Capacitors or diodes on RC speed controllers (ESC)? - Page 1
Forum section: electronics / projects

author: compet17
author_rank: Regular Contributor
timestamp: June 22, 2020, 08:40:36 am
content: I do have a nice RC model of a Super Constellation (40 years old now) and I want to convert that from gasoline engines to electric... So I need to have long power lines from the batteries to the motors.Long power lines on the motor side of the ESC are not a problem but putting 4 ESCs into the fuselage close to the batteries could cause heat problems.On the other side, placing the ESC close to the motor where cooling is easy, requires long battery cables.Speed controllers (ESC's) for RC models always have pretty large capacitors connected to the battery input side. These capacitors are primarily used to suppress voltage transients generated by the fast swichting of the MOSFETs. These transients are especially bad on the battery input side because they can be high enough to destroy the FETs.Transients also exist on the motor side but the circuits are designed to handle that.The question is this: When I use longer battery cables, I will get more powerful noise and I will have to add more capacitors to handle that. This is a common practise among RC builders.My idea is to use suppressor diodes instead or additional to the capacitors. They are lighter and smaller and cheaper.Is there something that speaks against those diodes?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 22, 2020, 08:52:02 am
content: Definitely place the controllers near motors, run battery DC wires. Not only you have one power wire less, these wires carry DC so less EMI (given that the controller has proper capacitors), and also run at higher voltage and lower current than the motors. (At speeds less than full, motor wires carry lower voltage and higher current).Battery itself has quite some inductance in it already, making the wires longer should not cause a big difference unless the design is already marginal. I understand the cheap ESCs save costs in capacitance; then add the missing capacitance to fix them. There's really no way around it as far as I know. Hopefully the ESCs already use enough low-inductance DC link capacitors (such as ceramic caps) so that only thing left is to add a larger electrolytic for damping. Maybe post an image of the ESC PCB so we can make some guesses?I do not know of any way to solve the problem with diodes. Where would you connect the diodes and why and how do you think it would help?

author: exe
author_rank: Unknown
timestamp: June 22, 2020, 10:54:36 am
content: Can you please provide a schematic and photos for your setup?

author: Niklas
author_rank: Frequent Contributor
timestamp: June 22, 2020, 08:13:18 pm
content: It depends on the current. I have not been active with RC flying for little over 10 years, but there were limitations on wire length, both on the motor and battery sides, on the brushless speed controllers back then. The LiPos were on the way in, but for the high currents needed for the F5b and F5f gliders we still used pushed and matched NiMh cells. The electrolytic capacitors are not fast enough, so an additional varistor at the controller's battery input could be an option. The current in the battery wires is far from DC and make sure to tie the wires together to minimize inductance and radiated emissions.

author: compet17
author_rank: Regular Contributor
timestamp: June 30, 2020, 08:04:18 am
content: My idea was to add so called "suppressor diodes" on the battery side of the ESC between the + and - wires to clamp down any voltage spikes higher than the allowed voltage range.That's what all the input capacitors do.... reducing the overvoltage created by the fast switching of the MOSFETs. The suppressor diodes act like z-diodes but they are much faster and can handle more powerful spikes. I would add two diodes in parallel with opposite polarities. In fact the reverse diode could be a normal diode, clamping reverse spikes at 0.7V already.Mounting the ESC close to the motor saves a lot of wire, thats a benefit. But all RC forums warn of long battery cables because of the said inductance effects. But you are certainly right when you say that a good ESC should be immune to that by design. The problem is.... what is a good ESC and what not?

author: compet17
author_rank: Regular Contributor
timestamp: June 30, 2020, 08:40:46 am
content: BTW schematic diagram.....

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2020, 09:38:19 am
content: Oh, you mean TVS diode. Sure, you can use those, but note their voltage curves are pretty lame, i.e., you need to pick one with nominal voltage = at least maximum battery voltage, and then the clamping voltage will be some 30-50% higher.Even if they were infinitely quick, the voltage still needs to rise to that clamping voltage for anything to happen. And the capacitance of the bus limits voltage rise.For catching "battery input suddenly disconnects and the controller regeneratively dumps momentum-stored mechanical energy into the DC bus" case, TVS is great because this would require quite a lot of capacitance. TVS can take quite a lot of heat before it blows up. Of course, proper action is to prevent any regeneration in this case, but this is up to the implementation of the controller.For catching "battery input suddenly disconnects and the controller does not regenerate still but dumps motor inductance-stored energy into the DC bus" case, electrolytics usually are enough and not too big of a solution, but in some cases, a TVS might be better.For EMI, it does nothing (except for the effect of its parasitic capacitance). The capacitors do that. If electrolytics are not "fast enough" - they do have maybe 10nH of inductance, and ESR also limits the voltage ripple absorption, add small ceramics (for really high frequencies).Remember that EMI originates from the energy stored in layout loop inductance resonating with the parasitic capacitance of open MOSFET. It does not require a massive amount of capacitance to absorb that, the problem is minimizing the impedance, which is caused by L and R of the capacitance. Electrolytics are used to dampen the remaining oscillation, here the key is to haveenoughR so that change in voltage per time causes actual heating loss, removing the energy instead of putting it back to the oscillation. Although too small of electrolytics, even if they had enough C for the job, havetoo muchR again. Hence it's typically some combination such as 10pcs of 1uF ceramics right in the layout, plus 1000uF of low-ESR electrolytic or 2200uF of "traditional" electrolytic for damping. (This is chosen so that Celectrolytic > 2-3 times Cceramic, and then chosen for the correct amount of ESR.)If you are having EMI problems, adding a bit of differential-mode LC plus common mode filtration could be inevitable, but this sucks if you need to minimize the solution. Tight layout and proper ceramic caps rightinthat layout is the key for a successful yet small design. A TVS could work but it should be in that tight layout to have any chance of catching the voltage overshoot.Randomly placed TVS does not hurt, as long as the nominal voltage (not clamping voltage) is above maximum battery voltage, but as it likely doesn't help either, I would start by verifying what problem you have, if any, then try to solve that.If you can't analyze/modify the controller, adding electrolytics outside does not hurt (except for size/weight) because their ESR is always large enough it's almost impossible to cause oscillation by adding electrolytics. So it costs nothing to try adding a beefy cap and measure what happens. Add some 1uF small ceramic in parallel and see again. If you really see spikes exceeding 30-40% the battery voltage on the oscillosscope screen (probed right from the DC wires, without creating a loop neither with the DC wires, nor with the scope ground clip), and these spikes are more than a few dozen nanoseconds, a TVS can possibly help.

author: compet17
author_rank: Regular Contributor
timestamp: July 03, 2020, 10:05:48 am
content: Wow.... that was a very detailed answer Siwastaja! Thank you!I don't have any problems with my ESCs yet. The aircraft model I have is about 40 years old and was originally equipped with gasoline engines. I know pretty much about real airplanes and electronics but when it comes to RC planes, I'm a complete newbieI'm in the progress of converting this airplane to electric motors and repairing a few cracks and damages. So my goal is to do everything as good as possible and to avoid the biggest beginners errors.When reading through the RC forums, there are all kind of theories about ESCs and long power lines (with long, I mean 50cm to the outer motors).I like your proposal to use ceramic capacitors. I should have come to this idea by myself.....They are small, light and have a very low ESR. Two 470uF capacitors are already attached to the ESC from factory. But many RC forums recommend to add more capacity when using longer battery lines.So I think I will try to add a small board with ceramic caps and also a TVR diode. I hope the ESC can handle a small overshoot in supply voltage. I think it should...I attached some pictures from my projectThanksChris

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2020, 11:27:58 am
content: Yes, with ceramics remember,Larger SMT types are prone to cracking and shorting out,They have such low ESR that they may create resonances with layout inductances, causing EMI instead of solving it. Dampen with lossy C.A 1000uF elcap + 1uF small ceramic (or a few) with minimized lead length is a good all-around combo for such extra low voltage motor control.