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[wullie/mk2]

If you use a BEC Rx with a 4.8 or 6 volt battery pack powering the whole system (2 servos + Rx) will the BEC cut power from the pack fairly soon because of the voltage drop??

[Netleyned]

I run my yachts on 6V 2600mA nimh's and after two or three hours I still have plenty of battery left.
This includes my marblehead with a 6V whirlwind winch and heavy duty rudder servo plus receiver.

Ned

[CF-FZG]

If you use a BEC Rx with a 4.8 or 6 volt battery pack powering the whole system (2 servos + Rx) will the BEC cut power from the pack fairly soon because of the voltage drop??

Terry,

I don't understand what you mean by BEC Rx - a BEC is fitted to most ESCs to provide power from the main drive battery to the Rx without having to have a seperate Rx battery, (and they are available as a 'stand alone unit' for those ESCs without a BEC).


Mark.

[Netleyned]

Some receivers have BEC in their description or on the case.
I have always thought that this was denoting that the Rx
could be run on a BEC the voltage being slightly above the old
4pack nicad standard of 4.8V when most bec's are around 5V.

Someone will know for sure and probably shoot me down in flames and smokin rxs 

Ned

[wullie/mk2]

If you use a BEC Rx with a 4.8 or 6 volt battery pack powering the whole system (2 servos + Rx) will the BEC cut power from the pack fairly soon because of the voltage drop??

Terry the original poster here. Maybe I am confusing myself guys. I just remember that in my electric flying days with BEC and a 7.2v power pack on board, once the voltage had dropped to a certain level, power to the motor was cut by the BEC so that there was enough juice left to control the plane's rudder, elevator, etc. for a controlled glide and landing. Maybe the ESC took care of this, I don't know.

[Netleyned]

The ESC would have contained a BEC to drop the 7.2V to 5V for the Receiver.
The ESC would be monitoring the battery voltage and cutting the motor
if the voltage dropped below a set point leaving the receiver to control the flight
servos.
No ESC in a Racing Sparrow so no probs.

Ned

[john s 2]

From my own experiance my boats slow down so i know when to bring them in the bec keeps working ok despite the voltage drop. The exception to this is lipos which can not be discharged to far. Pb cells again should not be taken to far down. If the bec is in the esc the same applies. On the one time i pushed things to far then the esc shut down because of low volts. Rowboat time. John.

[malcolmfrary]

I am fairly sure that the BEC on the RX label is best ignored.  It was probably put there at the insistence of a marketing man who thought he had a new buzzword to boost sales. It probably means that there is a voltage regulator in there that gives a stable voltage for the RX (but nothing else) to run on from a relatively wide range of battery voltages (say 4 to 6 NiCad cells), but it is difficult to say for sure since I have yet to find any manufacturers data saying just what it really does do.

[Netleyned]

Shoot the marketing guy then
If BEC means Battery Eliminating Circuit
then the RX is self powered.
And I walk on water 

Ned

[Martin (Admin)]

I used to know this stuff.....

 .... anyway, found this:   http://traxxas.com/support/receiver-equipped-BEC-What-voltageamperage-it-rated

A BEC is used to "step down" battery voltage so the receiver does not send too much voltage to the servos. In the early days of R/C, electric R/C cars used to carry a separate battery for the receiver because plugging the car's "main battery" into the receiver would overload it. The circuitry used to eliminate the need for the receiver battery was known as a "Battery Eliminator Circuit," or BEC. Like most mini receivers, the Traxxas 2.4GHz receiver is not equipped with a BEC. Whatever voltage your receiver battery or speed control puts into the receiver is what the receiver will deliver to the servos. The Traxxas 2.4GHz receiver is designed for use with 6-volt receiver packs (4 alkaline cells or 5 NiMh cells) and any electronic speed control.

[triumphjon]

i would imagine that the bec rx relates to the days before we all used an esc to control the motor , remembering the old tamiya wiper board type controller they were fitted with a small red bec type plug to fit into the rx battery socket , we would have had 7.2 volts into the rx , so maybe the rx had the circuit to drop the power down ? ?  jon

[CF-FZG]

i would imagine that the bec rx relates to the days before we all used an esc to control the motor.

You didn't read what several posters including Martin, right before your post said.....

BEC - Battery Elimination Circuit  it removes the requirement to have a seperate battery to power the radio gear.
I've not quoted your bit about the pre-ESC controllers as it's irrelevant to the discussion.


Mark.

[Martin (Admin)]

OK, getting back to the original question, yes a Receiver with BEC will continue to power the servo until the main power pack drops below a predetermined voltage and then shut down everything.

Some were programmed to shut-down the motor channel only allowing rudder but that was always a silly idea, what use is rudder only on a powered boat? Maybe it was intended for the fly boys....

A speed controller with BEC will also shut itself down on a predetermined voltage, again leaving power to the Rx & servos, just cutting power to the motor.

Some caveats;

 1. You need to ensure that the speed controller knows what type of battery it's connected to ( ie. Nicad/NiHd or Lead acid or LiPo)
     so it shuts-down at the right voltage.
 2. Some servos don't like 6v.
 3. Probably not a good idea to run high power servos through a BEC (eg. big sail winch, 1/4 scale etc. )

[CF-FZG]

i would imagine that the bec rx relates to the days before we all used an esc to control the motor , remembering the old tamiya wiper board type controller they were fitted with a small red bec type plug to fit into the rx battery socket , we would have had 7.2 volts into the rx , so maybe the rx had the circuit to drop the power down ? ?  jon

Okay, time for an apology

After mucho question asking and research - the BEC Rx came into being a long time ago, for exactly the reason triumphjon suggested - where it allowed the use of the main drive battery to power the Rx from a 6 or 7 cell pack when the SC didn't have it's own BEC, either because it was designed to run on a max of 5 cells or it was a non-electronic, (mechanical), SC.

So in effect they are a voltage regulator built into the Rx.

I can't find any definate info this, so I'm speculating that if the pack voltage drops below a set voltage they act like a failsafe and reduce throttle to zero, not much use if your boat's stuck in the middle of the lake, (discounting the normal feature of a BEC that will allow restarting of the motor at low power settings only), but then these BEC Rx were designed for RC car use - and it seems just migrated over to boat use not for the BEC facility, but because they are on the same frequency band(s).

So, once again, my apology jon.

Incidentally, the reason I've never heard the term before - was it seems that they came in after my RC car times in the 90's when I'd gone back to hairyplanes.


Mark.

[knoby]

Receivers with bec built in were indeed designed for RC cars to allow direct connection to the main battery, which at the time ( late 80's to early 90's)  was always 7.2 volt.The actual bec circuit were not sophisticated, usuall consisting of no more than 2 diodes connected in series to give a 1.2 volt drop. Therefore the don't cut power or switch to safe mode or anything else that complicated, they just drop the supply voltage down so as not to fry the Rx.

The electronic esc of that time were not capable of delivering the amperage required without a significant voltage drop, & so top speed were compromised. They got round this by using full power by pass relays ( i.e a relay gave direct connection to the battery when full power was required). This was not a great solution as some of the battery power was used up by the relay, so many top racers still used mechanical resistor type speed controllers.

I used to race mini stox which ran on 4 cells 4.8 volts & used a futaba Rx with built in bec for many years without incident, just when the battery voltage dropped below a certain level the motor would take all the power & the Rx would stop working, know as 'dumping', not a problem with a car as you just walk across the floor & pick it up.Obviously in a boat it can be more of a problem, although you would notice a significant drop in speed before this occurred.

Glenn

[roycv]

Hi all, as has been said the bec fitted in the receiver reduced the voltage from say the drive battery and supplied 4.8 volts or so to the servos including the esc.  As has been said else where you only need one ESC in a Rx system.

Earlier Rx's had discrete components as against a single chip for voltage reduction.  The discrete (separate components) circuit could operate at 0.75 volts above the 4.8 volts Rx supply.  With the single chip approach the input needed to be 2 volts above the Rx for the output power.
You may find that after a cut off you can operate at very low speed at get back to the bank.

I used to make my own bec's from a circuit in the old Model cars magazine.  It is just a few components on a vero board with a zener diode (this gives a fixed voltage) on the control (base) for the output transistor.

As has been mentioned there was a simple bypass circuit that called a relay at full speed on older type Rx's.  This just connected the battery to the motor.  You can detect full speed from the servo chip (i.e. the pulses stop and you get a DC o/p).
The relay was always switching at high current perhaps not a good idea but I seem to remember that the relay only took about 50 milli amps. (0.050 amps)  Not much current against the 15 - 20 amps of the car motor.  But it does make a significant difference to motor speed when it kicks in.

With modern Field Effect Transistors the voltage drop across the component is extremely small.  A transistor would have 0.75 volts and with early circuits there were 2 transistors in the output (the H configuration, 2 times 0.75 volts).

As power (watts) is current times voltage then say 6v (0.75 ) X current (say 10 amps) = 7.5 watts.  Quite a high loss of power which appeared as heat.  Hence the large heat sink of older ESC's.

Now, with FET (field effect transistor) the voltage drop is about 0.1 volts the power loss is a lot less and the heat sinks are much smaller and the ESC is lighter in weight. Also a bypass relay is not worth the trouble as there would be some slight loss in the relay contacts.

regards to all,
Roy