You are making me work on a Sunday. This is the bad idea I was talking about. It will shunt and absorb as heat any voltage between about 13 and 200V, and shunt up to 50A through the power resistors.
The board is 10cm X 10cm. You can use it as a starting point to build your own active shunt. Please do not attach a battery in parallel, itās a bad idea.
This board below is hypothetically working, in my mind only, 12V shunt. You need to manufacture and test.
No need to do anything in a hurry. Thatās the great thing about forums versus real-time communicationā¦ whether you write your response immediately or a week from now, it remains well organized.
That board does look like exactly what I was originally trying to come up with. Great use of a zener diode for the voltage sensor. Does it need one more resistor between the diode and transistor gate, or does the diode limit the current enough by itself?
This is an NPN power transistor in a voltage following configuration (not a mosfet), and is controlled by current. The answer is no. The current is already controlled by the resistor. The diode I selected breaks down at 15V and has excellent resistance at 11.5V, with current of about 300mA and pulse current of 6A. This should easily exceed any current a power transistor would need to follow. So you are kind of protected between 12 up to 15V, then it shunts at 15V but then the transistor will drop to a little under 15V (not sure how much you need to read the specs). The transistor itself is good up to 400V but the zener will bomb at 200V, so if you need higher voltage you need to pick a different diode. Also be careful with the zener resistor, it needs to dissipate a lot of power the higher the voltage, may be you can put a few resistors in parallel to dissipate the power better. Also if you expect a really high voltage surge, I suggest you add a high power TVS or a MOV, may be also limited by a low ohm resistor. The high voltage surge will be picked by the TVS and then if the TVS is still alive and the surge front is dissipated the rest will be taken care of the zener. Also, adding a ferrite core outside wil help the TVS/MOV. As you can see there is no limit how deep you want to go into the proverbial rabbit hole.
Iām sure the circuit needs to be tested, as I said, this is hypothetical. The zener resistor needs to be carefully calculated and match the zener and transistor. The zener current also needs to be calculated and appropriate zener selected. You can also add a second transistor to form a darlignton pair, or even a triplet, which will work better. Obviously the lower transistors donāt need to be high power and will be much smaller, but then you need to worry about the extra resistors and diodes to discharge the capacitance.
I had some time and soldered a very crude 80s version of the idea and tested it. Iāve been watching Stranger Things lately so Iāll call it āThe Thingā. It kind of works, it does shunt quite a lot of current, but the transistor overheats a lot, so active cooling may be required. Also this is a small transistor (10A/40V), and I found only a 5ohm 15W resistor, so the real big one is 50A/1ohm and will be a lot bigger to shunt 50A. Bigger like, huge, 10cm big.
Depending on the application itās possible to use the motor as a break resistor. The idea is you monitor the bus voltage and if it goes above a threshold you command a non-zero current to the d-axis to dump the energy. Please refer to section 4.4.3 of my friendās awesome thesis for more information.
Needless to say, this does not protect against BEMF generated when the system is not powered.
I was having my lunch and thinking. You can add a small ultracapacitor, and a low-power MCU and a low-power circuit, then when someone turns the motor by hand it will use the generated power to boot the MCU, start the protection algorithm and charge the capacitor then keep running the protection for as long as there is charge left in the capacitor!
I read, very long time ago, a Sci-Fi short story, some alien planet had this advanced civilization with incredible economy and seemingly endless supply of energy with no visible means of making their own energy, then everyone from the Universe was coming on space-ships to find out how they do it and no one could. Eventually one guy figured it out, they were harvesting the energy of the landing space-ships and storing it and powering their planet.
Cheers,
Valentine
PS Assuming you can recover 100% of a large space-shipās energy, the energy from an Artemis rocket is enough to power a small town of 50,000 houses, or about 100,000 people for one year. If your planet is 1 billion people, that would mean you will need about 30 spaceships landing every day. Pretty normal, not really out of the ordinary, to have 30 spaceports across the entire planet and have one spaceship per spaceport landing once a day. Of course you will need extra allowance for industrial activities, etcetera, however it is still very realistic.
I was also thinking yesterday that an ultra-capacitor could be used as a buffer to protect a higher ohm but lower amp shunt resistorā¦ current spikes could first charge the ultra-capacitor and then discharge more slowly through the shunt. Not sure its actually possible or practical, but if it is, it might be a way to make the circuit smaller.
This is used in electric cars to quickly charge and discharge when braking and accelerating. Also this puts less stress on the battery and prolongs the life because there is no chemical reaction involved.
I was just thinkingā¦ maybe the reason these boards die so easily is due to lack of bulk capacitance. They only have 14 1206 capacitors on board, which I would guess are 10uF 50V. The manual doesnāt say anything about adding any more, and all the photos Iāve seen of them in use didnāt have any electrolytics added, but 140uF seems awfully skimpy to me.
Itās possible the onboard capacitors are 25V 47uF in which case the total would be 658uF, but itās rated for 6S lipo which is 25.2V fully charged, and those capacitors cost about 20 cents each so that would be a significant portion of the total board price.
The board suffers from very poor thermal management. Itās only designed for quad-copters with strong forced air cooling. Unless you use liquid cooling for other use cases, it fails very quickly.
It is very nice to see how my simpe question above stirred such highly interesting conversation here and in the new thread on regenerative breaking. I learned a lot by the answers and the links to papers. Since I started this thread, I wanted to update you on my findings on the probably death cause for those boards. After finally hooking up the scope, I am now pretty sure that the real reason was not the BEMF, but more the weak, cheap power supply I wanted to use, which seems to be simply too weak, shuts down when high current is pulled and restores power very quickly after that again (so quick that the CPU does not reboot in most cases). My suspicion now is that this leaves the whole circuit in a somewhat undefined state. As soon as I use my Lab PS, it works fine.
Regarding the temperature, I had no problems so far with the boards, but I donāt pull more that 9A per phase @ 24V and use small heatsinks on the MOSFETS and a tiny fan. I also monitor the temperatur using the onboard sensor and normally it doesnāt get hotter than 35 degrees C or so.
I could well imagine, but honestly, I donāt want to rinsk any more boards. My plan is to go for a larger PSU and add capacitance to the individual boards (which is a bit of a space issue, but solvable). Since my hardware experience stopped in the early 90ies, do you have any suggestion as to which values I should use or is there anything else I should observe? I thought of something like 470uF/35V @Vbattery=24V. The new PSU I chose is however not yet available, so this needs to wait. Right now I am waiting for new CAN bus transceivers, since the ones I had were fake and died, so no real progress any time soon anyway. No real fun with components these daysā¦
So, since I started this thread, here is some update on the current status. The good news first is that I did not break any boards since this discussion started , but of course, I faced problems from all directions. One of the biggest issues was a loose magnet, which turned out to have a very strange effect ( we discussed that in a different thread). I only noticed this, becaue I modified my B-G431B-ESC boards to support a full SPI and now I can monitor the diagnostic outputs of the sensor and when the magnetic field deviates too much from the expectations, then I stop the motors. In addition, I now have sensor calibration and use the same routines but much coarser for a quick sensor test at startup. These steps eliminated one source for errors that might have killed my ESC boards in the past. On the BEMF side I reduced the supply voltage to 12V and added code to closely monitor the voltage on each board. The results were surprising in that the generated EMF is much higher than I expected. I measured up to 16V for a 12V PSU and that was not even close to an extreme operation of my device. Adding 470uF per ESC did not change much, thatās simply to little in this case. I will go for 4700uF, but need to order capacitors first. Also, I implemented a circuit similar to @Valentine 's, which solved the overvoltage issue, but now I have a problem with undervoltage instead, as soon as the protection circuit kicks in . Hopefully the higher bulk capacitance will solve that. I will also go for a higher supply voltage again, as soon as I manage to get my hands on a 20V/5W zener diode, but these are hard to get in small quantities and at reasonable prices these days. Apart from that I fought with my CAN bus, redesigned the mechanics of my paragliding simulator, use torque instead of position mode to control the break lines and all this together makes me very happy since operation is very smooth now and feels quite real - as long as I switch off the over/under voltage protection (which I only do under very controlled conditions and at 12V now, I learned some lessonsā¦).
As an advice for other users of the B-G431B-ESC1: Be very carefull with your power supply, take generated EMF into account, do not, really never ever, exceed the max specified supply voltage, not even by a little and not even for a very short time! Use the boards option to monitor VBus and if you have the chance implement sensor diagnostics, then do so! The B-G431B-ESC1 is really good, but requires some love, good soldering skills and the on-board aux power supply is very, very sensitive to overvoltage.
Meanwhile I experimented with a different method which should be more efficient and less space consuming obn the PCB. It is a shunt regulator based on a TL431. The schematic is below.
I have designed it for 24V, but for testing it is all set to 23V and the power supply is limited to app. 60mA. Still I killed already 3 MOSFETS. The first two I can explain (exceeded Vgs max), but the last case not at all. I can see that the TL431 switches properly, the LED reacts and the MOSFET seems to switch as well, but after a few seconds the MOSFET died silently (at 60mA, 23.4Vā¦) and is permanently shorted. Since I am not good at all in analog design, what do I miss here, any ideas? The simulation looked ok. My setup is an awfull perfboard, of course, I cannot exclude anything stupid there, but I checked it a hundred times. One last resort idea I had is that the TL431 might oscilate, but can that really cause dammage to the MOSFET?
, but of course, I faced problems from all directions. One of the biggest issues was a loose magnet, which turned out to have a very strange effect ( we discussed that in a different thread). I only noticed this, becaue I modified my B-G431B-ESC boards to support a full SPI and now I can monitor the diagnostic outputs of the sensor and when the magnetic field deviates too much from the expectations, then I stop the motors. In addition, I now have sensor calibration and use the same routines but much coarser for a quick sensor test at startup. These steps eliminated one source for errors that might have killed my ESC boards in the past. On the BEMF side I reduced the supply voltage to 12V and added code to closely monitor the voltage on each board. The results were surprising in that the generated EMF is much higher than I expected. I measured up to 16V for a 12V PSU and that was not even close to an extreme operation of my device. Adding 470uF per ESC did not change much, thatās simply to little in this case. I will go for 4700uF, but need to order capacitors first. Also, I implemented a circuit similar to 
. Hopefully the higher bulk capacitance will solve that. I will also go for a higher supply voltage again, as soon as I manage to get my hands on a 20V/5W zener diode, but these are hard to get in small quantities and at reasonable prices these days. Apart from that I fought with my CAN bus, redesigned the mechanics of my paragliding simulator, use torque instead of position mode to control the break lines and all this together makes me very happy since operation is very smooth now and feels quite real - as long as I switch off the over/under voltage protection (which I only do under very controlled conditions and at 12V now, I learned some lessonsā¦).