Electricity for Model Flyers
Part XI: Currents, Wires and Connectors
I’d like to thank my friend Keith Eldred for his help with this article. He has had a long career in electronic design and maintenance, including aviation, and he put me right over several things. The sections on soldering techniques and crimping are largely his. — PS
DC and Low Frequency AC Only
In a future article I will cover the way that wires react to high frequency alternating currents. This article will be about wires that handle direct (one-way) currents and low frequency alternating currents, say 50 or 60 hertz up to a few hundred. Names for types of wire are used loosely so let’s sharpen up. A made up wire with connectors is called a lead in the UK and a cord in the US. The raw wire on a reel without connectors is called cable or just wire. Normally it really doesn’t matter, but I’ll use lead and wire in this article. I will also cover our connectors and soft soldering tools and techniques. Why is it called soft soldering? Because lead is soft and relatively weak. For stronger non-wire joints you need silver soldering or brazing.
Wire Dimensions
There are six data you need to know about a wire:
- Material that the conducting part is made from, normally for us copper. This metal part is called the core.
- Number of individual wire strands that make up the core.
- Material that the insulation or sheath is made from.
- Overall size of the core given as a diameter or as a cross-sectional area of the whole core. It can also be specified as the number of, and area of, the individual strands. There are three ways to specify the thickness or gauge.
- Material used for the sheath.
- Current it can safely carry. This one is very uncertain as you will see.
Core Material
Copper is usually used as it is a very good conductor. It is also fairly plentiful though increasingly expensive, and is mined mostly in Chile, the Congo and the US. Aluminium is sometimes used but there are problems making connections to as it oxidises easily which makes an insulating layer and it gradually deforms under pressure. It is a poorer conductor so the cores have to be larger. Silver and gold are also used, mostly by high fidelity audio buffs with more money than sense.
Strands
The wires we use always have many strands. As the individual strands are thinner this makes them more flexible and less likely to break. It does mean more care is needed when making the connections, especially when soldering. Here are some examples of wire specifications:
- Hook up wire — 14/0.2 mm meaning fourteen 0.2 mm strands
- Very flexible wire — 55/0.1 used as probe leads for meters
- Heavy duty wire — 1050/0.16 for thick ‘4 gauge’ wire in car audio
Wire Gauges (Sizes)
There are three in common use: American wire gauge (AWG) in the USA, standard wire gauge (SWG) in the UK, and metric used everywhere. AWG and SWG sizes are given a number, eg 16 AWG. Strangely, the larger the number the smaller the wire.
Metric sizes are given as a cross-section area in mm². You can calculate actual resistances from them and hence safe currents and voltage drops, so metric sizes are much more useful and this time larger numbers mean larger wires. However most of our wires are still given in gauges, usually AWG.
There are lots of tables on the web so I will just add a few sizes typical to us. For a more complete table, see Resources at the end of this article. The AWG sizes are correct and can have several different sizes and numbers of strands. The SWG and metric sizes are not exact equivalents but are close.
When you look at the current carrying capacity, sometimes called ampacity, you will see that we modellers routinely and seriously overload our wires, but it doesn’t seem to matter as they are usually in ventilated free air in the fuselage. The specified currents are for when the wires are restricted, for example bundled in a conduit. Most ESC leads are 12 or 14 gauge and we run many tens of amps, or even a hundred, through them. I always now only use 22 AWG servo leads but of course retracts or powerful servos can go up to five amps.
Resistance
If you push a current through a wire it will absorb some of the energy (voltage) and heat up. Sometimes this is desirable as in fuses, but normally it is a waste. You can calculate these voltage losses by finding out the resistance of the wire. Typical values for ohms per metre for the AWG sizes are in the above table. Say you were pushing 50A through a total length of 0.25 m of 14 AWG wire. What would be the voltage drop?
V = I R = 50 x 0.25 x 0.008 = 0.1 V
Nothing to worry about then.
Sheath Material
As always there are many materials used, including those for hazardous environments. I will just deal with those relevant to us, where the main hazard is over-heating, though abrasion can cause wear if the wires are free to vibrate.
- Polyvinyl Chloride (PVC) — This is generally the most common as it is cheap and easy to form. It is water and oil resistant though it can be damaged by high temperature. It can be recycled.
- Silicone — This is much more expensive and cannot be recycled. It is less resistant to abrasion than PVC. However it isn’t affected by high and low temperatures so is the preferred material for us. We only use short pieces so the cost isn’t important.
- Polytetrafluoroethylene (PTFE or Teflon) — This shrugs off attack by heat and chemicals but isn’t very flexible so probably isn’t useful for us. Or do gas turbine modellers use it near their jets? Must ask.
You can tell what the sheath is made from by touching it with a hot soldering iron. If it quickly melts or burns it is PVC.
Radio Control Electric Power Connectors
Wherever you look in electronics you see a bewildering number of different connectors. Some manufacturers even invent their own that no-one else uses. Apple is bad for that as is my Waterpik tooth cleaner. The latter has the most weird mains lead connector. I dislike that ‘Daddy knows best’ attitude. Why can’t everyone just use the larger (kettle) three pin or small (figure of eight) two pin mains connectors and the micro USB for low voltage charging leads? OK, there are now two micro USBs but that’s better than a zillion.
I must clear up one thing. A connector with one or more metal pins is the male part. A connector with one or more hollow sockets is the female part. Please don’t get wokey on me for that nor for the fact that joining them is called mating. It is what it is. Some suppliers use the shape of the plastic body for the male/female name, especially for servo leads. Always look at the part, or a picture of it, before buying.
These are what we use in radio control. Yes, only eight! I’ll cover soldering later in the article. Current ratings are shown in square brackets — however, please note these are a guide and only for modelling applications. How much current the connection will carry depends on the cable size, how much ventilation there is and for how long the current flows.
My Preferences
I wish I could like Deans. They are small and light, but easy to damage when soldering and more difficult to make fatigue-proof even with heat shrink. I like EC connectors. You solder the connectors to the wire then push them into the housing when still warm. They lock into place. However I mostly use XT60 or XT90 as they are the usual LiPo battery fitting. In the shrouded version they are very resistant to fatigue, especially when sleeved with heat shrink.
Soldering
Always use resin core 60:40 tin/lead solder, not the lead-free type used for plumbing. The resin acts as a flux, improving adhesion and keeping out the oxygen. Lead solder is best for electric joints and circuit boards. There were experiments done with low- or non-lead solders a while back but they didn’t work well in electronics due to whiskering and other factors. Good sizes for electronics are 1.2 mm diameter for larger joints like connectors and 0.7 mm for finer work on circuit boards.
The Tools You Need
First you need a soldering iron. For small joints a temperature-controlled solder station is best. For larger joints get a 100 or 150 W solder gun. For really thick wires the very best is a gas soldering iron. I had a lot of trouble soldering 12 and 10 AWG wires into connectors even with a 175 W gun until a fellow flyer suggested a gas iron. None of these cost a lot. Probably no more than £20 ($25) for each, yes, even the station. The supermarket Lidl (see Resources), which is now almost worldwide, often has such equipment. You can of course also use the gas iron at the field.
Secondly you need a support for the things you are soldering. Helping hands with crocodile clips are useful, as are the jigs that take standard size connectors.
If remaking joints a spring loaded solder sucker is useful to clean up first. Or you can touch copper braid onto the melted solder and it will be removed by capillary action.
Soldering Techniques
Bullets can just be held in croc clips on a helping hands stand. Plastic bodied connectors like XT60 are best put in a jig ideally with the mating part fitted as a heat sink.
When you start using a new soldering iron the bit is likely to have a coating of another metal, usually iron. The worst thing you can do is clean it on something abrasive. That will immediately destroy the coating and lead to shortened life. There’s a flux for every application you can think of. For our purposes use a flux that is a grease-like paste that’s in a flat tin. I just dunk my soldering iron in it to clean it and use the damp sponge to wipe the stale solder off and reapply fresh solder and flux. And do it frequently. Also the parts being soldered benefit from being smeared with flux or have a liquid flux dripped on. It can be applied with something like a cocktail stick. Be sure not to use an acid base flux like the plumbers used to. Tin the iron tip by waiting until it is hot then wiping some solder on.
Never carry a blob of solder on the iron to a joint as it is almost certain to form a poor joint. As the tips get old, especially the cheaper ones I use, they will need cleaning up with a fine file or emery paper. I am too mean to replace them earlier.
Now prepare the wires. Make sure your hands are clean. After removing the insulation from the end, twist the strands to firm them up. Then clip the wire into the helping hands, heat the strands with the iron and push on some solder all round so they look solid. This is also called tinning. The name helping hands is a good one. Without its clamps most soldering jobs require you to be an alien with three hands.
Now the connector. Don’t touch the metal parts where the solder goes. There will be a hollow, sometimes called a bucket, into which the wire end goes. Heat it and push solder in until half full or so. Immediately heat it again and insert the tinned wire end. Wait till the two lots of solder merge and form a shiny surface. Hold steady while it hardens. If it goes wrong, reheat and try again. The silicone sheath should take a bit of abuse. However if the plastic part starts to distort throw it away and use a new connector.
Some people prefer to push the untinned stranded ends into an unfilled bucket then to solder both together. I think this heats the joint up for longer but you might find it preferable.
Never try to solder anything that is not shiny clean. Beware so-called solder tags plated with some coating that is years old and so oxidised that it is impossible to solder. Usually you can tell just by looking whether the soldering is successful. You can see if it is bright and has ‘wetted’ the parts fully.
Soldering is a skill that needs practice. To start with you might get bad dry or ‘cold’ joints that have a cloudy dull surface or you’ll melt a few connectors. Speed, flux and cleanliness are the keys and you will soon acquire the skills.
Desoldering for Repairs
A solder sucker, or desoldering braid, performs badly without flux so use generous quantities of the stuff. The aim should always be to get in and out as fast as you can. Flux will enable you to do this without running the risk of overheating. The most important aim with desoldering is to not do more damage. Struggling for a long time with a hot iron to clear the work site of unwanted solder will often result in just that. But flux it well and the problem goes away, leaving you with a reusable device which might otherwise have been destroyed. Initially I [Keith] was taught always to make a wire joint mechanically strong before applying the solder. Then it was realised that so much damage occurred trying to separate it for servicing/repair that that practice was banned. From then on a joint had to come apart as soon as the solder melted. Result was a huge reduction in scrapped parts during workshop time. The saying, “The equipment is good despite the servicing,” lost its meaning.
Heat Shrink
Always strengthen your connector joints with heat shrink sleeving. You need several different sizes so it is probably best initially to buy a box with lots of sizes until you know what ones you use most. Wait until the joint is cool. At this point you discover that you should have slipped the sleeve on the wire before you soldered the wire on. You will only make this mistake once, perhaps. The sleeve needs to slip on to the connector easily so will usually be much larger than the wire. It will shrink to half or even a third of its size depending on make. You can heat it with the iron or a cigarette lighter but the best tool is a heat gun. Again there is no need to pay more than £20.
Heat shrink is useful when joining two wires together for example to extend a lead or make up a special one from two different leads. Slide the sleeve onto one of the wires if there are connectors at the other ends. Remove about 10 mm of insulation from each wire then push the strands together to merge them. Twist to firm them up then it is easy to fill them with solder. Finally heat shrink the joint. Oh no! I forgot the heat shrink.
Crimping
This doesn’t apply to our power connectors but could apply to others such as servo leads. To me crimping is counter-intuitive. I used to think that solder was always best but Keith put me right. It is of course vital to use the correct crimping tool, so buy the best.
In applications in high vibration situations, the moment you tin a high flexibility cable it becomes the least reliable link in the chain and a crimped joint becomes the method of choice. The auto industry didn’t choose it just because it is quicker to do, cheaper and stronger. It is far more capable of withstanding the vibrations found in cars, etc. The act of tinning a multi-stranded cable turns it into a single strand and if flexed at the point where the solder has wicked to, it becomes highly susceptible to fracture. In fact, you will find a requirement to crimp written into most military specs.
Heat Shrink Soldering
One exception to the solder ban is where you join two wires. The heat shrink sleeve will resist the bending stress and bending will be at the ends of the sleeve. You can now buy heat shrink joiners with solder built in. These were new to me until Keith Eldred told me about them. The idea is a piece of heat shrink sleeving with a ring of low melting point solder in the middle. You trim the two wires to be joined and push them in so the bare bits are meshed together inside the solder ring. The ones I bought shrink to one third the diameter.
I messed up the black wires due to too low a temperature. The data said minimum of 135C. I tried 150C but it was much too low. In the end I did the red join at 270C. The coloured part spreads as the shrinking happens.
This is the kit I bought. Sizes are colour coded from 26–10 AWG (0.25–2.5 mm²) with one size covering two sizes of wire. This set will cover just about every size we are likely to need. It cost £7 on eBay. No point in adding dollars at present and I don’t see the point of the black heat shrink either.
That’s it for now. Good luck with your project, thanks for reading and see you next time.
©2022 Peter Scott
Resources
- American Wire Gauge (AWG) Chart — Wire size and ampacity table. From the website: “AWG is one of the important and standard tools in the US NEC (National Electrical Codes) used to sizing different cables and wires for multiple applications.”
- Lidl — From the website: “The first Lidl GB store opened its doors in 1994 and 28 years later, we now have more than 920 stores and 13 regional distribution centres across Britain, employing over 27,500 people. We take great pride in providing our customers with the highest quality products at the lowest possible prices…”
All photos by the author unless otherwise noted. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
