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how about an episode on measuring servo amp draw?

bicyclemonkey

Flying Derp
Mentor
#1
As the title says, how about an episode showing how to check how much amp/current the servos are pulling? This way, one will know if the internal BEC is capable of handling all the servos on your plane. I got this idea when researching how many servos I can run on a 60a ESC...

This one from HeadsUpRC says on 4s, it can only run two servos (60amp w/5v/3a BEC)

http://www.headsuphobby.com/HURC-60-Amp-Brushless-Motor-ESC-G-650.htm

While the Turnigy Plush 60amp (5v/3a BEC) just says it can run "larger planes with bigger servos"

http://www.hobbyking.com/hobbyking/store/__2166__TURNIGY_Plush_60amp_Speed_Controller.html

Good idea?
 

earthsciteach

Moderator
Moderator
#2
Good question. Hadn't really even thought of this sort of thing before. But, I try not to think about much, in general.

Here's a question that I have: Why can the HURC esc handle 4 servos on a three cell, but less on batteries of higher voltage?
Seems counter-intuitive to me. Ouch, you made me go and think.
 

OutcastZeroOne

Fly, yes... Land, no
#4
I did a static test of amp draw of the servos i plan to run for my FT cruiser (4 servos). No load, not even coneccted to anything. By rapidly moving the controls to get every single servo running I got 1.3 amps of draw useing 4 HKSCM9-6 Singlechip Digital Micro Servo at 5v of power. But if I was flying the way I was moving the controls the thing would be out of control. Under just simple easy movements I only saw about 130mah. but once the servo had reached the point I was holding the control stick it was only about 30mah.
 

OutcastZeroOne

Fly, yes... Land, no
#5
Good question. Hadn't really even thought of this sort of thing before. But, I try not to think about much, in general.

Here's a question that I have: Why can the HURC esc handle 4 servos on a three cell, but less on batteries of higher voltage?
Seems counter-intuitive to me. Ouch, you made me go and think.


The BEC on an ESC uses a very simple and cheap LM7805 5v voltage regulator. The LM7805, in the case of a Turnigy 18amp Basic ESC, uses two 78D05F made by KIA. Each one of these 5v regulators can handle 1amp of power. The way any 78XX series voltage regulator works is it needs atleast 1v higher then what it regulates. That reduction in power generates a lot of heat. So the reason why some ESCs can handle less load at higher battery powers is becuase the regulators have to dissapate more heat, but the heat cant be dissipated enough without killing the regulators. The heat sink on an ESC is ment for the motor side of things, and can not be used on the BEC side of the circuit.

Yeah, and then you have this 60amp ESC with a 6amp BEC and states it can handle nine standard size servos or 12 micro servos!

http://www.headsuphobby.com/Sky-Power-60A-ESC-with-Switchmode-BEC-H-110.htm
Larger servos have larger motors. Smaller servos have smaller motors. Differance of load means amperage useage and more strain on that BEC system.

Im not sure I explained everything clearly enough. Im a bit of an electronics nerd and even built my own BEC seperate from the ESC's BEC onto a power board for my FT crouser that I am going to build. I will post an article about it once I am able to give it a full test out.
 
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OutcastZeroOne

Fly, yes... Land, no
#7
I was surprised at just how much power these little 9g servos can pull, and with no load on them. Would be interesting to have an amp meter inline to the Rx and servos in flight to see how much pull happens in flight.
 

Ak Flyer

Fly the wings off
Mentor
#9


The BEC on an ESC uses a very simple and cheap LM7805 5v voltage regulator. The LM7805, in the case of a Turnigy 18amp Basic ESC, uses two 78D05F made by KIA. Each one of these 5v regulators can handle 1amp of power. The way any 78XX series voltage regulator works is it needs atleast 1v higher then what it regulates. That reduction in power generates a lot of heat. So the reason why some ESCs can handle less load at higher battery powers is becuase the regulators have to dissapate more heat, but the heat cant be dissipated enough without killing the regulators. The heat sink on an ESC is ment for the motor side of things, and can not be used on the BEC side of the circuit.



Larger servos have larger motors. Smaller servos have smaller motors. Differance of load means amperage useage and more strain on that BEC system.

Im not sure I explained everything clearly enough. Im a bit of an electronics nerd and even built my own BEC seperate from the ESC's BEC onto a power board for my FT crouser that I am going to build. I will post an article about it once I am able to give it a full test out.
Dude, welcome and get ready for sooo many questions lol.
 

OutcastZeroOne

Fly, yes... Land, no
#10
Dude, welcome and get ready for sooo many questions lol.
I am no expert on electronics, but I do know a few things. I did do an article that is still pending aproval for a home built BEC I made. The closest expert I know of would be my Mom, since she was formaly trained in electronics, has worked for many many companies. You can thank her for my interest in electronics. When I was a kid she showed me a picture of one thing she was assembling for Watkins Johnson, part of the navigation system for the B-2 Stealth bomber! All I remember of it was it was a metal ring of some kind. I was only like 8-9 years old at the time, so 23 years later things are a bit fuzzy :p

and now back to blasting heavy metal and working on my battery tab welder :p
 

Ak Flyer

Fly the wings off
Mentor
#11
Nice, thanks outcast's mom!! My main flying hombre and I have been figuring a lot of things out lately as we get into more complex and expensive models. We seem to be less willing to take chances or make assumptions will lots of dollars flapping around in the air. He was having an issue with his 10 amp bec on his Trex 600 nitro heli. Ended up using a 20 amp to cure it. Your post cleared up some things about it. I'm not exactly green in electronics, I know enough to be dangerous anyway lol.
 

OutcastZeroOne

Fly, yes... Land, no
#12
I can be really dangerous with electronics :p I finishing up building the boards to make my own battery tab welder. But me being me, I cant just slap everything on a board and call it done, I have to make it FANCY! With a spiffy houseing and lots of LED's, an internal power supply and cooling system.

One up side is that I work at a high end Car Stereo shop so I have some good tools to play with. How I still have all my fingers toe's and my job is beyond me. I am the crazy one at the shop too. just finished building a nice compact Vacuum Form set up that closes up into a suitcase too. More on that later :D
 

OutcastZeroOne

Fly, yes... Land, no
#15
I never said anything about a worst case. I was simply trying to get as much load as I could generate. Right now I do not have a plane that is flight ready so that was the best I could do at the time. The closest "real world" situation to what I was doing would be if you had a plane that had a gyro that was constantly trying to adjust the planes flight. But again, what I was doing was no load on the servos, so when in the air and pushing those control surfaces against air at speed would generate a much higher load per servo. I just wanted to see if I had enough amperage on my board for the number of servos I plan to run on the FT Cruiser.

The best test would be to get the thing into the air and use that load tester bicyclemonkey found.