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The RX Dual Power is a safety device for radio controlled models allowing to power the radio system (receiver, servos, sensors, etc.) from either the normal electrical source or from a backup battery.
Battery failures are one of the most common failures affecting RC models. When this happens, the model becomes uncontrollable resulting most of the time in its loss or destruction. For electric models, the radio system is often powered by the BEC1 present on the ESC2. In this case, another relatively common failure is the destruction of the ESC due to overload3 or other reasons, resulting in the associated BEC being destroyed as well.
The RX Dual Power:
- Can use any electrical power source used in RC models both for normal and backup sources: BEC or battery (LiPo, LiFe, NiMh).
- Uses telemetry to communicate to the transmitter whether normal or backup power source is used. The transmitter can then be programmed to output vocal messages to reflect this. It will also transmit the voltage of both power sources.
- Uses a magnet to power off the receiver and servos (you can use the buttons if you forgot your magnet!).
- Is small enough (60x40mm) to be placed in most models.
- Can power the receiver and servos up to 10A.
- XT30 or XT60 connectors can be soldered directly on the printed circuit board (PCB) for super easy connection of batteries.
The RX Dual Power will initially only use the S.Port protocol to communicate with the receiver and will only work with FrSky or compatible receivers.
Future developments will include other protocols:
- SBUS2 (Futaba)
- X-BUS (Spektrum)
- M-LINK (Multiplex)
- EX Bus (Jeti)
- iBUS (FlySky)
A transmitter using OpenTX is ideal but not mandatory.
1 Battery Eliminator Circuit, a device reducing the battery voltage to a level acceptable by the radio system. Most of the time, the BEC is a buck converter: a switching mode step-down DC-DC converter.
2 Electronic Speed Controller. Electric motors used today on RC models are usually of the brushless type. Brushless motors need a specific speed controller called an ESC and generally use LiPo (lithium polymer) batteries consisting of 3 cells or more in series (labelled 3S, 4S, etc.). Very often, ESC's are equipped with a BEC on the same PCB. When they are not, they are usually labelled "opto"
3 Overloading the ESC can be due to motor/propeller mis-match, using an ESC of an insufficient current rating, using a battery with too many cells, etc. Sadly, many modellers have no real idea of what combination of motor, propeller and battery to use (although an excellent power drive calculator is available online here: https://www.ecalc.ch). Too often, I have heard modellers having a perfectly matched LiPo 3S power drive say "Oh, I will try this with a 4S to have more power". I have seen an ESC fail because the motor was inavertently controlled to run while the glider was on the ground and the propeller was blocked by contact with the ground.
The normal primary power source is labelled MAIN PWR on the back of the PCB and the backup power source is labelled STBY PWR on the back of the PCB.
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As long as MAIN PWR is above its minimum voltage (according to the type of power source), MAIN PWR is selected and STBY PWR is isolated from the system.
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When MAIN PWR is below its minimum voltage - but not disconnected or in short-circuit - STBY PWR is selected and MAIN PWR is isolated from the system.
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If MAIN PWR and STBY PWR are both below minimum voltage - or if any one is disconnected4 or in short-circuit - The source with the highest voltage powers the model, the other one is isolated from the system. The available voltage - the highest of MAIN PWR voltage and STBY PWR voltage - will further decrease due to the discharging batteries. However, the receiver and servos will remain powered until this voltage becomes so low that the receiver and/or servos fail.
4 This continues to apply after a temporary disconnect, i.e. in the case of an intermittent bad contact.
Green LED (LED1) = MAIN PWR source
Yellow LED (LED2) = STBY PWR source
An external LED can be connected to the PCB. It will illuminate together with the yellow led to indicate when the magnet is detected or if STBY PWR is being used.
| LED (yellow or green) | Meaning |
|---|---|
| OFF | Source is OK but not used (power source isolated) |
| DIM | Source is OK and powering the model |
| BLINK SLOW | Source below minimum voltage |
| BLINK FAST | Source is disconnected or has a bad contact* |
* The LED will keep blinking fast until power off, even if it was a temporary disconnect (intermittent bad contact). This makes it possible to see the bad contact condition after landing. From the moment of the first disconnect, source selection is from Step 3 above (source with the highest voltage).
Note: whenever the two LED's blink simultaneously - fast or slow - it is a critical condition: a CRITICAL message is transmitted by telemetry. It is imperative to land as soon as possible.
| Green (LED1) | Yellow (LED2) | Meaning | Fix |
|---|---|---|---|
| DIM | OFF | Using MAIN PWR, STBY PWR is ok | |
| BLINK SLOW | DIM | Using STBY PWR, MAIN PWR LOW | Replace MAIN PWR battery |
| BLINK FAST | DIM | Using STBY PWR, MAIN PWR disconnected or bad contact | Connect a battery or verify MAIN PWR battery if connected |
| OFF | BLINK FAST | Using STBY PWR, but it was temporarily disconnected* | Check STBY PWR battery |
| BLINK SLOW | BLINK SLOW | Critical condition, both batteries have a low voltage | Replace both batteries |
| BLINK FAST | BLINK SLOW | Critical condition, MAIN PWR disconnected or bad contact and STBY PWR low voltage | Replace STBY PWR battery and check MAIN PWR battery |
* A source may have a bad contact but still power the model: if the source did reconnect and it has a higher voltage than the other source, it will be selected because we are now in Step 3 mode (the source with the highest voltage powers the model)
| Green (LED1) | Yellow (LED2) | Meaning |
|---|---|---|
| OFF | OFF | Power off |
| x | BRIGHT | Magnet detected |
| BRIGHT | BRIGHT | Software error or bad configuration |
To power off the receiver and servos, use a neodymium magnet of sufficient size and power. To be detected, it must be positioned either directly above or directly below the hall effect sensor on the PCB but not on its side. The hall effect sensor location is clearly indicated on the PCB with the label 'MAGNET' and an arrow.
Approaching the magnet close to the hall effect sensor twice within 2 seconds will power off the model. When the model is powered off, approaching the magnet once will power the model back on.
Position the RX Dual Power PCB appropriately in the model to be able to power off/on without opening any canopy or cover. Using a stack of magnets allows to adjust the detection distance by adding or removing magnets5.
Alternatively, if you don't have a magnet, press both buttons (SW1 and SW2) simultaneously for 2" to power off the model. Press button SW2 to power it back on. However, using those tiny buttons is not really convenient 6 and must be considered exceptional, using the magnet is the normal way.
When the model is powered off, the current consumed is minimal (about 50 to 100 micro amperes7). A small 300mAh 2S battery would completely discharge in about 2 weeks8. Nevertheless, it is best to always start a flight session with fully charged batteries.
5 A stack of 8 round magnets of size 12x5mm will be detected when it is about 3cm (1 1/4") from the sensor. Be very careful when adding magnets to the stack: the pull force is very strong and they break really easily. Otherwise, a single round magnet of 20x10mm seems to be just right with a detection distance between 30 and 40mm.
6 If this is inconvenient due to the PCB location in the model, you still have the option of disconnecting both power sources from the PCB.
7 This current depends on the highest voltage connected to the RX Dual Power. Measured currents are 45µA at 5V to 130µA at 8.3V
8 When powered off, most of the very small remaining current will be drawn from the highest voltage source. As an example, for an electric model using the ESC's BEC and with the BEC voltage lower than the backup battery, the RX Dual Power will draw this current from the STBY PWR battery. The main power drive battery, if remaining connected, will only provide the quiescent current of the ESC and its associated BEC (probably a few micro-amps as well). Even if the STBY PWR battery is very small (300mAh) and it provides the power off current, it should last the whole flight session and probably the following day as well, provided there was never a condition where it was used in flight
- MAIN PWR voltage
- STBY PWR voltage
These are transmitted using sensor ID: 26, to be confirmed
| Message | Value | Condition |
|---|---|---|
| USING MAIN PWR | TBD | MAIN PWR is the power source |
| USING STBY PWR | TBD | STBY PWR is the power source |
| STBY PWR LOW | TBD | STBY PWR is a battery and it is below its minimum voltage |
| MAIN PWR LOW | TBD | MAIN PWR is a battery and it is below its minimum voltage |
| MAIN PWR DISCONNECTED | TBD | MAIN PWR is below 0.5V |
| STBY PWR DISCONNECTED | TBD | STBY PWR is below 0.5V |
| CRITICAL | TBD | Both MAIN PWR and STBY PWR are either LOW or DISCONNECTED |
Note: these are non standard ad-hoc messages transmitted using sensor ID: 26, to be confirmed. Adequate programming of the transmitter using OpenTX must be done to have audio and/or visual messages corresponding to each reported message.
If using a non-OpenTX transmitter, e.g., FrSky Tandem X20, an adequate audio and/or visual message must be configured in response to the MAIN PWR voltage value reporting.
TODO (Future development)
TODO (Future development)
TODO (Future development)
TODO (Future development)
TODO (Future development)
MAIN PWR and STBY PWR can be any of the following:
- BEC from 5 to 8.4V
- LIPO 2-4S (7.4 to 14.8V nominal, 8.4 to 16.8V fully charged)
- LIFE 2-4S (6.6 to 13.2V nominal, 7.2 to 14.4V fully charged)
- NIMH 4-5S (4.8 to 6V nominal, not recommended)
The maximum allowed voltage is 16.8V (fully charged LIPO 4S). The minimum source voltage is 5V. A NIMH 4S (4.8V) can be used but NIMH batteries are generally not recommended as they tend to be unreliable.
Any voltage present at the power source inputs can be present at the output. So, if one or both of the power source voltages is above the maximum voltage of the receiver or servos, a BEC must be placed after the RX Dual Power and both power source voltages must be sufficient to drive that BEC.
Note: when using a BEC as input to MAIN PWR or STBY PWR, the RX Dual Power will measure the BEC's output voltage and will not detect a low battery voltage situation until the BEC fails due to low battery.
A non-powered glider will use 2 batteries.
- If all servos are of "HV" type (high voltage)9, you could use for example a 2S 1500mAh LiPo or 1600mAh LiFe ("18650" cells) as MAIN PWR and a 2S 500mAh LiPo or 300mAh LiFe ("CR2" cells) as STBY PWR.
- If any servo is not of HV type, you could use10
- Any 3-4S LiPo or LiFe battery for both MAIN PWR and STBY PWR and place a BEC after the RX Dual Power
- A 3-6S LiPo or LiFe and a BEC as MAIN PWR and a 4S NiMh as STBY PWR
- 4S NiMh batteries for both MAIN PWR and STBY PWR
Most of the time, you will use the BEC associated with the ESC as MAIN PWR.
For STBY PWR, use
- Any 3-6S LiPo or LiFe and a separate BEC
- All servos are HV: a 2S LiPo or LiFe
- If any servo is not HV: a 4S NiMh
If you use an "opto" ESC (it has no BEC), or you don't want to use the ESC's BEC (maybe because it has an insufficient current rating), then same as for glider above.
Same as glider above. Size the batteries according to the model: using a 300mAh on a big gas engine model is not enough!
For a glow engine helicopter: as for glow engine airplane.
An electric helicopter could still be landed in autorotaion after a failure of the main power source and the RX Dual Power can be used with a STBY battery just like an airplane with electric motor.
The RX Dual Power is probably not very useful for multi-copters. Should the main power source fail, the motor(s) will stop and the model will crash anyway.
9 Most servos are not HV and have a maximum voltage of 6V, some as low as 5.5V. HV servos usually have a max voltage of 8.4V but some are limited to 7.2 or 7.4V. In this last case use a 2S LiFe instead of a 2S LiPo. Check the specifications of your servos !
10 There are other possibilities. For example a 3-6S LiPo or LiFe and a BEC as MAIN PWR and a 3-6S LiPo or LiFe and a BEC as STBY PWR.
Configure:
- MAIN PWR source
- STBY PWR source
- Telemetry protocol (S.Port, SBUS2, X-BUS, etc)
Many existing commercial dual power devices do not require configuration but they do not work the same way as the RX Dual Power. They can measure the source voltages but they can't determine their battery types - how could they differentiate between a fully charged LiFe 2S and a fully discharged LiPo 2S? - and consequently, they use both power sources in parallel. If the batteries are not of the same type, one could deep discharge and sustain damage.
The RX Dual Power can conserve the STBY PWR battery charge if you replace the MAIN PWR battery before it has discharged to a low level and the STBY PWR battery has not powered the model. In this case, the STBY PWR battery doesn't need replacing during the flying session. You could locate a very small STBY PWR battery in a remote location in the model if you extend its charging wires so that they are accessible.
Furthermore, the RX Dual Power can transmit through telemetry a battery discharge or bad contact situation and - more importantly - a critical condition if both power sources are not ok. Existing devices cannot do this, they just transmit the voltages.
Configuration through the telemetry connector: TBD
- Using a USB to serial adapter and a Windows/Mac OS/Linux app
- Using a dedicated multi-device (RX Dual Power, SBUS Decoder and other future devices) programmer with OLED screen
- Using a Bluetooth LE device with an iOS or Android app
The Gerber files of the PCB will be published here after testing in real conditions has been completed.
It is a 4 layer PCB with generous copper fill areas and plenty of vias to connect the layers.
Click on the picture for the pdf file
On each input, two P-channel MOSFET's (Q1/Q2 and Q3/Q4), mounted back-to-back11, switch the associated power source on or off.
This circuit, using a STM32L021 micro-controller ("MCU" hereafter), is based on the Linear Technology LTC4412 "ideal diode" IC. This IC will drive the MOSFET's and will never allow a reverse current to enter the connected battery when the voltage at the output is higher than the battery, even if the MCU attempts to open the MOSFET's on both inputs.
By driving the CTL pin high, the MCU forces the LTC4412 to swicth its associated MOSFET's off. If the MCU drives the CTL pin low - or put its corresponding GPIO pin to high impedance - the LTC4412 will open its associated MOSFET's but only if its source voltage is at least 20mV above the output voltage (which could be at a higher voltage if powered by the other source).
So, refering to the "Power source selection" section here above:
- Step 1: the MCU drives the CTL2 signal high, forcing U3 to swicth off Q3 and Q4. This isolates the STBY PWR source from the output. The MCU puts its PC14 pin (CTL1) to high impedance. U2 will drive its CTL pin to low, opening Q1 and Q2, connecting MAIN PWR to the output.
- Step 2: the MCU drives the CTL1 signal high, forcing U2 to swicth off Q1 and Q2. This isolates the MAIN PWR source from the output. The MCU puts its PC15 pin (CTL2) to high impedance. U3 will drive its CTL pin to low, opening Q3 and Q4, connecting STBY PWR to the output12.
- Step 3 (when both sources are below minimum voltage or if any one is disconnected13 or in short-circuit): the MCU will put both CTL1 and CTL2 pins to high impedance. U2 and U3 will manage the source selection, connecting whichever source is 20mV above the other to the output.
The STAT_STBY signals to the MCU when STBY PWR is powering the output.
The input source voltages are measured by the MCU through R8/R15 and R9/R16.
The AH180 hall effect sensor signals to the MCU when a magnet is in close range to the sensor (MAGNET signal is driven low).
The HT7533 3.3V linear regulator will provide the 3.3V supplying the MCU, the Hall effect sensor and the LED's. It is powered by any of the two input voltages through the diodes D1 & D2. So the highest voltage input powers the HT7533. If the available voltage drops below 5V, the MCU VDD supply will gradually become unregulated but will initially remain at 3.3V. When the available voltage drops below about 4V, the MCU VDD will drop below 3.3V and the power source voltage measurements will become invalid. Anyway, well before this happens, the MCU will have put CTL1 and CTL2 to high impedance state and the LTC4412's will select whichever power source has the highest voltage to power the receiver and servos. When the available voltage drops below about 2.5V, the MCU will shut down and the LTC4412's behaviour becomes uncertain but most receivers and servos will have failed before reaching that voltage.
Powering off is achieved by the MCU setting both CTL1 and CTL2 lines to high so that all 4 MOSFET's will be closed and virtually no current will be drawn by the receiver and servos. The LED's are off. The STM32L021 will enter STOP mode drawing only a few micro-amps. The only remaining currents are due to the quiescent currents of the LTC4412's (about 20µA total), HT7533 regulator (< 5µA), AH180 hall effect sensor (< 15µA) and the high value resistances associated with the voltage sensors (about 10µA total).
When the model is powered off, if the AH180 detects a magnet or if the SW2 button is depressed, the SW2 or MAGNET signals will trigger an interrupt that will awake the MCU from the STOP mode. The MCU will then simply perform a restart, like if a power source was first connected to the PCB.
The configuration of the RX Dual Power is stored in the EEPROM of the STM32L021.
11 This is required: if a single MOSFET was used, a reverse current would flow through the MOSFET's body diode and into the connected battery when the output voltage coming from the other input source is higher than the battery voltage.
12 Note that we have to do this instead of putting both CTL pins to low or high impedance because MAIN PWR below its minimum voltage can still be above the STBY PWR voltage: consider the case of a discharged LIPO 2S as MAIN PWR (< 7.2V) and a fully charged LIFE 2S (7V) or NIMH 4S (5.4V) as STBY PWR .
13 We do this if a battery is disconnected because it can be an intermittent bad contact and we don't want the MCU to constantly switch between step 1 and step 2 with annoying and confusing aural messages on the transmitter. Once a disconnect situation happens - past the first 30" after power up to allow for the 2nd battery connection by the user - the LTC4412's will automatically switch to the highest voltage source and, if it has a higher voltage, the intermittent contact one will be used whenever the contact is made.
TODO
To connect your STM32 SWD programmer (e.g. STMicroelectronics ST-Link/V2) to the PCB, use a "DYKB 1.27mm spacing Test stand PCB clip, double row, 1.27mm-3P", available on Aliexpress.
