Wattflyer RC Network: RC Universe :: RCU Magazine :: RCU Forums :: RCU Classifieds :: RCU User Reviews :: RCU YouTube
Home Who's Online Calendar Today's Posts RealTime Post Spy Mark Forums Read
Go Back   WattFlyer RC Electric Flight Forums - Discuss radio control eflight > R/C Electric Power - Batteries, Chargers, ESCs and More > Power Systems
Register Members List Wattflyer Extras Articles Search Today's Posts Mark Forums Read Social Groups

Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

Thank you for your support (hide ads)
Reply
 
Thread Tools Display Modes
Old 09-04-2011, 01:40 AM   #1
kyleservicetech
Dennis V
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 7,933
Thanked 683 Times in 666 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
Awards Showcase

Outstanding Contributor Award  3kW  2kW  100mph Speed Demon 
iTrader: (1)
Friends: (20)
Default ESC Linear BEC Current Rating???

I've posted this thread in a different location in www.wattflyer.com, but IMHO it's important enough to have its own thread.

This posting started when looking at another thread where the Mfg specified that the linear BEC in their ESC had a peak current rating of 0.7 Amps, or 700 milliamperes, that when used with a 3 cell LiPo with 11 volts DC output.

This max BEC current rating of 700 Ma is scary!!

Lets take a look. This ESC has a 3 cell LiPo on it, for a voltage of about 11 Volts DC. So we have two voltages to be concerned with in this linear BEC in the ESC. First is that 11 Volts DC into the linear voltage regulator. Second is the 5 volts DC out of the same linear voltage regulator. So simple subtraction of 11 VDC minus 5 VDC indicates we have to account for the difference of 6 Volts DC in the regulator itself.

Now, they have a published maximum current rating of 0.7 Amps on that BEC. So we have the watts lost inside the BEC itself as W=Volts times Amps, or 6 VDC times 0.7 Amps. That is 4.2 watts.

Anyone that has ever taken a 5 watt resistor, and run 4.2 watts through it will find out it's temperature will quickly increase to high enough to brand your fingertips if you ever touch it. (Running a typical wirewound resistor at its rated maximum wattage results in a resistor surface temperature on the order of 300 - 400 degrees F.)

So, if you run 4.2 Watts through a linear voltage regulator, you'd better have a pretty good sized aluminum finned heat sink on it, or it will heat up to several hundred degrees in only a few seconds.

Problem is, these linear voltage regulators in many ESC's do not have any sort of a heat sink to remove heat from the regulator itself. And, most linear voltage regulators have built in protection that shuts them off if they overheat, protecting themselves. (And crashing your model. By the time you get to the model wreckage, that regulator has cooled off and is working again.)

Bottom line, those "Peak current ratings" provided by the ESC manufacturer for their BEC's should IMHO be taken with a big grain of salt. Or perhaps the entire salt shaker. Those peak current ratings are only valid for a few seconds at best.

Out of curiosity, I took an LM7805 linear voltage regulator, hooked it up to 11.5 Volts DC on my variable power supply, and connected a 0.7 Ampere load to it. No heat sink was used. That regulator heated up to 175 degrees F within 25 seconds. That LM7805 regulator measures 0.37 by 0.60 inches. The regulator on one of my ESC's measures 0.25 by 0.25 inches. Since this ESC regulator has only 1/4 the area of the LM7805, that little regulator is going to get very hot, very fast when trying to radiate 4.2 watts.

Bottom line, if you think you might have a problem with the linear BEC in your ESC getting to hot and shutting down, you are probably right.

What's the solution? If you've got any question on whether your linear BEC can do the job, put in one of those switching power supply BEC's such as Castle Creations 10 amp uBEC.

How these uBEC's work is a little involved, but basically their internal parts are never in their "linear mode". Just think of taking a toggle switch, put it in series with a 20 Volt battery and a light bulb. Then switch it on and off with exactly a 20% on, 80% off cycle. At 100,000 times per second. (Difficult with a toggle switch, easy with electronic switching) So, with a little electronic smoothing circuitry, your output voltage would be that 20 VDC times 20/80 or 5 VDC. That switch will never get hot, since it is either a dead short (no heat) or open circuit (again no heat).

You can actually build up a simple switching power supply with only a half dozen cheap easily available electronic components.

DennyV
Retired and the days are just too short, busier than ever!
kyleservicetech is offline  
  Reply With Quote
Old 09-04-2011, 01:44 AM   #2
rcers
Community Moderator
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,160
View rcers's Gallery57
Thanked 501 Times in 491 Posts
Awards Showcase

WAA-08 Pilot 
iTrader: (4)
Friends: (9)
Default

External switch mode BEC's rock.

For this reason - they are sure worth a close look. This is why so many blame their radio for an issue created by their cheap ESC and linear BEC.

Mike
rcers is offline  
  Reply With Quote
Old 09-04-2011, 01:45 AM   #3
gramps2361
Community Moderator
 
gramps2361's Avatar
 
Join Date: Dec 2008
Location: oxford, MA
Posts: 6,610
View gramps2361's Gallery9
Thanked 598 Times in 593 Posts
Club: Rocky Hill Flying Circus
iTrader: (3)
Friends: (21)
Default

Denny is this why a switching bec is better and produces less heat, with better reliability.
gramps2361 is offline  
  Reply With Quote
Old 09-04-2011, 02:00 AM   #4
kyleservicetech
Dennis V
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 7,933
Thanked 683 Times in 666 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
Awards Showcase

Outstanding Contributor Award  3kW  2kW  100mph Speed Demon 
iTrader: (1)
Friends: (20)
Default

Originally Posted by gramps2161 View Post
Denny is this why a switching bec is better and produces less heat, with better reliability.
You are correct. IMHO, I'd only use a built in linear BEC on a model with no more than a 2 cell LiPo for battery power.

A while back, I had a 3 pound model with a 4S A123 pack, and four small servos. The BEC was hand made with a pair of those LM7805 Voltage regulators in parallel. Those regulators were bolted to a piece of 2 1/2 inch square, 1/8 inch thick aluminum plate for a heat sink.

After flying for 6-8 minutes, that aluminum plate was very warm!

DennyV
Retired and the days are just too short, busier than ever!
kyleservicetech is offline  
  Reply With Quote
Old 09-04-2011, 02:18 AM   #5
gramps2361
Community Moderator
 
gramps2361's Avatar
 
Join Date: Dec 2008
Location: oxford, MA
Posts: 6,610
View gramps2361's Gallery9
Thanked 598 Times in 593 Posts
Club: Rocky Hill Flying Circus
iTrader: (3)
Friends: (21)
Default

Good info to know for any new person and myself included. Seems like I have been on a learning curve this flying season, and it is not ending. Hope more gets added to this thread very good info for a guy like me who is not electronic savvy.
Did I ever mention that I had a exploratory at my trade school in electrical and the teacher kindly told me this might not be the best trade for me.
Why I have been very cautious about setting up my next plane I do have a hard time wrapping my head around the electronic part of this to make it all happen with out a failure.
gramps2361 is offline  
  Reply With Quote
Old 09-04-2011, 02:30 AM   #6
kyleservicetech
Dennis V
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 7,933
Thanked 683 Times in 666 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
Awards Showcase

Outstanding Contributor Award  3kW  2kW  100mph Speed Demon 
iTrader: (1)
Friends: (20)
Default Switching Power Supply Design Software

"Simple" Switching Power Supply with 5 Volt DC and 3 Amp output.

http://www.national.com/en/webench/power.html


Attached Thumbnails
Click image for larger version

Name:	Five Volt Three Amp Switching Power Supply.jpg
Views:	225
Size:	50.8 KB
ID:	152561

DennyV
Retired and the days are just too short, busier than ever!
kyleservicetech is offline  
  Reply With Quote
Old 09-04-2011, 09:38 AM   #7
slipstick
Super Contributor
 
Join Date: Oct 2005
Location: N.Staffs, UK
Posts: 2,350
Thanked 197 Times in 191 Posts
Awards Showcase

Outstanding Contributor Award 
iTrader: (0)
Friends: (1)
Default

OTOH it's worth realising that the load on a BEC is generally VERY intermittent. Servos only draw lots of current while they're moving and how often do you have every servo in your plane moving constantly for even 3 seconds never mind 30 ?

If you have the BEC correctly sized to take care of the peak servo load then most of the time it's delivering less than 5% of what it's capable of. That's probably why many people have been successfully using linear BECs, even on 3S lipos, for years.

Of course switch mode regulators are more efficient and cooler running and I too prefer them for 3S or greater particularly with more than 3 servos. But they're not perfect, they cost more and also cause more RF interference, though this is only likely to be important if you're still flying on FM than on 2.4GHz.

Steve
slipstick is offline  
  Reply With Quote
Old 09-04-2011, 11:17 AM   #8
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,323
Thanked 477 Times in 445 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

Also worth pointing out that these days most ESC's with a rating greater than about 40A have built in switching mode BECs that in most applications should be just as reliable as external uBEC's
JetPlaneFlyer is offline  
  Reply With Quote
Old 09-04-2011, 01:45 PM   #9
rcers
Community Moderator
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,160
View rcers's Gallery57
Thanked 501 Times in 491 Posts
Awards Showcase

WAA-08 Pilot 
iTrader: (4)
Friends: (9)
Default

Originally Posted by JetPlaneFlyer View Post
Also worth pointing out that these days most ESC's with a rating greater than about 40A have built in switching mode BECs that in most applications should be just as reliable as external uBEC's
I don't know that I agree with "most" at least not yet - but they are heading toward that.

Most of the cheapo Chinese still use linear and that seems to be what most folks are buying.

And my only brownout crash was with a switch mode BEC in a Castle Ice 75 running 4 medium size digital servos. That 5 amp BEC gave up on landing not a super servo intensive task. Nothing like the aerobatics I was doing during the flight.

That is why I don't use BEC's on my larger models at all. Gone to LiFe batteries and servos that can handle that voltage. One less thing to fail (so long as you remember to charge them).



Mike
rcers is offline  
  Reply With Quote
Old 09-04-2011, 02:47 PM   #10
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,323
Thanked 477 Times in 445 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

Originally Posted by rcers View Post
I don't know that I agree with "most" at least not yet - but they are heading toward that.

Most of the cheapo Chinese still use linear and that seems to be what most folks are buying.
I guess it depends how 'cheapo' you go.. Of the Chinese ESC's most of the recognised names have gone to switching BECs on their larger rated ESC's.. For instance Hobbywing, Turnigy Plush(re-branded Hobbywing), Mystery, these all use switching BECs.

If you look on 40A -79A ESC's on Hobbyking s everything above $19 has a switching BEC, and a few below that price too.

Of course even a switching regulator can occasionally fail. Even batteries can let you down, it only takes a solder joint to be bad, wire to break or a cell to go down, or as you say, you might forget to charge it.

Steve
JetPlaneFlyer is offline  
  Reply With Quote
Old 09-04-2011, 03:00 PM   #11
rcers
Community Moderator
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,160
View rcers's Gallery57
Thanked 501 Times in 491 Posts
Awards Showcase

WAA-08 Pilot 
iTrader: (4)
Friends: (9)
Default

Don't want to give folks the impression "most" esc over 40 amps have switch mode BEC's. I would certainly look for that when buying so you can be better educated.

Issues still remain. Switch mode BEC does not mean, all is well. What you may not know is how much capacity those switch mode BEC's can deliver. And then most don't even know how much power their servos will draw under load. I certainly didn't when my 1.5amp EACH servos took down my plane.

While I agree batteries are not perfect they work well and models that use 40amp ESC's or above can generally easily take the few ounces of extra weight.



And with all this we tend to do OK - planes don't just fall from the air in large numbers.

Mike
rcers is offline  
  Reply With Quote
Old 09-06-2011, 05:19 AM   #12
kyleservicetech
Dennis V
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 7,933
Thanked 683 Times in 666 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
Awards Showcase

Outstanding Contributor Award  3kW  2kW  100mph Speed Demon 
iTrader: (1)
Friends: (20)
Default

Originally Posted by slipstick View Post
OTOH it's worth realising that the load on a BEC is generally VERY intermittent. Servos only draw lots of current while they're moving and how often do you have every servo in your plane moving constantly for even 3 seconds never mind 30 ?

If you have the BEC correctly sized to take care of the peak servo load then most of the time it's delivering less than 5% of what it's capable of. That's probably why many people have been successfully using linear BECs, even on 3S lipos, for years.
Agreed:
It's true that the current pulled by the servos in a model airplane are very low. But that current will change by an unknown amount during flight with wind loads on the models surfaces. And that extra current depends on how fast the model is flying, how much "Throw" the servo has on its connected surface, whether you're flying 3D, and so on.

But, IMHO, any valuable model should have a solid DC source for the receiver. And again, IMHO any model using more than two LiPos on the ESC's Linear battery elimination circuit might be pushing your luck. A lot of modelers have been flying very successfully with 3 cell Lipos and linear BEC's over the years. But, if you've ever had a sudden total loss of control during flight, consider you might have had issues with the linear BEC, and not the 2.4 Ghz receiver itself.

My CC 10 Amp uBEC has a 30 millivolt 50 Khz ripple frequency directly on its output terminals when connected to a one ampere load. That ripple frequency changes depending on the amount of load placed on the uBEC.

After four flying seasons with those CC 10 Amp uBEC's they've had no notable effect on my AR7000 Spektrum receiver's range testing, or flying.

DennyV
Retired and the days are just too short, busier than ever!
kyleservicetech is offline  
  Reply With Quote
Reply

  WattFlyer RC Electric Flight Forums - Discuss radio control eflight > R/C Electric Power - Batteries, Chargers, ESCs and More > Power Systems

« Previous Thread | Next Thread »

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
100+ MPH Club CHELLIE Hi-Performance and Sailplanes 1955 08-20-2014 02:55 AM
Radian Pro... Not impressed. firemanbill Hi-Performance and Sailplanes 38 02-05-2012 05:14 PM
ESC Failure or motor failure? payne9999 Power Systems 29 08-25-2011 07:47 AM
Park 480 current rating payne9999 Power Systems 4 08-07-2011 12:55 AM
60 amp e-flite bec brushless esc programming scott-p Power Systems 4 03-05-2011 11:12 PM


All times are GMT +1. The time now is 11:17 AM.


Powered by: vBulletin
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2005 WattfFlyer.com
RCU Eflight HQ

Charities we support Select: Yorkie Rescue  ::  Crohn's & Colitis Foundation



Page generated in 0.31960 seconds with 44 queries