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Old Jun 07, 2011, 07:46 PM
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Los Angeles, CA
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Question
when to have an external BEC and which amp or volt rating of UBEC or SBEC?

Hi all
How do you determine if you need an external BEC for a particular electric plane set up and if so, especially how do you determine what rating of eg. UBEC, you should get- eg. 3A, 5A, 10A..?

if anyone has a guide to know how to determine the amp rating, in general I'd appreciate it- I have a wattmeter if that helps..

More specifically, I just received my P-51D Mustang 1.2m EPO with Auto Canopy (PNP) from Hobbyzone http://www.hobbyking.com/hobbyking/s...dProduct=16235 (more info also available here: http://www.uniquemodelrc.com/proshow.asp?id=432)- stock specs include:
4 x 9g servo (pre-installed)
Servoless electric retracts (pre-installed)
800kv motor (pre-installed)
30A ESC (Pre-installed)

it is made to run off of 3S lipos, yet it has 6 channels- it doesn't come with an external BEC with the stock set up and on contacting them they say it doesn't need one, yet I read in another thread someone had their ESC burnout - so not sure now...was thinking of getting a UBEC as extra insurance but also with the possible intention of upgrading the ESC in the future and possibly motor and running 4S lipos, I'm certain it would need one then..it's just how do I determine what amp /volt rated UBEC I should get..?- thanks for any feedback, particularly on determining the rating in general with any other planes as well- thanks so much for any info!
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Old Jun 07, 2011, 08:06 PM
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E-Challenged's Avatar
United States, CA, Garden Grove
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Go to Dimension Engineering's website and read info on "switch-mode" vs linear BEC's , you can use their "Park BEC". Adding it to your setup will require soldering the red and black wires to your ESC's red and black wires, plugging your ESC's receiver cable to the BEC and connecting the BEC to your receiver's throttle channel following the instructions.

http://www.dimensionengineering.com/BECFAQ.htm

Some of the ARF model companies are admitting the need for switching type BEC's for models using 2.4ghz receivers, 3S lipos and four or more small servos.
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Old Jun 07, 2011, 08:10 PM
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Harry H's Avatar
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Hey Self- nice plane there. Looks like fun. The problem is that the PNP, BNF... mostly use cheaper parts and the details do not list what the internal BEC will put out. For the most part, these planes fly just fine the way they come, what can be a problem is what sort of RX you are using. I only know about Spektrum and know you need to have sufficient power to avoid a brownout if the voltage drops too low. With small 9g servos, you could use a Castle Creations 10 amp BEC and would be very safe. I like the idea that if you burn your ESC, at least you still have control of the plane and at least have a chance to get it down in one piece.

Harry
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Old Jun 07, 2011, 08:12 PM
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Challenger 413's Avatar
Dickinson, Tx.
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I would run the retracts on an add on BEC, like the Park BEC.

If they hang up, then it won't run down your ESC BEC.

Also, you probably will have to add an add on BEC if you run higher voltage than 3S, unless the stock ESC can handle that voltage.

Challenger413
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Old Jun 07, 2011, 08:13 PM
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Quote:
Originally Posted by E-Challenged View Post
Go to Dimension Engineering's website and read info , you can use their "Park BEC". Adding it to your setup will require soldering the red and black wires to your ESC's red and black wires, plugging your ESC's receiver cable to the BEC and connecting the BEC to your receiver's throttle channel following the instructions.

http://www.dimensionengineering.com/BECFAQ.htm
thank you but I was more looking at purchasing from Hobbyking's great value range which starts at less than $5 for a 3A UBEC- all UBECs listed here: http://www.hobbyking.com/hobbyking/s...&ParentCat=182

the question I guess is is this (3A UBEC) enough for my P51.. or is something bigger required)?
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Old Jun 07, 2011, 08:19 PM
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Originally Posted by Harry H View Post
Hey Self- nice plane there. Looks like fun. The problem is that the PNP, BNF... mostly use cheaper parts and the details do not list what the internal BEC will put out. For the most part, these planes fly just fine the way they come, what can be a problem is what sort of RX you are using. I only know about Spektrum and know you need to have sufficient power to avoid a brownout if the voltage drops too low. With small 9g servos, you could use a Castle Creations 10 amp BEC and would be very safe. I like the idea that if you burn your ESC, at least you still have control of the plane and at least have a chance to get it down in one piece.

Harry
Hi Harry- thanks for your reply- I'll be using a spektrum AR6200 with satellite RX- so you think a minimum 10Amp rated UBEC would suffice or would a 3A or 5A still be enough?
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Old Jun 07, 2011, 08:27 PM
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Originally Posted by TheSelf View Post
Hi Harry- thanks for your reply- I'll be using a spektrum AR6200 with satellite RX- so you think a minimum 10Amp rated UBEC would suffice or would a 3A or 5A still be enough?
I think you might be ok with the 5A, but the comment about the retracts makes me lean to a 10A. Better to have extra than not enough. Some of these are rated at their peak output but do not operate constantly at the advertised amps.

Harry
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Old Jun 07, 2011, 09:40 PM
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thanks for everyone's input- I ended up ordering a Castle Creations 10 Amp uBEC to be safe- don't know if overkill, but the hobbyking ones only go up to 7.5A, which may be enough, but better safe than sorry- that said I'm still unclear how in general and in the future how to determine what amp rated UBEC to get, generally speaking- ie. what info do I need to determine this?
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Old Jun 07, 2011, 09:59 PM
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What you need to know is the amp draw from the servos you are using. This can be challenging to find out from the manufacture. You can always do a search to see if someone has tested your particular servo or do a test yourself. Multiply that by how many servos you have and the amp draw of any other electronics installed, add fudge factor and you get a ball park. Your type of flying will also come into play. A 3D will draw more amps than say a scale plane. There are some here that might have a better explanation, but hope this helps.

Harry

By the way, I too got confused and just changed BEC's in 3 planes with CC 20 amp Pro's - but these were all standard size digital servos.
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Old Jun 07, 2011, 11:01 PM
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Originally Posted by Harry H View Post
What you need to know is the amp draw from the servos you are using. This can be challenging to find out from the manufacture. You can always do a search to see if someone has tested your particular servo or do a test yourself. Multiply that by how many servos you have and the amp draw of any other electronics installed, add fudge factor and you get a ball park. Your type of flying will also come into play. A 3D will draw more amps than say a scale plane. There are some here that might have a better explanation, but hope this helps.

Harry

By the way, I too got confused and just changed BEC's in 3 planes with CC 20 amp Pro's - but these were all standard size digital servos.
thanks Harry -the 4 servos in my plane are TowerPro SG90 9g servos- I do not know if they are digital or conventional..and what about the servoless electric retracts and the electric sliding cover?

I ordered this servo tester from Hobbyking, though I don't think they will tell the amp draw on the servos: http://www.hobbyking.com/hobbyking/s...dProduct=8296- what other product would you use to test the servo amp draw?
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Old Jun 07, 2011, 11:06 PM
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Quote:
Originally Posted by Harry H View Post
What you need to know is the amp draw from the servos you are using. This can be challenging to find out from the manufacture. You can always do a search to see if someone has tested your particular servo or do a test yourself. Multiply that by how many servos you have and the amp draw of any other electronics installed, add fudge factor and you get a ball park. Your type of flying will also come into play. A 3D will draw more amps than say a scale plane. There are some here that might have a better explanation, but hope this helps.

Harry

By the way, I too got confused and just changed BEC's in 3 planes with CC 20 amp Pro's - but these were all standard size digital servos.
do you think there is a way to measure servo amp draw using the servo tester somehow combined with my wattmeter?
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Old Jun 07, 2011, 11:21 PM
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Originally Posted by TheSelf View Post
do you think there is a way to measure servo amp draw using the servo tester somehow combined with my wattmeter?
If you just plug the BEC into the wattmeter, leaving out the ESC, then you can use your receiver to move the sticks and run the retracts. Setting the wattmeter to capture the peak power should give you close to the max draw, unloaded. Of course, the servos will draw more in the air because of the air pressure on the surfaces, but the retracts shouldn't change much.

I do this to check if any of my servos are dragging, or hitting the stops at maximum.
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Old Jun 07, 2011, 11:29 PM
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Here is another way to check. Also, try a search on "measuring current draw in servos" or some variation of that.
http://www.rcgroups.com/forums/showthread.php?t=951119

Harry
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Old Jun 07, 2011, 11:30 PM
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thanks all!
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