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Help! ES9251II servos vs. ES9051

mayan

Well-known member
#21
I have also never had a problem with 4 9g servos with the current setup. However I was planning on adding another one or two servos to do goffy things. Like drop bombs or parachutes
 

Keno

Active member
#22
I still don't see a problem. When I said that servos and motors will demand more pwr under stress , consider the loading that a sticky control surface and hard flying adds heavy to that stress or load. Control surfaces need to have free movement if stiff problem.
 

Hai-Lee

Old and Bold RC PILOT
#23
There are 2 aspects to the BEC that must be understood.

First is the maximum current capacity and the simple fact that there is a current limiting protection circuit that will drop the output volts if the current limit is exceeded.

Secondly there is the temperature handling of the BEC, If the BEC gets too hot it will protect itself by reducing the voltage and current output to protect itself.

There are 2 basic types of BEC. (SBEC and UBEC). SBECs are the simplest of circuits and are extremely sensitive to high current loads at high battery voltages. The S means serial! An SBEC running on 8.4 volts, (2S), and passing one Amp would be required to dissipate 3.4 Watts as heat. On 12.4 volts, (3S), at one Amp it would be required to dissipate 7.4 watts as heat and running on 16.7 volts, (4S), it would be required to dissipate 11.7 watts as heat.

With an ESC normally the SBEC and the motor Drive circuitry are very close to each other and a significant amount of heat transfer between the two occurs. If the motor drive circuitry also generates heat that causes the ESC to become hot the dissipation of heat by the BEC is hampered and the result can be an over temperature "Brown out" or loss of Rx volts with catastrophic results.

On an ESC that has a UBEC the heat generated and therefore which needs to be dissipated is markedly reduced, (it is a switchmode regulator which is highly efficient and its dissipation rises in a linear fashion to applied battery voltage). Normally UBEC type ESCs start with a BEC current capacity of around 3Amps though there are some SBECs that are also capable of supplying 3 amps. As battery voltages increase beyond 3S the use of SBECs ceases due to heat dissipation issues.

When using ESCs with a cheap SBEC I limit the number of servos to 4 maximum, (9gram), and do not use motors that push the motor drive circuitry to maximum. If I need to push the ESC to its limits, (an EDF installation for example), I will fit an external UBEC and mount it as far as I possibly can from the ESC and the airflow that cools it.

On "Full House" installations I will use an external UBEC to power the Rx and the primary flight controls installed as mentioned above and I might connect the other controls to the ESC/SBEC. This includes things such as flaps, retracts, bomb drops, bomb bay doors, and even nav lights.

Just what I have learned from research and many bad experiences.

Have fun!
 

mayan

Well-known member
#24
Thanks for all the wonderful replies on this thread it helped me learn a lot. Will do some testing of my own to figure out what works best with my setup.
 

Hai-Lee

Old and Bold RC PILOT
#25
Thanks for all the wonderful replies on this thread it helped me learn a lot. Will do some testing of my own to figure out what works best with my setup.
Always a wise move! Listen to all advice, think it through, test the info for yourself, and discard anything that does not fit your application!

Have fun!