the example shown, the SK720 is supplied with 8.4V from the LiPo battery through the "BUS-A" port. The satellite receiver gets its rated power from the SK720's internal
voltage regulator circuit.
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