This article was published in the FMT model magazine
in February 2002. You can download the german text (sorry).
Why you should connect your buffer battery ”just so”
and not using any other arrangement: this is explained in the document.
Which servos are suitable for BEC operation?
and
How many servos with which speecontroller?
If you think these questions imply a problem, you are quite right: this is it in a nutshell:
Not all servos are suitable for use with BEC systems.
and
You cannot connect any number of servos to a speed controller.
In the operating instructions supplied with our new speed controllers and governors
we discuss this problem in detail.
1) This is the basic rule: the stall current of a BEC-suitable servo should not be higher
than about 500 mA.
This is where the first problem arises: establishing the stall current:
For Graupner servos you can look up the servo specification in the Graupner catalogue.
Robbe does not publish this value (but Robbe will answer telephone queries).
The accuracy of the published data is quite another matter - for example, a manufacturer
may make modifications to a servo type, and its technical data will then differ from those
published in the earlier catalogue.
What happens if you connect four BEC-suitable servos (500 mA) to a 1.5 A BEC system?
Quite simple:
If all four servos run simultaneously or are operating under load, the voltage of the BEC
system will collapse even though the servos are BEC-suitable types.
Why?
Four servos times 0.5 A = 2 A. However, the BEC system is rated at only 1.5 A,
so it collapses under the strain (like a flat battery).
As a result the receiver gives up the ghost:
It will briefly send ”wild signals” to the servos, then shut down completely, together with the servos.
However, the servos are now no longer moving, so the BEC voltage recovers to 5 V again,
the receiver resumes working, and the ”in-house” receiver interference passes - until the next time.
Please note:
The sum of all servo stall currents must be lower than the maximum permitted BEC current.
Note: don’t rely on the fact that the two airbrake servos are usually left alone until the landing
approach. If interference occurs, they will also move involuntarily ...
2) It is equally true that you cannot just keep on connecting servos to a BEC system until
the maximum (calculated) current load of the BEC system is reached.
Used with a large number of cells, the BEC circuit would heat up and die unless protected against
thermal overload.
The BEC system has to reduce the full battery voltage to 5 Volts, and losses inevitably occur
in this process (in layman’s terms, the excess voltage is ”destroyed” or ”burned up”).
The magnitude of the losses varies according to two factors:
the voltage difference mentioned above (i.e. drive battery less 5 V BEC voltage),
and the servo current (Volts x Amps = Watts ),
and if you bear in mind how hot a 25 Watt filament bulb gets, it is easy to see why a speed
controller or governor fails under such conditions.
Typical calculation: 12 cells = 14.4 Volts; 14.4 V - 5 V = 9.4 V: this is the figure which the
voltage regulator has to ”destroy”.
Let us now connect four 9-gram servos to the BEC system: four servos times 1.0 A = 4 Amps,
9.4 V * 4.0 A = 37.6 Watts.
If the servos are working hard, or if the controls are constantly in use (servos continuously
changing direction, either due to control commands or due to problems towards the limits
of range), this BEC system will quickly heat up and then switch itself off ”for safety reasons”.
However, the receiver now has no power, and can do nothing ...
Note: apologies for the calculations; there is no way of avoiding a little arithmetic
if you wish to operate a BEC system and keep your model ”on the safe side”.
3) Note:
Caution when using 6-gram and 9-gram servos.
Under certain circumstances many sub-miniature servo motors are only half as efficient
as the motors fitted to the recommended servos.
In practice this means that such servos draw twice the current in order to generate a given
torque at the servo output arm. This reduces the maximum possible number of servos
which can be connected to the BEC system.
4) The solution
In our experience we can recommend the following BEC-suitable servos:
BEC suitable servos (selection)
| DYMOND | D 60, D 54 |
| FUTABA | 5102 |
| GRAUPNER | C261, C341, C351, C3041, C3321 |
| HITEC | HS 55 |
| MEGATECH | MTC FX200 |
| ROBBE | FS40 #8433 |
| VOLZ | Microstar, Wingstar, Zip |
We have also drawn up a table for the various schulze speed controllers in
conjunction with particular numbers of cells.
This table is only a guide; it is simply intended to help you obtain a reasonably accurate idea
of the load capacity of your speed controller.
The actual load capacity may be higher or lower according to your set-up.
This depends on many factors: not least the cooling (ventilation) of the speed controller,
the friction in the control linkages, and the type of flying: i.e. whether the controller is
installed in a glider which usually circles in thermals, requiring very small control movements,
or in an aerobatic model with continuous control corrections.
The final factor is the current drain of the drive motor.
If the temperature of the speed controller rises to 60 - 80°C due simply to the drive motor current,
there is very little chance of the BEC circuit remaining cool.
Guide values: speed controllers - cell count - number of servos
Using a controller in a helicopter, we can only recommend the use with less
than the maximum allowed number of cells
(8 instead of 10 cells, 10 instead of 12 cells).
In a helicopter 3-4 servos are permanent in use (as apposed to a wing aircraft)
and the BEC system could be overheated.
| Controller types | BEC data | Practical maximum load
with BEC-suitable servos |
for brushed
motors | | |
slim-05
slim-10
slim-105 | 1.0 A/ 1 W | up to 6 cells max. 2 servos
up to 7 cells max. 2 servos
up to 8 cells max. 1.5 servos*
up to 9 cells max. 1.2 servos*
up to 10 cells max. 1 servo*
[*] 2 extremely low current draw
slowflyer servos
or 2,5qmm negative battery cable
for BEC-heat dissipation |
slim-20
slim-26 | 2.0 A/ 2,5 W | up to 6 cells max. 4 servos
up to 7 cells max. 4 servos
up to 8 cells max. 3.5 servos
up to 9 cells max. 3 servos
up to 10 cells max. 2.4 servos |
slim-40
slim-55 | 3 A/ 3 W | up to 6 cells max. 6 servos
up to 7 cells max. 5.6 servos
up to 8 cells max. 4 servos
up to 9 cells max. 3.4 servos
up to 10 cells max. 3 servos
up to 11 cells max. 2,5 servos
up to 12 cells max. 2 servos
|
| brushless | | |
| future-9.xx | 2 A/ 1.5 W | up to 6 cells max. 4 servos
up to 7 cells max. 3 servos
up to 8 cells max. 2.2 servos
up to 9 cells max. 1.7 servos
|
| future-11.xx | 2 A/ 3 W | up to 6 cells max. 4 servos
up to 7 cells max. 4 servos
up to 8 cells max. 4 servos
up to 9 cells max. 3.4 servos
up to 10 cells max. 3 servos
up to 11 cells max. 2.5 servos
up to 12 cells not allowed
|
future-
universal | 3 A/ 4 W | up to 6 cells max. 6 servos
up to 7 cells max. 6 servos
up to 8 cells max. 6 servos
up to 9 cells max. 4.5 servos
up to 10 cells max. 4 servos
up to 11 cells max. 3 servos
up to 12 cells max. 3 servos
|
| slim-66 | 5 A/ 5 W | up to 6 cells max. 10 servos
up to 7 cells max. 9 servos
up to 8 cells max. 7 servos
up to 9 cells max. 5.7 servos
up to 10 cells max. 5 servos
up to 11 cells max. 4 servos
up to 12 cells max. 3.6 servos
|
| elder types | | |
slim-08
slim-18 | 1.5 A/ 1 W | up to 8 cells max. 2 servos*
[*] 2 extremely low current draw
slowflyer servos |
slim-15
slim-24 | 1.5 A/ 1.5 W | up to 6 cells max. 3 servos
up to 7 cells max. 3 servos
up to 8 cells max. 2 servos
up to 9 cells max. 2 servos
up to 10 cells max. 1.4 servos*
[*] 2 slowflyer-servos |
slim-25
slim-35 | 1.5 A/ 3 W | up to 6 cells max. 3 servos
up to 7 cells max. 3 servos
up to 8 cells max. 3 servos
up to 9 cells max. 3 servos
up to 10 cells max. 3 servos
up to 11 cells max. 2.5 servos
up to 12 cells max. 2 servos
|
slim-35
slim-50 | 3 A/ 3 W | up to 6 cells max. 6 servos
up to 7 cells max. 5.6 servos
up to 8 cells max. 4 servos
up to 9 cells max. 3.4 servos
up to 10 cells max. 3 servos
up to 11 cells max. 2.5 servos
up to 12 cells max. 2 servos
|
future-18
future-20
future-25 | 1.5 A/ 2 W | up to 6 cells max. 3 servos
up to 7 cells max. 3 servos
up to 8 cells max. 3 servos
up to 9 cells max. 2.3 servos
up to 10 cells max. 2 servos |
future-45be
future-Ce | 3 A/ 3.6 W | up to 6 cells max. 6 servos
up to 7 cells max. 6 servos
up to 8 cells max. 5 servos
up to 9 cells max. 4 servos
up to 10 cells max. 3.4 servos
up to 11 cells max. 3 servos
up to 12 cells max. 2.6 servos
|
Power supply with down converter
- pulsed BEC
- switched BEC
- switched voltage controller
- "Safety power supplies"
This is probably the most dangerous system which you can install in your model.
The basic technology has been tested and is reliable, and the results are amazing if
1500 mAh can be extracted from a 1000 pack.
However, the converter switching frequency can generate interference which affects your receiver,
with a resultant substantial loss of effective range. Even the less powerful voltage converters
which are installed in some piezo gyros can cause interference to the receiver if located too
close to it; a 5V BEC system involves much higher powers, and therefore represents
much higher interference potential.
Choice of Controllers/Regulators:
1) The motor connected to the controller should under static conditions and full load not exceed the allowed nominal current of the controller. For current measurements use a clamp amperemeter with digital indication, not a busbar!
We determine the nominal current of our controllers for a given motor running time at full load with a 2000 mAh battery.
For the starting phase a motor consumes a higher current than at full load. This starting current is provided by the controller up to a maximum value specified for the given controller.
A cold controller can be loaded for short periods with currents between the nominal and allowed maximum values. But with a warm controller there is no guarantee that the motor will achieve full throttle. The overcurrent recognition circuit protects your motor as well as the controller for a short time against overloading and too high temperatures.
2) The motor stall current with the applied number of cells should distinctly lie above the allowed maximum current of the controller (i. e. minimum twice the allowed nominal current draw), otherways the protective function mentioned in paragraph 1) will be lost.
In other words: The controller must simply fit the motor. Practically the motor current draw should at the minimum correspond with one half of the nominal controller current, at the maximum with it's nominal current. For example: The controller f31-33bes fits well to motors with current draws between 17 A and 33A. Manufacturers or wholesalers very often specify the allowed maximum current of the controller/regulator only. Usually these controllers cannot be continuously loaded with this current. The current draw of the used motors should in that case not exceed 50-75% of the stated current.
For a judgment of the motor current through the controller/regulater is the number of used cells unimportant, on the other hand draws the motor at different cell numbers different currents. But there may appear some restrictions to that at low cell numbers (below 10 cells) if there is no increase of gate voltage provided or if there are not logic-level FETs used.
3) But for the estimation of the controller operating time at partial load the number of cells is significant. If the controller is used in a scale or multi engine model which mostly flies half throttled, stronger or more powerfull types should be preferred. The load capability of controllers can approximately be estimated by the following practically established empirical formula: If the controller has to work with the maximum allowed number of cells, the total current consumption of the connected motors - measured at fully open throttle - must not exceed at 100% half throttled operation half of the nominal current. With lower cell numbers or short time half throttle operation the engine current draw may be higher. A capacitor of 220µF or better 470µF with an allowed voltage of at least the voltage of the unloaded battery, soldered between + and - of the battery cables may slightly improve the partial load capability.
Optocoupler or 5V-supply (BEC)?
As a matter of principle a controller with 5V-supply can transport noise even from a well interference suppressed motor directly via the (copper-) connection cables of the controller into the receiver. An efficient noise suppresion is only possible by means of an optocoupler, which does not allow wirebound noise to pass through it's separating light path. Sailplanes are often flying to their range limits and should therefore always be equipped with optocoupler controllers. Small pylon models (Schnuppis) or small sport models - which usually fly within ranges of 700-1000 ft - can be in most cases operated without any problems with 5V/1A BEC as long as the supply of not more than 3 microservos has to be provided.
Motor interference suppression:
A primary interference suppression of motors must be provided in any case; for the most part this already happens during manufacturing and consists generally of 3 ceramic multilayer capacitors of approximately 47 nF / 63V (10 nF...100nF and 63 V - not less!) connected up as follows:
1) one capacitor across both connecting tags (+,-) of the motor;
2) from each connecting tag one capacitor to the motor housing.
This primary interference suppression is sometimes insufficient for controllers or switches with built in 5V-receiver supplies. Interference peaks reach - due to the fact that the separating light path of the optocoupler is missing - via the connections of the motor directly the receiver!!! Suppressor modules, consisting of two coils with approximately 10 windings of enamelled copper wire and 3-5 capacitors directly soldered to the motor connecting tags, show the best suppressing effect.
Antenna routing:
Antennas routet to the tailplane or placed outside or inside of the fuselage have not the best reception conditions. A remedy in case of interferences - which usually occur at close range when the pilot is flying toward or away of himself - can mostly be achieved by the following antenna routing scheme: half of the receiver antenna (approx. 18") laid straight inside or outside of the fuselage, the other half (approx. 18") hanging free down!
Copyright © 1996-2003 Schulze Elektronik GmbH. All trademarks shown are trademarks of their respective owners. All rights reserved.
http://www.schulze-elektronik-gmbh.com, designed by matthias schulze
