This page was last edited on March 2nd, 2003; thanks for visiting!
It is best to have a low wattage soldering iron with a fine tip and a three wire cord / plug (with a grounded tip) when working with CMOS ICs; otherwise, you don't need any special equipment to build the SC7.
This control has the following specifications:
Completed ESC size: 1.47"L x 1.16"W x 0.55"H; 1/16" Fiberglass PC board used
(Etched & drilled Bare PC Board available for $14.00 including full instructions plus delivery)
Weight assembled, without leads: approximately 3/8 oz (12g)
Current Handling : ~45A continuous, to 60A intermittent (higher with a modest 36 gauge sheet copper heat sink).
Internal Resistance : "RDS ON" rateing = .002 Ohms using two IRL1404L MOSFETs).
Solid state brake when throttle is off, .11 Ohms; allows folding props to fold.
Throttle stays off when transmitter is off.
6 to 10 cell operation as shown; to 14 cells with some modifications and considerations, as noted in the instructions.
The circuit begins with an input signal buffer, consisting of C1, R1, and Q1. This provides some isolation between the radio receiver and the rest of the circuit, and makes circuit operation somewhat independent of the model or brand 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.
The TLC272CP is more stable than the commonly used LM393 dual compatator, and is my choice for this application.
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, MOSFETs Q2 and Q3 are turned off via R12 through R15. When Z1B is high, the MOSFETs are turned on via R9 and R12 through R15.
D1, C4, R11, and Q4 form the prop brake. Whenever Z1B is high, C4 is quickly discharged through R9 and D1. When Z1B is low, C4 is slowly charged through R11. This charging occurs so slowly that it will not get very far before the next time Z1B goes high. Only when Z1B does not go high for about 50ms (i.e. the throttle has been off for 50ms) does C4 make any significant progress. When C4 does charge fully (the lower side reaches close to 0V), the P-channel MOSFET Q4 is turned on, effectively shorting out the motor, and acting as a brake. Notice that this can't happen as long as the throttle is on even a little bit, so there is no danger of Q4 and Q2&Q3 being on at the same time. Because Q4's on-resistance is about 0.11 Ohms, the brake is more than adequate to stop a wind-milling propeller.
THE BEC feature is implemented using the latest generation L4940V5 low dropout IC Voltage regulator. Two compact, lightweight tantalum capacitors provide filtering on the input and output; be sure to watch the polarity when installing these in the PC board, as the + side lead is marked. On higher cell count battery systems (11 or more cells) with heavier servo flight loads on these size battery systems, you might consider the possible need for some more heat sinking; adding a bit of 36 gauge copper sheet soldered to the tab on this voltage regulator will allow it to handle the extra heat dissipation required when reducing higher applied voltages down to 5 volts for your radio system. (The thin lightweight copper 36 gauge sheet I use for extended heat sinking is available from craft and hobby stores, and is referred to as 'tooling copper sheet'; it's very light weight, and carries away heat very well! )
On a hot 12 cell or larger system , I'd consider leaving out the BEC Voltage Regulator and the 10uF 16 volt input filter capacitor, and use a separate receiver battery (such as a 4 cell 300mAH NiMH.) The 33uF 6 volt tantalum capacitor could then also be down-sized if you wish, too. On a 12 cell system where you are planning to use the BEC, it would be best to increase the voltage rating on the C7 10uF filtering capacitor - 16 volts is a bit too low in such a system.
The Low Voltage Cutoff is used when the BEC is implemented to insure that the motor can not be run until the voltage is so low as to cause the radio system to malfunction. (More on this feature later.)
Power from the battery pack is filtered by C7. The L4940V5 produces 5V on its output. C6 provides output filtering, and also stabilizes the regulator. C5 provides additional filtering for higher frequencies.
D2, D3, R14, 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 7 x 600 AA 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 D2, D3, R14, 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: (This information is duplicated from Stefan Vorkoetter's web site; please also visit there for other Electric RC flight information, as well as designs for two other ESCs.)
Number of Cells = Zener Voltage = Cut-off Voltage per Cell
6 = 4.7V = 0.90V
6 = 5.1V = 0.97V
7 = 5.6V = 0.90V
7 = 6.2V = 0.99V
8 = 6.8V = 0.94V
8 = 7.5V = 1.03V
9 = 7.5V = 0.91V
9 = 8.2V = 0.99V
10 = 8.2V = 0.89V
10 = 9.1V = 0.98V
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 Low Voltage Cutoff (LVC) approach implemented in this circuit 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 when a load on the battery reduces the supply to the BEC below the level you've set with your Zener diode selection. 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 promptly.
In Flight Observations
Recently I had a chance to observe this mode of LVC in action on my newly completed VIENTO, an electric powered sleek 2 meter aileron sailplane with a thin fast RG15 airfoil. The SC7 I installed in the VIENTO is set up with a 1N4733A zener diode, so I can fly the 'Eight Cell wind' 05 Cobalt motor on a 6 cell 3000 NiMH pack or the 6 cell RC2400 packs, (as well as on 7 or 8 cell packs if I choose), using a set of CAM 9x5 folding prop blades instead of the 8x4 blades that would be typically used at lower altitudes.
I was typically powering up to climb to about 300 feet altitude within about 15 seconds, then chopping the throttle and cruising with the motor off and the prop folded, working the sky and getting used to the handling on landing approaches, playing with flaperon mixing variations, then going to full throttle briefly to climb up again for another circuit. Late into a flight on one 7 cell 1250 SCR battery pack, I noticed that when I would move the throttle stick to full ON quickly, I began to notice a 'stutter' on the motor (during the higher current start-up surge) that would dissappear after three or four pulses over the first 2/3 of a second, with the motor then running smoothly and strongly.
When I noticed this , I still did three or four more brief power applications (maybe 5 second motor runs) to come around each time for another pass at the landing zone, noticing this 'stutter' briefly at the beginning of each power application. Upon landing, the 1250maH battery pack took just over 1200mAH on the charger before peaking; It was time to land and change battery packs, but there was plenty of reserve capacity to run the radio system reliably.
Bottom line: I like this mode of 'Low Voltage Cutoff' operation better than the rude and sudden 'cut to zero' that's used on virtually all of the commercial ESCs I've used.
The BEC is provided by the L4940V5 voltage regulator. Without a heat sink, 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.
[Note: The following table indicates the ‘continuous load' current limits when using 6 to 10 cells with the earlier model 2940 regulator; the 4940 can handle more than this, but I don't have specific data at this time, so I'm leaving this information in for now. Bottom line: with higher cell count battery packs, provide adequate cooling when using the BEC.]
6 cells= 0.91A ; 7cells= 0.58A ; 8 cells= 0.44A ; 9 cells= 0.34A ; 10 cells= 0.28A Using this info, 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). The BEC can also provide muchhigher currents occasionally for short periods- it simply generates heat while doing so which needs to be dissipated.
Above is the component placement diagram, as well as the parts designations which corespond to the SC7 circuit diagram above. 7 pages of Assembly, testing, and installation instructions, and a parts list are included with each PC board ordered. DIGIKEY is my preferred source for these special MOSFETS; all of the other electronic components are also available from them.
Yes, I have built these for my own use, and fly them; they do work well- particularly for powered sailplanes with folding props. Stefan builds and flies similar circuit designs with some of these features in each. No, unfortunately I don't have time to offer full parts kits, and I do not offer completed ESCs at this time, and my consulting business keeps me busy so that I don't have time to answer a lot of questions- sorry!
If you're not already familiar with electronic circuit assembly, I would suggest you look at some of the fine products on the market, including those from Castle Creations, and consider the Astro Flight 215D ESC for a comparable commercially available ESC with BEC and Brake which is rated for 30 Amps (if you're using at least a seven cell pack; mine does not run on 6 cells however- it cuts out on power application.)
Bruce K. Stenulson
P.O. Box 69
Fairplay, CO 80440