Spectrum brown out-servos and mods

[flydiver]

First-disclaimers: I'm not a Spectrum user but I act like I know what I'm talking about. I do have friends with them that are pretty 'electronics deficient' so I help them sort it out. I also spend too much time reading so I've found some interesting stuff. Bits and pieces have come together in this forum and I've had some long discussions with [admiralev] over his Spectrum disaster so I'm trying to put in in one place. If you are using a stock Cub you can quit now or salt this away for future reference.

Ready? You'll have to work a bit ;D

All modern ESC have a built-in BEC (battery eliminator circuit). This sorts the electricity out between the motor (high voltage and the rest of the electronics (low voltage). The voltage step down for the RX and servos produces heat. The higher the voltage that needs to be stepped down the more the heat. So, weird as it sounds at first as you go from 7-cells to 3S lipo you stress the ESC MORE! More voltage DOES NOT HELP this step down!
More power is NOT helpful.

Read this link-you'll need the info:
www.rcgroups.com/forums/showthread.php?t=748220

More info on the CC BEC (read the FAQ):
www.castlecreations.com/products/cc_bec.html


If you fly WOT the motor is sucking more juice. This makes the battery voltage 'sag' more. The ESC has a bit lower voltage to draw down and it's job is actually a bit easier. If you fly part throttle the voltage is higher and the ESC will actually have to work HARDER. And you thought you were getting away with it because you only use part throttle. That concept took me awhile to grasp fully.
A seeming contradiction that makes sense once you understand the electronic 'physics'.

Now you are all happy you've got 3S in the Cub and you want to mod the crappy electronics and replace it with SPECTRUM! (Spectrum cures everything ;D). And ailerons-got to have ailerons.

So you get the stuff but it's expensive with that new DX6i so you get a GWS 480li and 4 Hextronic 9gm servos since they are cheaper (you can fill in with other products but these are common). Everyone says the Spectrum is WONDERFUL, so that other stuff, it should work OK, right? Looks just like the more expensive electronics.

Just having 4 servos ends up pulling more power from the system (extra load). If you ACTUALLY READ MOST ESC INSTRUCTIONS you'll find they TELL YOU to use a SEPARATE BEC if you exceed 3 servos on 3S.

Most All ESC have a BEC limit. Cheaper ESC may have a lower limit or just plain fabricate some pretty specs for you. GWS makes a crappy ESC but they don't seem to lie so I'll pick on them:

GWESC480/ICS-480Li www.gwsus.com/english/product/speedcontroller/480.htm
LVC = 5.4 & 8.1V] (should be at LEAST 9.0v)
BEC = 5V/1.2A[/color][/b]

2 problems-the LVC sucks and the BEC is only 1.2A. You'll easily find other ESC with more reasonable LVC and BEC from 1.5A > 3A (claimed).

Now the Servos. I just ran across this little gem yesterday. It's a MUST read:
homepages.paradise.net.nz/bhabbott/Servo.html

Notice how different servos have different electrical draw. Those inexpensive Hextronics are kind of hungry [HXT-900 (750mA)]-so are digitals. (BTW-I use the Hextronics all the time, they're cheap and work OK)

Put in 4 x 750mA = 3A if you use them all at once.
Even 3 x 750 = 2.25A
Or 2 x 750 = 1.5A

You've just exceeded the current draw ability of the ESC with the servos but you still have to 'feed' the RX.

Guess what >>>> BROWN OUT!!

You don't have enough juice to go around for the critical control electronics.

The solution > use a separate BEC (sometimes referred to as UBEC). This is essentially taking the load off the ESC and putting it on a secondary device that is designed specifically for that purpose. The link above for the Castle BEC goes over that but there are other devices that do the same job. You'll also find a range of prices and specifications. My own experience is limited there so you are on your own.

You can also add an RX battery but that's extra weight and more suitable for larger planes.

Read your specifications!!! They are there for a reason. If your plane falls out of the sky for no apparent reason it isn't necessarily a 'glitch'. I flew a stock Cub for months and it never glitched on me once.

I bring this up specifically in this forum because there are a lot of enthusiastic newbies that really don't have a clue what they are doing electronically. It IS confusing, and is WAY more complex than I imagined when I started out to simply learn to fly an airplane. Now we have to learn to keep them from falling out of the sky. Ignorance CAN crash your plane.

fly

[admiralev]

my eyes hurt....

this is exactly what happened to me. i was running 3 hextronics from hobbycity on an old aileron wing that only took to the air once. i was using the GWS480 ICSli ESC, which fly says has a 1.2 amp BEC (confirmed ). the 3 hextronics at 750 mA * 3=2.25 amps. BAM! there went my specktrum.

here's the important part from that link fly found

[High peak currents can temporarily overload the BEC, which tries to limit current by dropping the receiver's power supply voltage. This may cause glitching, and in extreme cases a 'smart' receiver may go into failsafe or even 'reboot', losing control for a few seconds. To minimise the chances of this happening you should choose servos whose combined peak currents won't exceed the current limit of your BEC. The only problem is:- how can you find out how much peak current your servos draw?]

its the perfect explanation.....a light shows down from the heavens and the information all clicks...

the thing with my setup is that my reciever (AR 6200) didnt just loose connection. it completley lost its bind. i never regained control. im now flying JR and am just as happy, but with this new 4-servo aileron mod i added the CC BEC to my CC Pixie 20P. it gives me great piece of mind.

now all of you run along with your calculators and do some math work. if your using GWS or electrify and specktrum (or anything for that matter) i would suggest you look into these.

www.castlecreations.com/products/cc_bec.html

they are cheaper on ebay

GREAT INFO FLY!

admiral

[flydiver]

The article with the servo draw that set this discussion in motion and [admiralev] is quoting from is here: www.wattflyer.com/forums/showthread.php?t=41328
The discussion is currently active and there is more info on the sort of failures that this can cause.

[Ellis]

Hey Fly,
I have an Exceed 30A ESC that claims 5V/2A. Are you able check your ESC with your amp meter? Also whats the word on the streets about Exceeds specs? Are they trust worthy?

Once again you've provided valuable info, saving some of us time and grief.

Thanks,
Ellis

(Spektrum cured my bald spot)

[flydiver]

You can check some of the parameters but you can't easily check the BEC Amp limit. It has to do with the 'switching' on/off it uses for controlling the current (PWM) through the FET's. These guys are having a mind expanding discussion about it right now I was reading few minutes ago:
supercubclub.proboards78.com/index.cgi?board=rg&action=display&thread=1858&page=1

Don't be put off by the title. The good stuff is a bit lower down. Bring coffee and a thinking cap!

Take a look at their post count!!! These guys have been around awhile. Vintage1 knows his stuff. Great on motors too.

Looked up Exceed:
www.xheli.com/exrcbrspco.html
Based on the look it's a Hobbywing, same as the Turnigy using the program card as a reference. If it is then it's one of the better units you can buy.

One interesting recommendation is to use the 40A (Turnigy) units since it has a 3A BEC. You get a larger unit (weight) and spend more money. OTOH, if you use a separate BEC you get 2 units (a bit more weight) and also spend more money. The separate BEC in theory should be more efficient.

I'm kind of cynical and don't expect much honesty out of Chinese components. Just because it might have a US sticker doesn't mean it's American either. Even CC is made out of Asian components though it's assembled here. Almost all of the ESC only come from a few sources.

[flydiver]

Sometimes serendiptiy helps....

Found this article that does a better explanation of it than I did:
The Battery Eliminator Circuit
www.stefanv.com/rcstuff/qf200312.html

There are 2 links at the end worth reading also:
An Electronic Speed Control Primer
To BEC or Not To BEC

These are older articles and mostly talk about cell count-just substitute 10-cells for 3S. The majority of the information is completely relevant today. Electricity is still behaving like electricity.

Thought you were learning to fly a plane, huh? Now you are subjected to an education on electricity.

[flydiver]

Spectrum users- READ THIS!!

Found this in another forum thread. Here is some specific info from a very experienced pilot and forurm moderator about Spectrum Brown Out and use of Hextronic 9gm servos. See posting #11. There's some other good info in the thread too.

www.wattflyer.com/forums/showthread.php?p=594004#post594004

[patmatgal]

fly
Yes, that pilot knows a lot (has probably forgotten way more than I'll ever learn) Thanks for the link.

[flydiver]

Some more to add to this discussion:

www.wattflyer.com/forums/showthread.php?t=45901

If you are running ANYTHING with 4 Hextronic servos and Spectrum you REALLY REALLY NEED to read this thread and the links posted.

If your plane falls out of the sky for no reason that you can think of, you were warned.

[duck9191]

you can always do a little digging on the fet used for the bec. you can find white papers for almost any fet. most sites like fairchild, has them on there site. the white papers will give you more info then you would ever care about, but some of the useful stuff is the thermal tolerance, and thermal efficiency, which will give you an idea of how they fet will perform as it heats up from use. it will give you its rated max and min current, voltage, efficiency and output voltage drop. most give you graphs of voltage vs current, current vs efficiency and alot of other stuff that you can look and say " at 5 volts it will provide X amps" which is useful. one good thing is that servos are momentary use, they don't constantly pull current, so if you do peak over the rated current by just a littel you will be fine, just dont pull over the rated current.

the gws 480 is a poor excuse for a built in bec, as mentioned its only good for 1.2 amps, which isn't a whole lot. most speed controls over say 20amps should have a 3 amp bec. you can put a heat sink on the bec fet (the motor drive needs one too) which will help some, but its not really gonna solve the problem. gws 480, over all is pretty much a bad esc lol.

one of the biggest problems with becs, well what kills them is binding in the servos or pushrods. if there is any binding at all in the linkage between the servo and control horn your already causing the servo to work hard which in turns draws alot more current and makes the voltage drop. worst case is that a servo sticks and shorts causing the bec to die and you come down with nothing, never good.

ubecs for what they cost are a great thing, even for the cub or other park flyers. another big benefit other then handling alot more current is that if your esc dies mid flight, guess what, you still have surface controls. been there done that and it ended alot better then it could have thanks to a ubec.

funny thing is that this problem has been present for along time, even maybe less so now because esc's are being equipped with better becs, but thanks to the design of the spektrum rx, they are alot more sensitive to low voltage conditions then most 72mhz rx its become more obvious.

[flydiver]

How do you know what FET is being used? Open it up and read the tiny writing? I can't recall any manufacturer info sheet ever divulge that. I think it would send many pilots screaming from the room with data overload.

[duck9191]

ya it would be the tiny little writing. some manufactures will scratch them off if they dont want you knowing what it is. there is a ton of info, but 95% is useless to the pilot, but if you look in the main specs and the neat little graphs you can normally find some relevant info

[duck9191]

i found one of my dead supersimple 40a esc that a shorted motor killed. it has two lf50a voltage regulators that they use for a bec. ill high lite the usefull info below, but each is a fixed 5v output @ 1amp, so 2amps total.

Min. Typ. Max.
Output Voltage V 4.95 5 5.05 V
Operating Input Voltage 16 V
Output Current Limit 1 A

so max input from the batt is 16v, output is 5volts at 1amp. they dont have a graph for the lf50a but a general one that shows that once you push it towards 1.5amp its efficiency drops fairly quickly.


get place to get data sheets and white papers

www.alldatasheet.com/

[flydiver]

Now that's interesting! I have a couple dead ESC from the same problem I can look at. I was still a newbie, had a TowerPro with a broken wire and kept trying to make the d**n thing run. Then I was pissed that the ESC fried and blamed it on lousy cheap ESC.

You ever listen to [All things that Fly] podcast?
www.allthingsthatfly.com/index.php

They have Lucien Miller (Innovative Designs/Scorpion Motors)
www.innov8tivedesigns.com/index.php
come on occasionally and answer questions. He did a segment on ESC, and how they work. He explains things well. It's a show a lot of folks on this forum might like.

[duck9191]

humm, the 40 amp super simple uses 50amp fets for the motor drive, that's good.

those pod cast look pretty informative. ill have to check out some more of them later