Build a Miniature High-Rate Speed Control with Battery Eliminator Circuit (BEC)
January 20, 1998
My first Speed 400 powered plane was the Sydney’s Special, an 80% scaled down version of Vernon Williams’ Fred’s Special, as published in the November 1991 issue of Model Airplane News. I chose this project because I was really happy with my original-size 46½” Speed 600 powered Fred’s Special. The scaled down version has a 37″ (94cm) wing span, weighs 18.5 oz (525g), and is powered by a Graupner Speed 400 6V motor, 7x600AA cells, and a Graupner 6×3 folding propeller. To keep the weight down, I decided to design a speed control with a battery eliminator circuit (BEC).
This design is based on one published by Milan Lulic in the German magazine elektroModell. Mr. Lulic’s design is for surface mount technology (SMT) construction, whereas mine uses standard off-the-shelf components, and is therefore better suited to construction by the hobbyist.
If you’re interested in a non-BEC speed control with higher current capacity, and a brake, please see my other article, A Miniature High-Rate Speed Control with Brake.
This control has the following specifications:
- Size: 1.3″L x 1.1″W x 0.7″H (3.3cm x 2.8cm x 1.5cm).
- Weight: approximately 0.5 oz (14g) without motor and battery leads.
- Current: 12A continuous, 18A intermittent.
- Voltage Loss: 120mV @ 12A.
- Battery eliminator circuit (BEC) with low-voltage cut-off.
- Arming/power switch.
- No power-on glitch.
- Throttle stays off when transmitter is off.
- 6 to 10 cell operation.
- Cost to build: approximately $40 Cdn.
The circuit begins with a buffer, consisting of C1, R1, and Q1. This provides some isolation between the receiver and the rest of the circuit, and makes circuit operation somewhat independent of the model of receiver (although you may have to adjust R8 if you change receiver types). R2, R3, and C2 form an integrator, which produces an output voltage proportional to the pulse width of the input signal. This output voltage varies from approximately 1.15V for a 1ms input to 1.45V for a 2ms input (at 50 pulses per second).
Z1A, together with R4 through R8, and C3, form a 2.5kHz triangle wave generator. R8 adjusts the upper and lower bounds of the triangle wave (it also affects the frequency, but within the range over which R8 must be adjusted, this is not significant). When properly adjusted, the triangle wave (which appears across C3) will oscillate between about 1.2V and 1.4V. This covers the middle 2/3 of the range that the integrator voltage covers.
Z1B is used as a comparator, which compares the integrator voltage with the triangle wave. When the integrator voltage is above the voltage of the triangle wave, the output of Z1B is high; when it is below, it is low. At zero throttle, the integrator voltage (1.15V) is always below the triangle wave voltage (1.2V to 1.4V), so Z1B remains low. At full throttle, the integrator voltage (1.45V) is always above the triangle wave voltage, so Z1B remains high. At half throttle, the integrator voltage (1.3V) is above the triangle wave voltage half the time, so Z1B is high half the time and low half the time.
When Z1B is low, MOSFET Q2 is turned off via R12. When Z1B is high, Q2 is turned on via R9 and R12.
The LM2940CT-5 low dropout voltage regulator provides the BEC facility. Power from motor battery flows through the arming/on-off switch S1, and is filtered by C6 and C7. The 2940 produces 5V on its output. C4 provides filtering, and also stabilizes the regulator. C5 provides additional filtering.
D1, D2, R11, and C8 form the low-voltage cut-off circuit. D2 is a Zener diode which must be selected based on the desired cell count and cut-off voltage. The value of D2 should be the desired cut-off level minus 0.7 volts. For example, with 7x600AA cells, a reasonable cut-off level is 6.3V, or 0.9V per cell. The desired value for D2 is thus 5.6V. As the motor battery voltage drops below the cut-off level, the voltage at the junction of D1, D2, R11, and C8 drops below 0.7V. This pulls the voltage at pin 5 of Z1 below 1.4V. R11 and C8 serve to filter any motor noise from getting back into the control part of the circuit. The following table shows suggested Zener diode values for 6 to 10 cells:
|Number of Cells||Zener Voltage||Cut-off Voltage per Cell|
In each case, the closest commonly available Zener voltages are shown. Cut-off levels of around 0.9V per cell (the green rows) are suitable for high internal resistance cells such as 600AA or 600AE. Cut-off levels of around 1.0V per cell (the yellow rows) are suitable for low resistance cells, such as the 1000SCR.
Note that the cut-off is not a sudden all-or-nothing type of cut-off. Instead, the cut-off lowers the integrator voltage on pin 5, thus reducing the throttle. The throttle will continue to be reduced until the battery voltage rises above the cut-off level. So, as the battery runs down, the speed control will reduce the throttle to keep the voltage high enough to run the BEC. When you notice this lack of power while flying, it’s time to cut the throttle and land.
The BEC is provided by the LM2940 voltage regulator. Without a heatsink, and with reasonable cooling airflow, this regulator can dissipate about 2W of heat without overheating. Power dissipation is equal to current times voltage, where voltage is actually the voltage difference between the input (the motor battery) and the output (5V). This means that the amount of current that it can provide to your receiver and servos is limited, and goes down as the motor battery voltage goes up. The following table indicates the current limits when using 6 to 10 cells:
|Number of Cells||Maximum BEC Current|
Using this chart, and information provided by your receiver and servo manufacturer, you can determine the maximum number of servos that you can use with a given number of cells. A typical radio system with a receiver and three full-sized servos draws under 300mAh on average, but can draw up to 1A for brief periods (for example, when pulling out of a steep dive).
In the past, I’ve never been a fan of BEC systems. That’s because I mostly flew electric gliders (wherein I used 270mAh receiver packs). With a glider, one can spend a long time in the air after motor cut-off, and I wasn’t comfortable using a BEC to run my radio equipment for extended periods of time from a nearly-dead motor battery. So, my recommendation is that you use this controller for sport planes, which are generally flown power-on for the entire flight, and then landed before or shortly after the motor shuts off. In my opinion, using a BEC in a glider is asking for trouble.
The circuit is best built on a printed circuit board. Refer to my article on the subject, Making Excellent Printed Circuit Boards.
There are a few things to note in the construction. The leads to the receiver (a replacement servo lead) are connected directly to the pads on the bottom of the board (on the right side in the PCB layout shown above). Typically, the CH- lead is brown or black, the SIG lead is white, yellow, or orange, and the CH+ lead is red. The arming switch, S1, is connected with two short lengths of wire to the two holes marked S1 in the component placement diagram below.
Begin by installing all the resistors and capacitors. The resistors should be installed standing on end (except R12, which lays flat). Be sure to orient C2, C7, and C8 correctly, with the positive sides where indicated by the “+” symbols. Install D1, D2, and Q1, again making sure to orient them correctly (the negative ends of D1 and D2 will have bands on them).
Install the jumper that will end up underneath Z1, and then install a socket for Z1.
Install C4 last. Leave the leads long enough that you can bend C4 over and lay it down on top of Z1 once Z1 is installed in its socket. To prevent short circuits, put short pieces of heat-shrink tubing on C4’s leads. Be sure to orient C4 correctly.
Connect 14 or 16 gauge wire to the MOTOR+, MOTOR-, BATT+, and BATT- traces on the board. For each wire, strip off enough insulation that you can solder the wire along the whole length of the trace, since the trace alone is not heavy enough to carry the full motor current. The MOTOR+ and BATT+ wires can actually be a single length of wire, with 1.4″ of insulation stripped off the middle.
Install the MOSFET with its tab towards the MOTOR- side of the board. Bend the two power leads of the MOSFETs over so they are touching the MOTOR- and BATT- wires.
Install the LM2940, oriented in the same direction as the MOSFET (tab towards the MOTOR- side of the board).
Double check your work, making sure there are no solder bridges, and that you didn’t make a mistake copying the circuit board layout. Check that all the components are in place, but do not insert Z1 into its socket yet.
Connect a 6 to 10 cell motor battery to the BATT+ and BATT- leads, and use a volt meter to ensure that there are no high voltages on the servo leads (you don’t want to fry your receiver because of a wiring error). Also check that there is 5V between CH- and CH+.
Disconnect the power, insert Z1 into its socket, plug the servo lead into the appropriate receiver channel, connect the motor battery, and connect a 12V automotive lamp to the MOTOR+ and MOTOR- leads. Move the transmitter throttle stick to off, turn on your transmitter, and then turn on the arming/on-off switch. The lamp may or may not light. If it does light, use a small screwdriver to turn R8 counter-clockwise until the lamp goes out. If it does not light, turn R8 clockwise until it does, and then counter-clockwise again until it goes out.
Turn everything off, disconnect the motor battery, and hook up a motor (with a suitable propeller). Don’t forget to install a diode across the motor terminals, with the banded end connected to the positive terminal of the motor (the diagram shows the easy-to-obtain 1N4004, but a Schottky diode would be better). Make sure the motor is firmly fastened to something and that the propeller can swing freely. Turn everything back on in the following order: throttle stick to off, transmitter on, arming/on-off switch on. If you’ve adjusted everything using the light bulb as described above, the motor may be completely off, humming a bit, or turning slowly. Adjust R8 so that with the throttle stick set to off, the motor is not running, but with the stick advanced one or two clicks, it begins to hum. Keep clear of the propeller while making the adjustments. When the motor first starts, it will emit a high-pitched whine. This is simply the motor armature oscillating at the speed control’s 2500Hz rate and is quite normal.
When everything is adjusted so that the motor starts at the right point, try moving the throttle stick slowly to full power. Pay attention to the motor speed. It should speed up as you move the throttle stick, but it should stop getting faster before you reach full on. Once you reach full on, move the throttle trim forward to confirm that the motor won’t go any faster.
If you find that you can push the throttle stick all the way forward, and still get more speed by pushing the trim forward, then you may need to replace R5 with a 120kΩ resistor to narrow the throttle range to match your radio.
Installation is straightforward. Hook up everything as you did while testing. Install the arming/on-off switch in an appropriate place (I prefer the left side of the fuselage, just ahead of the leading edge of the wing, with forwards being ON). Make sure that the bottom of the circuit does not touch anything metallic. To prevent corrosion, I sprayed the bottom of the board with clear lacquer. Keep the motor and battery leads as short as possible. Also make sure your motor is equipped with a diode, and suppression capacitors (I use one 0.1µF capacitor across the motor terminals, and one 0.047µF capacitor between each terminal and the motor case; do not use electrolytic capacitors).
Be sure to use a fuse. However, do not install the fuse between the battery and speed control. If you do, and the fuse blows in flight, you will lose control of your plane, since the BEC will no longer provide power to your receiver. Do install the fuse between the speed control and motor. The best place to install the fuse is in the MOTOR+ lead (i.e. between the speed control and the motor). I use two 14-16 gauge female spade connectors, soldered at right angles to the wire, as a fuse holder.
Before flying with this control, do a range check. With the motor off, you should get the same range as without the control (for most radios, this is 100ft (30m) with the antenna down; check your manufacturer’s recommendations). With the motor on, you should get at least 85% of the range you got with the motor off. If you do not pass this range check, do not fly!
If you plan to use this control with 8 to 10 cells, you can replace the LM2940CT-5 with the more readily available LM7805 regulator. If you do, the cut-off Zener diode must not be less than 6.3V, or the cut-off voltage will be below the 7V level at which the regulator is able to provide 5V to the receiver.
If you need a particular Zener diode that you cannot obtain, you can “make” one out of of a lower voltage Zener diode and a regular diode (such as a 1N914 or 1N4148), wired back-to-back. Simply install the Zener diode, banded end down, in the hole for D2 closest to the edge of the board (the one marked “-“), and install the regular diode, also banded end down in the other hole for D2. Then solder the two remaining leads together. The Zener voltage of this back-to-back diode will be the voltage of the Zener you selected, plus 0.7V. For example, a 5.1V Zener diode back-to-back with a regular diode will give you a 5.8V Zener diode.
The following table lists all the parts needed, along with Radio Shack® part numbers for those components that are available there.
|C4||47µF tantalum or electrolytic||272-1027|
|C7||10µF tantalum or electrolytic||272-1436|
|D1||1N914 or 1N4148||276-1122|
|D2||Zener diode (see text)||276-561*|
|Q1||2N3904, 2N4401, or equiv.||276-2016|
|Z1||LM393 dual comparator|
|Q2||IRL2203N or SMP60N03-10L|
|Regulator||LM2940CT-5 or 7805||276-1770**|
|S1||SPST miniature toggle switch
or minature slide switch
* Part number given is for a 6.2V Zener diode, suitable for use with seven SCR type cells. Radio Shack has a limited selection of other Zener diodes, which may be of use (see the Modifications section).
** Part number given is for a 7805 regulator, suitable only for use with eight or more cells. Radio Shack does not carry the LM2940CT-5 (see the Modifications section).
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