As Bill points out, 2-sided 0.100" 1/2oz copper pour will handle anything a sailplane can throw at it. As a PCB designer however, I find it very difficult to imagine this same scenario handling 40A for more than a few seconds though (that is to say...40A applied instantaneously).
I investigated this and wrote at length for the large scale (40%+) powered aircraft groups, I looked back for a message on this, find some of it pasted below: > > As far as testing to destruction to determine current-carrying > capabilities of a typical receiver, this will only confirm what can be > extrapolated from existing data which is used by myself and other PCB > designers to determine trace width and thickness. > Add to this the fact that I could say "I can extract 100 amps from any > RX", and you'll see what I mean. Yes, I CAN push 100+ amps through any > RX...for about 20 milliseconds. I could also push 30A through any RX, > for about 600 milliseconds. The information is meaningless. > > Most modern RX's I have seen have power/GND buss traces both top and > bottom, and are ~0.100" in width and most likely a 1/2oz copper pour. > After > plating, this can increase to up to 1oz which equates to ~500 sq/MILs of > conducting surface area between both upper and lower buss traces...if > they are indeed plated. > > For our purposes, lets say the traces are not plated and stick with a > total MIL area of 120MILS (2 x 60 for upper/lower traces). So with no > plating, 120MILS of copper equates to ~5 amps @ 20C. This means it will > handle 5A all day without an increase in temperature. Unfortunately, > given the substrate material (the actual printed circuit board), is > actually acting as an insulator, we need to derate to say 4.5A. Given a > typical RX board's dimensions, and learned observations regarding heat > dissipation for a board of this size and mass, one could expect to be > able to pass 5 amps continuously with only a small increase in buss > trace temperature. Say ~30C, or 6A @ ~45C, or 7A @ ~60C, etc. > > As you can see, this is pretty good, but is it good enough? Who > knows...and who cares? OK, I do...sorta. Although I could modify a LoLo > to measure current, then download the data after landing, what would I > do with all this dynamic info? It's only going to change on the next > flight, and the one after that, and...well you get the point. One flight > could consume current that when average trending was applied the value > will be towards the top of the system's limit, while another more docile > flight that would see trending towards the lower end of current > consumption. > > All I can say for sure is that as the level of current consumption > increases "PAST" the nominal limit (if we use the info above, then % > amps), so does the RATE at which heat is developed. What level of heat > is acceptable? I would suggest ~60C+. After this the PCB itself and > surrounding components begin (notice I said begin) to become affected. > > So, at what point in a 40% Fantasia XX345's aggresive flight maneuvering > would this occur? It's not that simple: > a) not all servos simultaneously, or any for that matter, are drawing > anywhere near their rated power. > b) Another reader (correctly) pointed out that similar to water pipes, > the total system will only pass current relative to the weakest link, or > more accurately circular MIL area before significant heating takes > place. Heat is lost electrical energy BTW. > c) Although the cells within battery pack(s) can pass significant > amounts of current (single cell NiCD AA=50A+, NiMH AA=40A+, LiXX > 18650=significantly less), the spot-weld tabs between cells are lossy. > d) From a servo's point of view, or worse 2 servos operating from a > single RX O/P, see only a shadow of the pack voltage and control signal > O/P. Here are some reasons for this inherent series-resistance: > - OEM (original equipment manufacturer) connectors. These are rated for > ~2A, and are not meant to handle lots of cycling (the act of > connecting/disconnecting). > - Differing pin and spring-contact material between differing brands of > OEM connectors causing an electrolytic reaction do to galvanic action > when plugged together. This creates deposits which significantly reduces > conductivity > - inadequate size (gauge) of electrical lead employed over significant > distances > > The electrical losses within large-scale aerobats are significant enough > to allow the RX to at least continue to operate without experiencing > undo heat-loading. The actual voltage present at the servos will dictate > current consumption. More voltage, more current available, more > consumption. Here are some tips that will increase electrical > performance: > > - Current consumption is highest upstream. In other words, use adequate > gauge lead material (the term "wire" refers to a single strand in a > lead, and the term cable refers to >1 lead) for the expected load. > Depending on strand-count in 22awg lead, the circular MIL are can be > from 640 to 700, 20awg can be as much as 1216, and 18awg can be as much > as 1900. The current carrying capacity of some lead material (expand > your viewing window if misaligned) of a single conductor in Free Air at > 30C ambient temp): > > SIZE Polyethylene Polypropylene Polyvinylchloride Kynar Kapton > Neoprene Polyethylene PVC (irridated) Polyethylene Teflon > Polyurethane (high density) Nylon (cross linked) > Silicone > PVC (semi Thermoplastic > rigid) Elastomers > at 80C at 90C at 105C at 125C at 200C > > 26awg 4 5 5 6 7 > 24awg 6 7 7 8 10 > 22awg 8 9 10 11 13 > 20awg 10 12 13 14 17 > 18awg 15 17 18 20 24 > > ....without coming close to thermal cut-off. If > you do not prescribe to either of these theories, than don't use > regulators! > - DO NOT add electrical gadgets, for the sake of adding electrical > gadgets! The most efficient and trouble-free electrical systems > world-wide...are the simplest. > - Get rid of as many connectors as possible! This is "really" important. > The electrical losses through connectors is significant. Using OEM > extensions for convenience's sake is compromising the performance of > your onboard electrical system. It's that simple. Long extensions should > be soldered at the servo end at the very least, and remain unbroken all > the way to the RX. > - Ganging (paralleling) 2 servos and expecting some skimpy OEM > extensions or "Y" to supply adequate power is inadequate. Signal and > power will be split, with the resulting decrease in operating > performance. The lead material upstream from where the servos are "Y'ed" > needs to be able to adequately handle the potential current loads of > both servos. Don't skimp! > - If required, the backside of the power buss (within the RX) can have > 22-24awg buss wire soldered directly to the bottom of the O/P pins, more > than doubling it's ability to pass current. Never use solid conductor > for anything other than this though. > - Keep all extensions a short as possible > - The use of twisted cabling increases RF headroom at the RX. More > headroom is possible if properly grounded sheilded cabling is employed > instead. > - Plug-and-play type connectors that are cycle-rated that allow you to > plug/unplug stabs and wings without touching or manipulating any cabling > or connectors adds integrity, longevity, and safety. > > ...Next I would make extensions from twisted 22awg, making sure to twist > all of the leads going to the same physical location (say the tail), so > they form one bundle. I might consider using specialized shielded (or > double-shielded) cabling that enhances EMI/RFI suppression. > These extensions would utilize connectors with spring contact material > identical to the header material used on the RX PCB. These aftermarket > spring contacts would be configured to ensure that as much contact area > between the RX post and the spring contact itself was achieved, thereby > further reducing series resistance. > At the end of each of the extensions, I would directly solder the servo > pigtail (cutting off the existing OEM connector close to the servo > case). If the servo happens to be mounted in a removable portion of the > airframe (eg wing, stab), I would employ a high quality, low resistance, > high cycle-rate drawer-type connector that would allow automatic PWR/SIG > hook-up. This elliminates brain-fade, increases electrical system > reliablility (no physical manipulation), and therefore airframe > longevity and safety! > > Up to 40% aerobats power or opto isolation is not a valid requirement in > my opinion. Two RX's have demonstrated time and time again viability to > minimize paralleling (Y'ing) servos, enhancing power distribution, and > redundancy. Pretty simple actually, as the majority who already do > this...already know. I think I addressed all your points Stephen, if not > let me know... > > torquerolls "Andrew E. Mileski" wrote: > > Simon Van Leeuwen wrote: > > Typical 22-24awg PVC-jacketed lead material can handle 2A without > > appreciable rise in temperature. > > That's not what I'm worried about. It's the copper traces on the > circuit board that weren't designed to carry over C/10 currents. > > -- > Andrew E. Mileski > Ottawa, Canada > http://isoar.ca/ > > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and > "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and > unsubscribe messages must be sent in text only format with MIME turned off. -- *~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~* Simon Van Leeuwen, Calgary, Alberta RADIUS SYSTEMS Cogito-Ergo-Zoom IAC25233*MAAC12835*IMAC1756*LSF5953*IMAA20209 *~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~* RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
