CC BEC ,design flaw.
 

So here's the story.I was running my Erevo last week and the steering stopped working.I took it in and checked it out,and the servos had just stopped.Checked everything out and it seemed alright.So I thought OK stock servos it happens.Tried a old spare and it seemed to work fine.Put in a Digital Futaba,tested it with a 2s lipo and all seemed fine.Got ready to run on 4s,turned it on and ....smoked it.So I said to myself,could be a bad bec, grabbed my other 1 that I had from my Maxx.(The last time I had drove it my MM smoked)So tried it all out again on 2s and all was fine.Put in my JR servo,loaded up 4s and turned her on....smoke.So now I'm a little upset,slap my Novak system in my Maxx,only to find that it's servo's had been fried when the MM went previously,or perhaps the bad BEC had something to do with it.So at this point I ask on the brushless forum and someone said that when a CC bec fails it can just pass the input voltage stright through.So I finally got around to testing them today and that exactly what happened.Green light still on showing all is good,but they are putting out the same voltage thats going into them.I believe this is messed up.Why put out a product like this with no built in failsafe.I'm out a couple hundred in servos,all over something that IMO should not be able to happen.

this very subject has been discussed before and for this reason I will never use a CC BEC...here are a couple of links to good BECs....I run the Western Robotics High Voltage model

http://www.helidirect.com/product_in...oducts_id=4667

and the Koolflight is a good one also

http://www.hobby-lobby.com/ubec.htm

http://www.helidirect.com/product_in...oducts_id=4667

:oh: ^^^ That is a serious BEC!!!!

I've been running 2 CC BECs since they first came out with no issues on 4 and 5S lipos. I don't remember seeing any of those threads on problems, but I'm a bit concerned now, not so much about the servos but blowing my Nomadio React receivers which have become pretty rare and scarce to get for decent $$ w/o getting gouged on e-bay. I could easily see a fully charged 5S blow a receiver putting >20v through it.

If you are electronically inclined, a voltage sensing switch can be added to the output of the CCBEC so if the output goes above a set value (~6.5v should be good), the circuit simply cuts power. Basically just an op-amp with the CCBEC output voltage fed to the + input of the diff amp, and create a reference voltage (6.5v) using a zener and resistor on the - input of the diff amp. The output of the op amp can be a PNP transistor (probably a darlington) or a FET that actually does the switching. Should be simple enough.

I will never put another in my vehicles.These are both pretty new,probably neither ran 30 times.Pissed me off so much I put my Novak system in my Erevo just So I wouldn't even have to think about it.Not to mention not being able to get my MM replaced.I thought I was inclined,but you lost me. :) IMO that should be included with it.

My 2nd CC BEC crapped out on me this weekend.

Wow that bites asheck.

I'm not out to slam CC here but I did have a CCBEC fail with no use, after thoroughly checking and re-checking, my rx & servos were spared cuz it was running off 2S, (under the max for Spektrum) but for some reason the insulation on the rx lead of my (then) brand new MM almost completely melted off, I had no choice but to resolder a new lead on the PCB, which is not something I want to do again.

But....... those of you not racing- weight be damned- run an rx pack!

I really do not understand how weight is such an issue for plain bashing.

Just my 3 cents.

If i get some time this weekend, I'll throw together a prototype circuit and post the schematic. There's enough of these out there that some protection would be nice. I probably should go out and get a FET, but I'll most likely use a darlington transistor simply because I can make that out of the discrete transistors...

Well,can't help but grin here,Now that I have an open sevo spot that might be just the place to put a receiver pack. Brian I won't be using another 1 of these,but it sure sounds like it's needed,and you have me quite curious as to what it would take to make these safe.

That's a logical and pretty simple design, but it's been a long time since I etched a circuit and would have to spend as much on chemicals and supplies as buying one of those linked ones. Also - wouldn't the transistor have to support the full battery voltage so that it doesn't burn out just from overvoltage across terminals?

i have a dumb question for you guys, the MMM bec doesnt happen to have the same problem with passing input voltage through when it fails as does the cc bec?

I may be barking up the wrong tree, but is there any way a fuse of some sort could be added to the output? If it blows then the ccbec is toast anyway. I have played with the small buss fuses (slow blow) in my Cerwin Vega tower speakers... would something like that work? I know that fuses are usually used for over current purposes, any chance they could be used for over voltage?

I only ask as I have about 10 of the ccbecs, and all run $50+ receivers and $110+ servos. I can only imagine the flame that I would produce if one of them cooked either component.

I am a die hard cc user, but may have to move these becs along if they are questionable.

yeah, etching is a PITA if you don't have the stuff already. I would just breadboard it with the pre-clad PCB found at radioshack. There is a minimum amount of components so it should be clean. If you start adding things like status LEDs, and other fancy stuff, the circuit gets bigger. Enclose the whole thing (except the switching transistor/FET) in epoxy or something for protection.

And yes, the output transistor/FET would have to be rated for the battery voltage you plan to use and the max current you plan to draw. The good news is that ratings for 40v+ and ~10A are not difficult to come by.

It would depend on the design. IIRC, the MMMBEC also uses a discrete BEC design (as opposed to a BEC circuit IC like most BECs use). Patrick has said that ANY BEC that utilizes a discrete switching FET is subject to this type of failure. So, I would say that it is a possibility. However, we don't know exactly how the CCBEC fails to make it do that, so it's difficult to know if the same thing would happen to the MMM BEC.

A fuse is for over-current protection. We need over-voltage protection. The only way the fuse would help is if the CCBEC failed, sent full B voltage to the devices, and those devices pulled a lot more current. But, if that happens, the damage is already done. Many devices are voltage sensitive with little or no actual current flow. The only way I see to protect your expensive receivers/servos is to use an external voltage-sensing switch.

Oh this is nice to read about.
Anyone wanna buy my brand new and never used CC BEC?

Chances are nothing will go wrong. I've been using one in my Hyper8 with no issues. And I'm sure there are lots of them out there being used with no problems as well. It just seems bad because most people don't make new threads saying "my stuff works" - they just post when it doesn't.

Three of them have been used in my RCs, one in my rock crawler powering a Hitec 5955TG, one was in my Hyper 8 on 4s pushing an Airtronics 359 and is now in my MT2 on 5s2p A123 with an Airtronics 357, and the last one is in my CRT.5 on 3s with an Airtronics 359. And my SH buggy is using the internal MMM BEC with an Airtronics 359.

Guys, ALL SWITCHING REGULATORS CAN FAIL OPEN...THIS IS THE NATURE OF THE BEAST. If you don't like the idea of that, use a receiver battery.

Did you mean they can all fail closed? I assumed that the FET S-D terminals shorted (or were driven that way). Anyway, fair enough. It just seems that most other BECs fail by simply stopping to operate.

Knowing that, how hard/expensive would it be to incorporate a voltage switch at the output to cut power if it exceeds a certain value? Since the CCBEC is adjustable to 9v, it would have to be above that...

Well, we could use the microprocessor that is on the BEC to determine if the power FET failed, and then use another FET to cut off the output (or use an additional comparator output to drive the FET, and generate the compare voltage with the D/A converter.... etc...)

The issue is that the CCBEC shouldn't be failing -- not with all the over current protections it has. We need to look at the applications to see if we can reproduce the failures in the lab. There shouldn't ever be a failure in a car with only one servo -- people run the CCBEC without any issue on helis and aircraft with 6+ servos.

Patrick

I'd add I put 10s (twice) into one of mine (and a MM) and they both survived to tell the tale...

I had a Koolflight BEC fail on me and it took out all my electronics with it. I think it can happen with any BEC.

The only reason I got into cars was due to getting bored waiting for the weather to be good for flying. I thought it would be easy (&cheap), I can fly for a year without breaking anything... Not so, the bastards break something every 10min of driving. I guess the electronics don't like being thrown around much either and the conditions I imagine would be hard to replicate in the lab.

Patrick,I threw 1 away but I'm actually pretty close to you so if you want to test the other 1 to see why it failed your more then welcome to it.I need to come in to your shop once you get some MM in anyways.But what I'd like to know is if all these can fail like this,and you know how to fix it,why not do it?BTW 1 of them had only been used with 4s and the other all 4s except 3 times with 6s and a couple with 5s.The 1 I threw away had some slight wire fraying on the receiver wire at the board.Which I couldn't see until I unwrapped it.But it didn't appear like it had been able to short.

Wouldn't that be nice - living close enough to CC to be able to make personal visits. :yes:

Too bad that for the price of the CC BEC most people probably do throw them out rather than sending them in to be checked.

Patrick or Joe, do you want mine?

I bought it from Impakt in May I think, so too late on that one to return it anyway.

And I am using an rx pack so I don't care about an exchange, maybe a coupon for $20 off a Monster?

;)

Can't you just put an appropriately sized zener diode as a shunt across the + and - bec outputs?

That's called crowbar protection. As long as the BEC output voltage was lower than the zener voltage, things would run fine. If the BEC goes bad and the output voltage goes higher, the zener would basically short out the BEC causing it to either shutdown from overcurrent, burn up, or the zener would blow. Since the CCBEC can output over 7A continuously, you'd need one heck of a zener. Otherwise, as soon as it blew the voltage would go back up to whatever it is outputting. Not really an elegant solution IMO, but certainly simple enough.

Please explain, what in theory would that do in an overvoltage situation? (Im getting a headache form this thread...).

EDIT: its like magic....

Unless the zener was big enough to handle that kind of power, it would blow, and then your receivers and servos would get the full battery voltage again. Crowbar protection is not really the best solution IMO. And if the zener didn't blow, the BEC wires would likely fry.

lol, and there's a similar thread in the Traxxas forums about this very thing. :smile:

Just have the zener trigger a larger relay that will handle the current. Or, put a fuse in-line with the CCBEC power leads and put a relay in place to short those leads after the fuse. It will blow the fuse and then the BEC shuts off.

That's a possibility I suppose. I still don't like the whole shunting protection scheme, but that's just me. Leaves too much room for high enough currents to sizzle a wire, which may cause collateral damage depending on how bad it is.

I'm not saying my idea is better per se, but it is more elegant. The best thing would be to have some type of protection built into the BEC itself IMO.

The zener wouldn't be a shunt with my method, it would conduct into one terminal of the relay. Once the fuse blew, nothing would be conducting. This doesn't change the fact that some type of protection should be built in. My CC BEC has been working flawlessly. Let's hope it continues to do so.

Hey guys... I haven't kept up with this issue. Is it common that the CCBEC lets full voltage pass through if it fails?

Does it seem like there are quite a few going bad?

I was getting concerned a while back when there were a couple reports.

yea im running one in my buggy, its been goin a long time now, no issues, now i jynxed myself

I'm not sure what the failure rate is.It doesn't seem to be highly reported,but 2 out of 2 of mine failed in 2 different vehicles.

I dont think it happens a lot but if it fails it can fry your Rx and servos......that can be over $200 for a failed $25 componenet...I guess a Rx pack is the only way to be 100% sure it wont happen

j3110 try ebay bro otherwise you may not even be able to give it away. Ive never had one fail because ive been smart enough not to own one. I only run western robotics hecules bec or dimension engineering bec's they rule.......sorry sammus.

:lol: dont be.

j3110: you can give it away to me :P

You can increase the power handling of a zener by adding a power transistor. I think this is what what'snitro was saying except with a transistor. The transistor should be able to dissipate the power, but if you are afraid of melting wires, you could add a fuse. The nice thing about the zener shunt is that it is completely passive until needed.

http://i187.photobucket.com/albums/x...na_dog/ckt.jpg

its been a while since I saw a schematic....

Watching this thread......i dont want to waste an expensive RX/Servo