RX8R-Pro how many servos

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joefly

New member
Jan 23, 2018
14
0
#1
How many high voltage servos can the RX8R-Pro handle by itself? My Escapade has 6 HV servos. Do I need a RB10?
 

Konrad

Active member
Jan 23, 2018
636
38
San Francisco
#2
Not sure what you are asking. But the limit is the amp rating on the bus bar and maybe the voltage regulator. The nice thing about HV servos is that for any given power there is a lower amp draw. Now I think it is generally understood that the servo connectors are near their limit at 5 amps. As the RX bus bar looks to have about the same amount of copper area as the connector I'd guess 5 to 7 amps is about the limit for the RX output.

It really is a bus bar and connector issue.

I think FrSky is hoping your servos don't draw more than 2.5 amps (Digital holding).

I might go as high as 10 amps potential with the power coming from two servo (battery) connectors. I'm assuming that all my servos won't be at max draw at any one time.
 
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joefly

New member
Jan 23, 2018
14
0
#3
I was concerned about how much current the receiver bus could handle safely. I put a wattsup on and measured the current of 4 of the servos operating and found that they pulled 1.2 A. I would expect that 6 servos, all operating at the same time, would pull less than 2A. Thanks for your help.
 

Konrad

Active member
Jan 23, 2018
636
38
San Francisco
#4
I was concerned about how much current the receiver bus could handle safely. I put a wattsup on and measured the current of 4 of the servos operating and found that they pulled 1.2 A. I would expect that 6 servos, all operating at the same time, would pull less than 2A. Thanks for your help.
I think what you measured is 2 amps of idle current. If you are measuring this as the servos are moving on the bench your idle current is in all likelihood lower.

But this is not taking into account the air loads on the surfaces. How big are your servos? Can you measure current draw from air loads while in the air? Does the wattsup latch the highest reading?

I'd think about using 2 connectors from the battery and do a short test fllight measuring the current.

My gut says your are OK but not knowing the servo I can't say with any confidence, that all is well with your set up.
 
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joefly

New member
Jan 23, 2018
14
0
#5
Servos are in the plane. I’m replacing the receiver from Spectrum to a R8X8-Pro. Servos are the digital hitec sport servos. I think it’ll be ok since the plane is used for gentle sport flying.
 

Konrad

Active member
Jan 23, 2018
636
38
San Francisco
#7
Servos are in the plane. I’m replacing the receiver from Spectrum to a R8X8-Pro. Servos are the digital hitec sport servos. I think it’ll be ok since the plane is used for gentle sport flying.
Oh, this is a proven set up. Since both FrSky and Spektrum use the same style/type bus bar for the RX pin outs you should be fine, assuming you didn't use a power distribution module with the Spektrum.

"Gentle Sport Flying" is the type of flying that costs me most of my airframes!:eek:

All the best,
Konrad
 
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Wayne

Administrator
Staff member
Nov 29, 2017
806
90
Novato, CA USA
#9
My general rule is 10 amps for a normal receiver. Anything more and you better look to a power distribution option of some sort. To monitor this amperage you can easily add a 40 amp sensor to the receiver battery (or BEC output). You want to see the maximum the servos will draw, not idle. A good servo for a modern glider for example can easily draw 2 amp on its own, but they really should not, properly setup you should never be near the stall torque of the servo. In other words, make sure you have setup the radio correctly so your servos are not running into mechanical limits of the flight surfaces, etc. For a glider, our biggest draw tends to be throwing in a full crow mix while still carrying some speed. Even that, a max amp reading should only last a short period of time.

At a recent large scale gas event I was surprised to see many planes had no power distribution systems.