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sBEC / uBEC and stability question.

Old 04-02-2015, 10:09 AM
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solentlife
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Default sBEC / uBEC and stability question.

Lets say a 5v 5a sBEC is powered by a 3s 1800 lipo.

How stable is that 5v ? As load increases and amp draw increases - at what level does voltage drop and Rx may drop-out ?

Second - there are posts giving figures of servo demands of whole amps. This makes a 5a BEC look like its at limits.

Lets look at a model, excuse the fact bits a Glow powered model, but it has direct relevance.

58" model with 10cc motor up front. FrSky Rx, 2 x 20gr analogue, 1 x 40gr analogue, 3x 25gr digital servos. Flies reliably and without trouble on 4.8v Rx NiMh.

Let's assume I replace the NiMh with a sBEC on a LiPo - is a 5a sufficient ?

Denny - I know your bursting at the starting blocks to post about A123 / LiFe - my post is really to not only help me, but anyone else out there looking at power levels of sBEC's.

Nigel
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Old 04-02-2015, 11:42 AM
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fhhuber
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Depends if its a switching or linear BEC and the quality of the BEC

Generally its not a drop in amp output you have to worry about with differing voltage input but instead exceeding the ability of the BEC to cool and burning up the BEC, especially with linear BECs

Depends on the servo as to how much it will draw under normal loads and when stalled. Its possible to have a stalled Futaba S3003 pull over 2 amps. If you don't overload it it will draw a LOT less. But you should plan on worst case condition which is something going wrong with a linkage and stalling at least one servo.

Generally if it was reliable using a Spektrum RX on a 4 AA cell NiCd the 5 amp BEC should be OK, since 5 amps would pull the voltage of the NiCd pack down enough to put the Spektrum into its infamous "brownout". The standard switch harness typically used isn't going to handle 5 amps well either.

If it seemed reliable on 72 mhz... or some of the other brands you could be into an area where you really shouldn't be with NiCd powering the RX and not realize it. Most 72 Mhz RXs (except PCM) don't drop out until below the voltage to make a servo such as the Futaba s3003 move with no load. Similarly some of the other brand 2.4 ghz Rxs don't drop out until well below where the servos are only giving half their rated output power.

I'd want more RX (+servos) system power available than the standard 4 cell AA NiCd for the example aircraft. I've seen too many 10 cc (.61 ci) glow models have issues with just 5 servos and a 4 cell NiCd.

The only way to know you have enough RX system power is some sort of logger to tell you how low the lowest voltage (or the max current demanded) was at the RX during the flight. If you don't have the measurement then too much power is almost enough. Data logging and telemetry are getting cheap enough to consider them as basic test equipment.

https://www.dimensionengineering.com...ervosense-plus
(I was a tester of one of the prototypes)
https://www.commonsenserc.com/produc...oducts_id=1464
commonsenserc.com bought the rights to sell the device.
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Old 04-02-2015, 04:52 PM
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solentlife
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The original post was centered on Switch mode not Linear. Note use of terms sBEC and uBEC.

I'm aware that input voltage is important and not exceed designed levels.

My question is really does the voltage stay stable throughout the amp range demands ?

Second what about when demand exceeds switch modes maximum output ?

As to use of 4 cell NiMH, my Pitts (glow 53), Sky master biplane (glow 61), Extra (glow 91 / 61) - all fly fine on such. No glitches, no brownouts, no trouble.

The thread came about after a chat with guy locally who suggested swapping the Extra NiMH pack for sBEC with LiPo as I have box full of gear. My sBEC's are all 5A items. Being switch mode they cannot be doubled up to provide extra power without separating servo leads etc.

Nigel
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Old 04-02-2015, 06:53 PM
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Depends on which BEC... not all makers are equal.

Exceed the BEC rating...it gets hot and can melt down. Some might have a thermal shut-down to prevent the BEC from being damaged, but that's not going to help you maintain control.

You can put BECs in parallel... put a Shotkey (low voltage drop) diode in the + lead of each BEC. You lose a little voltage.
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Old 04-02-2015, 10:51 PM
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The built in linear esc in my 18 amp esc (rated at 1amp) wasn't brought below 4.97v's with 5 full sized servos.interestingly, the 9 gram servos pulled more under load then the full sized servos, but it was on my slowstick with only two 9 gram servos.

I bought a switchmode ubec from heads up after using a becless esc. It was rated for 5 amps and didn't drop below 5v's, even with 5 servos. I also never had any problems with .40 sized and larger models with a modern 4 cell spectrum battery pack and receiver with full sized everything. I didn't verify votages on those though.
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Old 04-03-2015, 08:39 AM
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Voltage shouldnt drop significantly unless you exceed the rating of the BEC. Not all 5A BECs are equal, many in fact are really only rated 3A but have a (claimed) short term burst of 5A. If it's a genuine 5A BEC that can deliver 5A constantly then it shouold be ok but you could remove all doubt with a bit of cheap overkill: http://hobbyking.com/hobbyking/store...dProduct=62915
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Old 04-03-2015, 11:15 AM
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Good god, that's cheap. I've just ordered 2 as spares.
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Old 04-08-2015, 03:57 AM
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Default A123s?

Originally Posted by solentlife View Post
Lets say a 5v 5a sBEC is powered by a 3s 1800 lipo.

How stable is that 5v ? As load increases and amp draw increases - at what level does voltage drop and Rx may drop-out ?

Second - there are posts giving figures of servo demands of whole amps. This makes a 5a BEC look like its at limits.

Lets look at a model, excuse the fact bits a Glow powered model, but it has direct relevance.

58" model with 10cc motor up front. FrSky Rx, 2 x 20gr analogue, 1 x 40gr analogue, 3x 25gr digital servos. Flies reliably and without trouble on 4.8v Rx NiMh.

Let's assume I replace the NiMh with a sBEC on a LiPo - is a 5a sufficient ?

Denny - I know your bursting at the starting blocks to post about A123 / LiFe - my post is really to not only help me, but anyone else out there looking at power levels of sBEC's.

Nigel
Hi Nigel
As for me, ALL of my 500 Watt and larger models use 10 Amp Castle Creations sBEC's for receiver power. (Don't have any glow or gasser models) Those CC uBEC's are all programmed for 6.5 Volts DC output voltage. Even my two giant scale models running near 3000 watts up front.

In fact, those two giant scale models are equipped with my home design undervoltage alarm on the sBEC, set to 6.0 Volts DC. Along with a two cell A123 pack for backup power. The way this system is set up is, if the receiver voltage for my giant scale models EVER drops down below 6.0 Volts DC to where the A123 backups take over, it sets of the undervoltage alarm. (The two cell A123's have a pair of silicon diodes in the red wire to drop their voltage down to around 5.5 Volts DC.)

And, after flying with those giant scale models for four years, those under voltage alarms have never gone off. FYI, my giant Big Stick is equipped with seven Hitec 645MG servos, that pull up to 2 Amps each.

I've tested my CC uBEC, it still put out the programmed 6.5 Volts DC while briefly pulling 15 Amps out of it. I've hung my oscilloscope across several of my CC 10 Amp uBEC's and watched voltage while hitting it with a 10 Amp load. The voltage didn't sag very much, pretty much matching what an A123 would do at the same current.

I did have one of those Common Sense uBEC's in one of my models. That thing suddenly decided to put out 4.5 Volts DC. It was junked. Lucky it didn't cost an airplane. So, IMHO, it's not wise to go cheap on a uBEC for a model that costs 20 times the uBEC.

As for using a LiPo battery for uBEC power, personally I don't know if you'd gain anything in reliability. From what I've seen, LiPos that have been damaged by overdischarge or what ever, tend to open circuit, last thing you want for receiver power. A123's don't do that, and I've tried!

Last edited by kyleservicetech; 04-08-2015 at 04:17 AM.
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Old 04-08-2015, 04:10 AM
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Originally Posted by fhhuber View Post
The only way to know you have enough RX system power is some sort of logger to tell you how low the lowest voltage (or the max current demanded) was at the RX during the flight. If you don't have the measurement then too much power is almost enough. Data logging and telemetry are getting cheap enough to consider them as basic test equipment.
Yeah
I designed and made up a bunch of under voltage receiver battery alarms for my giant scale models, along with some for club members. This thing used a little MicroChip PIC chip that measured the voltage 4000 times per second. And if the receiver voltage EVER dropped below 6.0 Volts DC, it flashed an LED as a warning.

Still using them in my giant scale models.
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Old 04-08-2015, 05:08 AM
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Hi Nigel,

I've attached a graph of Voltage at the RX from telemetry data from my Firestorm. It has 4 12gm metal gear analogue servos and 2 electric retracts. The retracts can be power hogs when stalled. Power is from an integrated 6 amp sBec in the ESC. This one from HeadsUpHobby. http://www.headsuphobby.com/Sky-Powe...-BEC-H-105.htm

The Specs say it should output 5.5 Volts. The graph shows 5.4-5.6 volts. I have no real idea how accurate the telemetry data is. It's Spektrum stuff. I am glad to see that see that voltage stays this consistent. All of my flights have fallen into this range. The telemetry data has been really handy tracking these types of questions.

Steve
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Old 04-08-2015, 05:13 AM
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fhhuber
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The Spektrum telemetry is actually using Eagletree sensors which is good stuff... proprietary software in the module that encodes it and sends the data to the TX for display and logging, so you can't use the Eagltree software to read the data.

There IS a guy who posted a free to DL program that will read the log file. forgot exactly where. I downloaded it though and its good.
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Old 04-08-2015, 09:35 AM
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solentlife
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Just a comment - with retracts, I would never power from same source as Rx and other servos. Retracts are most liable out of all to get stalled - therefore I would always have separate supply to them. It only needs a small pack in fact. I have a 250mA 4.8v pack that is ideal for the job. Charges up between uses quickly, provides enough for more than a couple flights.

As you guys realize, this thread is more as an information matter than actual change of gear. I'm sure I am not only one who is interested to know stability of BEC.

I have to admit that I do have a lot of LiPo's that could easily be put into service on at least one model, but the others where Rx battery is buried would be unwise.
I have a boxful of uBEC's all at 5v 5a .....

Nigel
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Old 04-08-2015, 09:01 PM
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fhhuber
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Modern electric retracts sense a jam and will shut down the motor. Special retract servos have had that feature also for several years.

Before this feature came out, I split the power for retract servos (for mechanically servo-operated retracts) to a separate battery. Now there's not much point. There never was a need to split an air retract system's servo to a separate source.
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Old 04-09-2015, 12:32 AM
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I am aware of e-retracts that cut power if stalled - I have two sets at home with such feature. But I still feel better having them separately powered.

It all goes back to a 40 powered Spitfire many years ago I had. Mechanical retracts with standard servos. Nearly lost her due to bent leg and stalled servo. Even though I know its different now - old habits die hard. And to carry a small pack just for retracts is no real hardship ....

Nigel
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