Switch Mode vs Linear Type BEC's - RC Groups
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Nov 10, 2011, 02:09 PM
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E-Challenged's Avatar

Switch Mode vs Linear Type BEC's

I would like to see a detailed and well written "Sticky" on this forum that explains what a "brownout" crash is , how to tell the difference from other types of crashes, and what to do to prevent them. I have tried repeatedly to explain this " phenomena " in simple terms aimed at beginners with 2.4Ghz systems.

Some 2.4Ghz model flyers still seem to disbelieve in the need for speed controls with switch mode battery eliminator circuits (BEC's) or add-on switch mode BEC devices since they have not had a suspected brownout crash (yet !!) with their foamy. They seem to think that somebody is trying to sell them something that they don't need. Others may be afraid of the process of installing an add-on BEC device.

Some of the 2.4Ghz RC systems makers and distributors are starting to admit (in small print) that when using 3 or 4 cell lipos with 2.4Ghz receivers and four or more servos, that you need to be using a speed control with switch mode BEC circuit that is rated high enough in amp output to avoid the low voltage to the receiver condition known as "brownout" and receiver reset causing temporary loss of control and a crash.

I don't feel qualified to write a sticky being "E-Challenged"
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Nov 11, 2011, 05:38 PM
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Not directly related to your post, but I thought I would share my most recent brownout experience.

I was flying an E-Flite 4-Site outdoors when I noticed that all of a sudden, the ailerons had reversed!

It turns out that it is possible to reverse the ailerons in the receiver, by turning the receiver off and then back on while holding the transmitter right stick in the 11 o'clock position.

What this meant to me was that the receiver must have reset while I was flying and just by chance the right hand transmitter stick was in the correct position to reverse the ailerons.

I am not overly concerned about brownouts on my little parkfly aircraft, but consider it to be an important consideration for helis and larger aircraft.
Nov 12, 2011, 10:07 AM
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E-Challenged's Avatar
I use DM's Park switch mode add-on BEC's in my small to medium size scale models if I am using a 3S lipo, a 2.4Ghz receiver and four or more servos. I was compelled to do so few years ago when I started using 2.4Ghz systems and crashing my GWS foamies and valued stick built scale models. With all the benefits of 2.4Ghz systems the brownout crashes are thankfully preventable with properly rated add-on switch mode BEC devices or modern speed controls with adequate switch mode BEC circuits built in.
Read the BEC specs carefully concerning use of 3S or 4S lipos and number of small versus standard servos.
Nov 12, 2011, 10:12 AM
Magicsmoke maker
Inflexo's Avatar
Well you have the two modes of "brownout";

1) the BEC linear regulators get too hot and go in to safety shutdown
2) the BEC linear regulators cannot handle the current demand directly and the over-current causes them to shutdown (internal current limiting safety)

#1 happens most frequently and gets worse as the cell count increases.
Nov 12, 2011, 10:12 AM
I don't want to "Switch Now"
pmackenzie's Avatar
3S is OK with linear BEC in many applications. A "wiggle" test where you move all controls full throw for 30 seconds to one minute should show up any problems.

4S will almost always need a switching BEC.

Having said that, I run a switching BEC on 2S in my DLGs!

Pat MacKenzie
Nov 12, 2011, 01:24 PM
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E-Challenged's Avatar

Beg to differ, brownout crashes don't happen until they do. Not worth the gamble due to variations in ambient temperatures, changes in servo current draw, drag in linkages, changes in EPA settings, etc.
Nov 12, 2011, 03:23 PM
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Doug Bartley's Avatar
Its my understanding that a linear Bec loses it rating as cell count goes up while switching Bec's don't.
EG: A linear Bec rated for 3A on 3S will only provide 2A on 4S, and 1.? on 5S. Switching Bec's I own will provide 4A on any cell count from 2-6S.
That said, I use separate Becs from Hobbyking (Megabec8 and Turnigy 8-15) that are linear Bec's, but designed for low cell counts either 2S or 3S. The Megabec8 is a 2 cell unit while the Turnigy 8-15 can be used on 2 or 3S ( I still prefer 2cell myself. I only use servos up to 6 volts, if you where to use the newer 7.4v servos, I'd run the 8-15 on 3S.
Some separate Bec are made to handle up to 42v -10S, they are usually switching Becs. I don't see why you would force a Bec to work that hard to reduce the voltage down to 5-6 volts. JM2C FWIW Doug B
Nov 12, 2011, 04:49 PM
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Lnagel's Avatar
Originally Posted by Doug Bartley
Some separate Bec are made to handle up to 42v -10S, they are usually switching Becs. I don't see why you would force a Bec to work that hard to reduce the voltage down to 5-6 volts. JM2C FWIW Doug B
That's the whole point of using a switching BEC. Unlike a linear BEC the switching BEC doesn't have to work to reduce the voltage to 5 or 6 volts. No work, no heat.

A linear BEC is on all of the time and to reduce 42 volts to 5 volts it has to drop the excess 37 volts across a series resistor (which is actually a variable impedance transistor) and that excess voltage is dissipated as heat. The exact amount of heat is dependent upon the load. The higher the load, i.e. the more servos there are pulling current, the more heat it has to dissipate. That is why it can drive only a limited number of servos.

A switching BEC however works just like the ESC. It reduces the voltage by turning on and off at a high frequency rate. It is that switching on and off action that gives the BEC its name. To reduce 42 volts to 5 volts the switching BEC simply turns on for approximately 12% of the time and remains off for approximately 88% of the time. Just like an ESC, it produces a 12% duty cycle square wave. The peak voltage of that square wave will be the battery voltage, 42 volts in this case. However, unlike the ESC the square wave output of the BEC uses a filter circuit to average out that square wave into a 5v DC voltage with very little ripple (ripple being remnants of the original square wave).

So, a linear BEC has to work continuously, and subsequently continuously produce heat, to reduce the battery voltage. Whereas a switching BEC actually rests for the majority time so it stays nice and cool. And since it is not burning off the excess battery voltage as heat it can safely supply power to a greater number of servos regardless of the ESC's input voltage.

Last edited by Lnagel; Nov 12, 2011 at 04:58 PM.
Nov 14, 2011, 11:27 AM
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E-Challenged's Avatar
See what I mean? This is a tricky subject especially for those just starting with electric power or upgrading from simple 3 channel electric powered gliders, etc. Looks like more are starting to "get it" but many more will have "unexplained" crashes.

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