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Old Nov 18, 2012, 02:30 PM
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Quote:
Originally Posted by dglo
Question: The more I research the subject, the more confused I get. Should I install a seperate switching bec? I'll be using a scorpion commander 70 amp esc with a 5v 4amp linear bec on 3s lipo. From what I can figure out this is kind of borderline as to wether to use a seperate bec or not. Any help? I'm using 2 Hitec HS635HB servos on elev and rud and 2 HS225BB servos on the ailerons.

Thanks,
Doug


Quote:
Originally Posted by E-Challenged View Post
Go to Dimension Engineering's website, read FAQ's on when you need to use a switch mode BEC and what amp rating you need for 3S and four full size servos. ( I use their Park BEC's rated at 1.5 amps for 3S lipos, 2.4Ghz receiver and four or more small servos. They have BEC's with higher amp rating for larger stuff/better safety margin to prevent brownout crashes. Alternative is to use separate flight battery pack to power receiver and servos same as in IC powered model and disable the Scorpion linear built-in BEC. Don't risk a crash with linear BEC.

225BB servos according to Servo City...
Control System: +Pulse Width Control 1500usec Neutral
Required Pulse: 3-5 Volt Peak to Peak Square Wave
Operating Voltage: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C
Operating Speed (4.8V): 0.14sec/60 at no load
Operating Speed (6.0V): 0.11sec/60 at no load
Stall Torque (4.8V): 54.15 oz/in. (3.9kg.cm)
Stall Torque (6.0V): 66.65 oz/in. (4.8kg.cm)
Operating Angle: 45 Deg. one side pulse traveling 400usec
360 Modifiable: No
Direction: Clockwise/Pulse Traveling 1500 to 1900usec
Current Drain (4.8V): 8mA/idle and 300mA no load operating
Current Drain (6.0V): 8.8mA/idle and 340mA no load operating
Dead Band Width: 8usec
Motor Type: 3 Pole Ferrite
Potentiometer Drive: Indirect Drive
Bearing Type: 1 Ball Bearing and 1 Oilite Bushing
Gear Type: Nylon
Connector Wire Length: 11.81" (300mm)
Dimensions: 1.27" x 0.66"x 1.22" (32.4 x 16.8 x 31mm)
Weight: 0.95oz (27g)

Granted under load, those current numbers are worthless other than idle and could be much higher.

HS635HB servos according to ServoCity:
Control System: +Pulse Width Control 1500usec Neutral
Required Pulse: 3-5 Volt Peak to Peak Square Wave
Operating Voltage: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C
Operating Speed (4.8V): 0.18sec/60 at no load
Operating Speed (6.0V): 0.15sec/60 at no load
Stall Torque (4.8V): 69 oz/in. (5.5kg.cm)
Stall Torque (6.0V): 83 oz/in. (6kg.cm)
Operating Angle: 45 Deg. one side pulse traveling 400usec
360 Modifiable: Yes
Direction: Clockwise/Pulse Traveling 1500 to 1900usec
Current Drain (4.8V): 5.3mA/idle and 400mA no load operating
Current Drain (6.0V): 6.6mA/idle and 500mA no load operating
Dead Band Width: 4usec
Motor Type: 3-Pole Ferrite
Potentiometer Drive: Indirect Drive
Bearing Type: Dual Ball Bearing
Gear Type: Karbonite Gears
Connector Wire Length: 11.81" (300mm)
Dimensions: 1.6" x 0.8"x 1.5" (40 x 20 x 39mm)
Weight: 1.83oz. (52.1g)

So, at no load, you are looking at minimum of an amp used by elevator and rudder without load but operating. The ailerons working will draw over 0.6A. Presuming the power needs of the receiver are 500 mA, then you are looking at conservative numbers of 1.0 + 0.6 + 0.5 or 2.1A What are your Rx specs?
Putting an amp meter on the ground on the positive from the Scorpion ESC's RX connection will give you a close to no load reading. Granted you have load from linkages, only when the servo has nothing but the horn/wheel will it see "no-load".
You would need something like an Eagle Tree system to get true flight loads on the servos to give accurate estimates of typical loads being recorded. With the design of the controls to be very light throughout the range of motion, if the model has similar setup for its flight controls the servo loads would stand to reason be less and more consistent throughout the range.
A servo at 60 degrees and holding may draw far less than between 0 and 60 and getting there...
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