Build a Versatile Miniature High-Rate ESC with BEC and Brake

July 1, 1999  for  Sailplane & Electric Modeler Magazine

This electronic speed control (ESC) for brushed motors combines the features of two of my earlier designs. One was a high-rate 30A ESC with a brake, and the other a high-rate 12A ESC with a BEC (receiver battery eliminator circuit).

This ESC is an analog one, using off-the-shelf components. There are no microprocessors (which would require specialized equipment to program), and no surface-mount parts. It’s not as small or as light as many commercially available ESCs, but it’s smaller than many other do-it-yourself designs. It’s also versatile, in that it can be built for many applications, ranging from small Speed 400 sport planes to "hot liner" sailplanes.

Specifications

This ESC has the following technical specifications (if built exactly as described):

• 6 to 12 cell operation.
• Size: 1.9" L x 1.3" W x 0.7" H (48mm x 33mm x 15mm).
• Weight: approximately 1 ounce (28g) without motor and battery leads.
• High-rate switching (1,500 Hz).
• Current: 48A continuous, 72A for 30 seconds.
• On resistance: 0.003Ω.
• Typical voltage or power loss: 52mV or 1.6W @ 30A.
• Battery eliminator circuit (BEC) on 6 to 10 cells.
• Soft start and soft brake.
• Arming/power switch.

Furthermore, there is no momentary burst of power when you first switch on as there is with some older analog designs, and the power stays off if the transmitter is off (unless another transmitter is operating on your frequency, or you’re using a PCM receiver and your failsafe doesn’t go to zero throttle).

How it Works

Figure 1 is a schematic diagram of the ESC. The main component is Z1, an LM339 quad voltage comparator IC. As its name indicates, this chip contains four independent voltage comparators, Z1A through Z1D. These have what is known as an open-collector output, which means they can be either low (near 0V), or floating (effectively disconnected). To go to a high voltage, a pull-up resistor is needed.

Z1A acts as an input buffer, isolating the rest of the ESC from variations between brands of receivers. R2 is a pull-down resistor which ensures that the ESC is off if no receiver is connected. R3 and R4 form a voltage divider, giving about 1.6V at pin 4 of Z1A. If the input signal is higher than this (as it is during a pulse), pin 2 goes to 5V thanks to R5. If the input signal is below 1.6V (between pulses), pin 2 goes to 0V.

D1, R6, R7, and C1 form an integrator, which smoothes the pulsing on-off signal from Z1A into a fairly steady voltage, which appears at pin 9 of Z1B. This voltage will vary from about 1.15V to 1.65V depending on the throttle stick position. Because of this smoothing action, the integrator takes some time to respond to changes in throttle stick position. For example, if you move the stick from off to full-throttle, it will take the ESC about one second to go to full throttle. You can use this ESC as a soft-start switch by controlling it with a switch-operated channel.

Z1C, together with C2 and R8 through R12, form a triangle wave generator. The triangle wave appears at pin 10, and oscillates between about 1.2V and 1.6V, depending on the setting of R12. The frequency also depends somewhat on R12, but is approximately 1,500Hz.

Going back to Z1B, we see that the triangle wave from Z1C is compared to the integrated voltage. When the integrator voltage is higher than the triangle wave, pin 14 is pulled to the motor battery voltage by R15. When the integrator voltage is lower than the triangle wave, pin 14 goes to 0V. The percentage of time that the integrator voltage exceeds the triangle wave depends on the setting of the throttle.

The output from pin 14 is used to drive N-channel MOSFETs Q1 through Q4 via resistors R17 through R20, which serve to ensure that the gate current is divided equally among the MOSFETs so they all turn on at the same rate.

Everything described so far constitutes a simple high-rate ESC with no BEC or brake.

The LM2940CT-5 is a low-dropout 5V regulator. Low-dropout means that the input voltage doesn’t have to exceed the output voltage (5V) by much in order for it to work. C3 and C4 filter the voltage coming from the motor battery (which can be quite electrically noisy due to motor noise). C5 stabilizes the voltage regulator (it won’t work correctly without it), and C6 provides some extra filtering on the 5V line. The 5V from the regulator is used to power the rest of the ESC, and also the receiver and servos.

Z1D compares a fraction (set by R14) of the motor battery voltage against a fixed reference of 1.6V. If the fractional battery voltage drops below 1.6V, the output of Z1D (pin 1) goes to 0V, and through R21 and D2, pulls the input of Z1B to about 0.7V. This in turn causes pin 14 to go to 0V, thus shutting off the motor. R13 provides positive feedback to prevent the motor from turning back on as soon as the voltage goes back up (which it will when the load of the motor is removed from the battery).

D3, C7, R16, and Q5 form the brake. Whenever Z1B’s pin 14 is high (due to R15), current flows through D3, bringing the input to Q5 high. Q5 is a P-channel MOSFET, which is off when it’s input is high. During the times that pin 14 is low, no current flows through D3, but the high input to Q5 is "remembered" by C7. As long as there is any throttle activity, C7 will keep being "reminded" about 1,500 times per second. However, once the throttle is off for a while, C7 will slowly "forget" as it becomes charged via R16. This will take about 1/10^th of a second. Once the input to Q5 goes low, it will turn on, effectively shorting out the motor and hence acting as a brake. Q5 turns on somewhat gradually, and it’s resistance is rather high compared to the other MOSFETs, so the brake is quite soft.

D4 is the freewheeling diode, which serves to circulate motor current during the times that Q1 through Q4 are off, greatly improving efficiency at part throttle. D4 also protects the MOSFETs from motor noise.

Finally, S1 is the combination arming and power switch. When S1 is open, no power is applied to the voltage regulator, so nothing happens in the ESC. Furthermore, there is no voltage by which R15 can pull Z1B pin 14 high, so the FETs cannot turn on. R16 of the brake helps ensure that pin 14 remains low. If you wish, you can install a separate high-current switch in one of the motor leads for additional protection.

Not shown, but very important, is a fuse. This should go into the MOTOR+ lead (i.e. between the ESC and the motor). Do not put it in the BATT+ lead between the battery and ESC, because if it blows, the BEC will cease to function and you will lose control of your airplane.

Construction

The ESC is best built on a printed circuit board. My article, Making Excellent Printed Circuit Boards, gives tips on etching your own boards. Figure 2 shows the board layout from the copper side, actual size (1.9" x 1.3", or 48mm x 33mm).

Figure 3 is an enlarged view showing the component locations as viewed from the component side.

First, install all the fixed resistors. Install R1 to R11, R13, R15, and R21, and R22 to R23 standing up over one hole, with the other lead bent over and going down into the other hole. R16 to R20 lay down flat on the board (you’ll have to bend the leads very close to the resistor bodies to make them fit). When you install R5, also install jumper J2, since it shares a hole with R5. Likewise, when installing R21, also install J1.

Next, install jumper J3, variable resistors R12 and R14, and a 14-pin socket for Z1 (you don’t have to use a socket, but I suggest you do since it is hard to unsolder a defective chip).

Install all the capacitors. Be sure to observe the polarity for C1, C3, and C5; their + leads are marked on the component layout diagram. Note that capacitor C8 is installed differently from all the rest. Rather than being installed into holes in the circuit board, it is soldered directly to the two outside legs of variable resistor R14.

Next, install diodes D1 to D3, observing their polarity. Install D3 with its banded end on the board, next to C7. The banded end of D1 and D2 should be oriented as shown in the component layout diagram. When you install D2, also install jumper J1, since it shares a hole with D2.

The next components to install are the drive MOSFETs, Q1 to Q4. These should be installed with their tabs towards the BATT-/MOTOR- side of the board (the bottom of the component layout diagram). Before soldering, the drain and source leads of each MOSFET should be bent over towards the BATT-/MOTOR- side of the board, so that they reach the corresponding leads of the next MOSFET (or the edge of the board for Q1). This will help distribute motor current more evenly between the MOSFETs, since the copper trace alone is not heavy enough to carry this much current without significant losses.

Next install Q5 and D4, which should have their tabs away from each other. Note that D4, although it looks just like a MOSFET, has only two leads.

Finally install VR1, which should be installed with its tab towards the MOSFET end of the board (i.e. facing the same way as D4).

If you’re unsure about any of the above, please refer to the component layout diagram and the photos of the completed ESC.

Cut a 12" (305mm) piece of red 12ga (3.3mm²) or 13ga (2.6mm²) flexible wire. With a hobby knife, remove about 1.9" (48mm) of insulation from the middle of the wire. Tin this with a heavy soldering iron (at least 45W). Also thoroughly coat with solder the BATT+/MOTOR+ trace on the circuit board. Lay the bare section of the wire on the trace, and clip it down at each end with alligator clips. Run the soldering iron slowly along the wire, allowing it to melt the solder in the wire and on the board. The wire should fuse to the trace along the whole length of the board.

Cut a 6½" (165mm) piece of black 12ga or 13ga wire and strip 1.6" (40mm) of insulation from one end and tin it. Coat the BATT- trace with solder, and solder the wire along its length as described above. Then cut a 5½" (140mm) piece of black 12ga or 13ga wire and strip 0.2" (5mm) of insulation from one end and tin it. Coat the MOTOR- trace with solder, and solder the wire to it.

Install the appropriate connectors on the BATT leads. If you prefer connectors between your ESC and motor as well, install them on the MOTOR leads.

Attach the receiver lead to the CH+, CH-, and SIG pads. The CH+ lead is usually red, the CH- lead is usually black or brown, and the SIG lead is usually white, orange, or yellow. The colors vary with the brand of receiver lead you purchased.

Connect the arming switch to the S1 pads on the board using short lengths of thin (22ga or 24ga) flexible wire.

Testing and Adjustments

Double check all your work, making sure all the components are installed the right way around, and that you haven’t inadvertently created any solder bridges between traces. A magnifying glass helps here. Do not insert Z1 into it’s socket yet.

Connect a 6 to 10 cell motor battery to the BATT leads and turn on the arming/power switch. Check that there are no high voltages on the receiver lead (measure between the CH- and the SIG leads). Also check that there is 5V between the CH- and CH+ leads.

Turn off the arming switch, disconnect the power, and install Z1 into its socket (the IC will have a semi-circular notch or small dot at the pin 1 end). Turn R14 fully counter-clockwise to disable the low-voltage cut-off, and turn R12 fully counter-clockwise to disable the triangle wave oscillator.

Attach a motor, without a propeller, to the MOTOR leads. Make sure the motor is restrained so it cannot move. Plug the receiver lead into your receiver’s throttle channel, and reconnect the motor battery.

Turn everything on, in the following order: throttle stick off, transmitter on, arming switch on. If the motor starts, turn off the arming switch immediately and reinspect the board for errors.

Now slowly turn R12 clockwise until the motor begins to whine. When you reach that point, turn R12 counter-clockwise slightly until the whine stops. If you advance the throttle, the motor should start, with the speed proportional to how far you’ve moved the stick. The speed of the motor should increase as you move the stick forward. It should stop increasing before you reach full throttle. Once you reach full throttle, move the throttle trim forward to confirm that the motor won’t go any faster.

If moving the trim lever at full throttle does increase the motor speed, then you aren’t getting the full range of control. Some transmitters have stops on the throttle stick; check that the throttle stick has the same range of travel as the elevator stick. If your transmitter has such stops, you can open it up and remove them. If you still don’t get full throttle range, replace R9 with a higher valued resistor (82kΩ or even 100kΩ).

If at any point during the testing and adjustment procedure, something doesn’t work the way it should, please refer to the troubleshooting section at the end of this article.

Low Voltage Cut-off Adjustment

The low voltage cut-off level is set by R14. The desired cut-off voltage should be about 0.7V to 1.0V per cell, depending on the type of cells you are using and the current level at which you are operating. The following table suggests some per-cell cut-off voltages:

	600AE/500AR	800AR/2000SCE	1700SCR/RC2000
10A	0.80V	0.90V	1.0V
20A	–	0.80V	0.9V
30A	–	–	0.8V
40A	–	–	0.7V

Multiply the desired per-cell cut-off voltage by the number of cells. For example, if you intend to draw 30A from a 7-cell RC2000 pack, your cut-off voltage would be 7 x 0.8V, or 5.6V.

Now, measure the voltage of a charged battery pack with the same number of cells (7 in our example). This will be your reference pack. Multiply this voltage by 1.6, and divide by the desired cut-off voltage to get an adjustment reference voltage. For example, if your pack reads 9.0V, and your cut-off voltage is to be 5.6V, the adjustment reference voltage would be 9.0 x 1.6 / 5.6, or 2.57V.

To adjust the cut-off voltage, connect a voltmeter between BATT- and the center terminal of R14 (or pin 7 of Z1). Connect the reference pack to the ESC, turn on your transmitter, set the throttle to off, and turn on the arming switch. Turn R14 fully counter-clockwise, and then slowly turn it clockwise until the voltmeter shows the adjustment reference voltage (2.57V in the example above).

After setting the cut-off voltage, bench test the entire system and confirm that the cut-off is activated when the battery reaches the desired voltage.

BEC Limitations

The BEC functionality is provided by the LM2940CT-5 regulator, which provides 5V from the motor battery for use by the receiver and servos. However, the difference in voltage between the motor battery and 5V, multiplied by the current drawn, must be dissipated by the regulator as heat. For instance, if your motor battery is at 9V, and your radio equipment is drawing 200mA (0.2A), the regulator must dissipate (9V – 5V) x 0.2A = 0.8W. With higher battery voltage and/or higher currents, the power dissipated by the regulator increases. With adequate cooling air, the 2940 can dissipate about 1.5W of heat. This limits the motor battery voltage and current that can be handled, as summarized in the following chart:

Number of Cells	Maximum BEC Current
6	0.7A
7	0.4A
8	0.3A
9	0.25A
10	0.2A

Use this chart and the information provided with your radio to determine the maximum number of servos that can be operated from the BEC. A typical system with a receiver and two micro-servos draws under 0.3A average, but can draw up to 1A under strenuous conditions. Make sure that all the control surfaces and pushrods move freely, without binding. Remember, if the BEC overheats, it will shut down, and you will lose control of your plane.

Modifications

As I mentioned in the introduction, this ESC is very versatile. You can omit or substitute various components depending on your requirements.

BEC Modifications

If you will be using 8 to 10 cells, you can use the less expensive LM7805 voltage regulator in place of the LM2940CT-5. This regulator requires at least 7.5V input to produce 5V, so be sure to set the cut-off to 7.5V or higher.

If you do not need BEC, you can omit VR1, C3, C4, C5, and C6. The rest of the circuitry will then be powered by your receiver battery via the CH+ lead.

You can disable the automatic low voltage cut-off by omitting R13, R14, R21, R22, C8, and D2. You should however then install a jumper between the top two holes shown for R14 in the component layout diagram to keep Z1D from oscillating.

Using the component values indicated, the BEC will completely shut down the motor when the battery drops below the cut-off level. If you increase R13 from 100KΩ to 150KΩ, the BEC will pulse the motor on and off for a while before shutting it down completely.

Brake Modifications

The brake can be made softer by using a higher resistance MOSFET, such as the IRF9530. With this MOSFET, the brake will probably not be strong enough to stop a direct-drive propeller.

If you don’t need a brake at all, you can omit C7 and Q5. Do not omit D3 and R16, as these are needed to ensure the ESC stays off when not armed.

Drive MOSFET Modifications

You don’t need to install all four of the drive MOSFETs (Q1 to Q4) and their corresponding gate resistors (R17 to R20). Each IRL2203N MOSFET can handle about 12A, so you can select the number of MOSFETs according to your current handling requirements. For a single Speed 400 motor, one MOSFET that can handle at least 10A would be sufficient. You can also use much thinner wire for the motor and battery leads (e.g. 16ga, or 1.3mm²).

You can also substitute different types of MOSFETs for the IRL2203N specified (but you must use all the same type on one ESC). The following table lists some possible substitutions, the minimum number of cells required, and the current handling capabilities per MOSFET:

MOSFET	Minimum Cells	Current per MOSFET
IRL3803N	6	13A
IRL2203N, SMP60N03-10L	6	12A
IRL3103	6	10A
SMP60N06-14	7	10A
SMP60N06-18	7	9A
IRLZ44N, ECG2986	6	8A
IRL3303	6	7A
IRFZ40, IRFZ44, SMP50N06-25, ECG2395	7	7A
BUZ11	8	7A
SMP25N05-45L	6	5A

Not all of the above are ideal for ESC use, but I’ve found that in some parts of the world, it’s difficult to get some of the more modern MOSFETs, so less-than-ideal parts are sometimes the only choice.

The current ratings above assume reasonable cooling airflow and no covering over the ESC. Under these conditions the MOSFETs can readily dissipate about 1.5W each. If choosing a different MOSFET, keep the power dissipation (current squared times on-resistance) below 1.5W.

Installation

Install the ESC in the plane near the motor, keeping the motor leads as short as possible. Make sure the ESC circuit board doesn’t touch anything metallic. Be sure to install interference suppression capacitors on the motor (a 0.1µF capacitor across the motor terminals, and a 0.047µF capacitor between each motor terminal and the motor case).

If you are using the BEC feature, install a fuse in one of the motor leads. If you are not using the BEC, install the fuse in one of the battery leads. For a fuse holder, I generally use two 14ga female spade connectors soldered at right angles to the wires, and insulated from one another with heat-shrinkable tubing.

Before flying, do a range check, both with the motor off, and with the motor running at various throttle settings. You should get at least 80-100ft (24-30m) range with the radio antenna down.

Notes

There are a few things you should know about the operation of this ESC.

The first thing you’ll probably notice is that response to throttle changes is not instantaneous. For example, going from off to full-throttle takes about one second. This is because it takes time for the integrator (R6, R7, and C1) to respond to changes in the pulse width from the receiver. Replacing C1 with a 1µF capacitor will result in faster response, but noisier operation.

Moving other controls, such as the rudder, will momentarily affect the throttle setting by a few percent. This is because during the movement of a control stick, the time between throttle pulses can vary, and the integrator is thrown off by this. This effect is minimal though. If the throttle level changes a lot during control movements, it’s likely you have a binding control surface which is causing high current drain from the BEC or receiver battery, which can adversely affect the operation of the ESC.

If you are using the ESC to operate two or more motors at once, you should install a Schottky diode on each motor. Suitable diodes can be obtained from your local hobby shop (they’re sold for R/C car use). The diode should be installed across the terminals, with its banded end at the positive motor terminal.

Troubleshooting

If you can’t get the ESC to work as built, use the following as a guideline for troubleshooting. Refer to the How it Works section to understand what is supposed to happen. A basic understanding of electronics would be helpful here. But don’t panic; the most common of problems are found by following step 1 below.

1. Check for solder bridges between traces on the board. If that checks out, look at all the components, and make sure they are the correct ones. For the diodes, MOSFETs, tantalum capacitors, and Z1, make sure they are installed the right way around. If everything looks right, proceed with the steps below.
2. Check that the BEC is working (unless you’ve omitted the BEC and are using a receiver battery). Measure the voltage between BATT- and CH+ (connect a voltmeter’s black lead to BATT- and the red lead to CH+). With BEC, it should read about 5V. With a receiver battery, it should be between 4.8V and 5.6V or so, depending on the state of charge of the battery.
3. Move the red voltmeter lead to Z1 pin 1. The voltage should be above 1.6V. If it is less than this, check the setting of R14 as described under Low Voltage Cut-off Adjustment, and check the wiring to Z1D and all the associated components.
4. Move the voltmeter’s red lead to Z1 pin 9. Make sure the voltage varies as you move the transmitter throttle stick. It should vary between about 1.15 and 1.65V. If it does, the input stage is fine. If not, check the wiring to Z1A and all the associated components.
5. Check that the oscillator is working. Move the voltmeter’s red lead to Z1 pin 10 (or pin 8). This should read about 1.4V. If it doesn’t, adjust R12 until it does. If that works, the oscillator is probably fine. If it doesn’t work, check the wiring to Z1C and all the associated components.
6. Move the red lead of the voltmeter to pin 14 of Z1. With the arming switch on, the voltage should vary between 0V and the battery voltage as you move the throttle stick. If that works, the control circuitry is fine. If it doesn’t work, check the wiring to Z1B, all the associated components, and the arming switch

If everything is okay up to this point, the problem must be with the FETs. Assuming they are all installed correctly, it might be best to remove them all, and reinstall them one at a time, using a small motor (such as a Speed 400 7.2V with a 6×3 propeller) to test the ESC after each FET is installed.

Parts List

The following table lists all the parts along with DigiKey part numbers. Radio Shack part numbers are also shown for those parts available at your local Radio Shack store.

Part	Description	DigiKey Part	Radio Shack Part
R1, R3	10kΩ ¼W resistor	10KQBK-ND	271-1335
R2, R16	470kΩ ¼W resistor	470KQBK-ND	271-1354
R4, R5	4.7kΩ ¼W resistor	4.7KQBK-ND	271-1330
R10, R15	1kΩ ¼W resistor	1.0KQBK-ND	271-1321
R6	220kΩ ¼W resistor	220KQBK-ND	271-1350
R7	39kΩ ¼W resistor	39KQBK-ND
R8	22kΩ ¼W resistor	22KQBK-ND	271-1339
R9	75kΩ ¼W resistor	75KQBK-ND
R11	68kΩ ¼W resistor	68KQBK-ND
R12	10kΩ trimmer	3316F-103-ND	271-282
R13	100kΩ ¼W resistor	100KQBK-ND	271-1347
R14	47kΩ trimmer (or 50kΩ)	3316F-503-ND	271-283
R17, R18, R19, R20, R21	100Ω ¼W resistor	100QBK-ND	271-1311
R22	47kΩ ¼W resistor	47KQBK-ND	271-1342
R23	1MΩ ¼W resistor	1.0MQBK-ND	271-1356
C1	2.2µF 10V tantalum capacitor	P2022-ND	272-1435
C2	22nF capacitor	P4953-ND
C3	10µF 25V tantalum capacitor	P2049-ND	272-1025 (electrolytic)
C4, C6, C7, C8	0.1µF capacitor	P4923-ND	272-109
C5	47µF 10V tantalum capacitor	P2030-ND	272-1027 (electrolytic)
D1, D2, D3	1N914 or 1N4148 diode	1N4148DICT-ND	276-1122
D4	MBR1645 Schottky diode	MBR1645-ND
Q1, Q2, Q3, Q4	IRL2203N MOSFET	IRL2203N-ND
Q5	IRF9Z34N MOSFET	IRF9Z34N-ND
Z1	LM339 quad comparator	LM339N-ND	276-1712
VR1	LM2940CT-5 regulator	LM2940CT-5.0-ND
S1	SPST mini toggle switch	CKN1003-ND	275-624

Addendum

Since the original article was written, much has changed in electric flight, with the two most significant advances being the widespread adoption of brushless motors and the introduction of LiPo cells. If you’ve switched completely to brushless, then you probably aren’t using this speed control any more, but if you’ve still got some brushed motor power systems and want to use them with LiPo packs, here are the relevant specifications for this ESC:

• Works with 2 to 4 series LiPo cells (i.e. 2S to 4S).
• 48A continuous, 72A for 30 seconds.
• Battery eliminator circuit (BEC) when used with 2 or 3 series LiPo cells.

The number of parallel cells doesn’t matter, so long as the power system doesn’t exceed the ESC’s current limits. Just be sure to adjust the cut-off voltage (as described in the article) appropriately so that the cells are not discharged below 3V per cell (i.e. 6, 9, or 12V).

                    

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