RC-systems, servo wire and connectors

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spinningmagnets
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RC-systems, servo wire and connectors

Post by spinningmagnets » Dec 13, 2010 3:31 pm

As an RC-system newbie, I encountered some confusing choices, and now that I've found some better info, I wish to share it. Several builders have used a Servo-Tester (ST) as the basis for a throttle, and as luck would have it (for us), RC manufacturers have also been using servo wire and connectors to make the connection between the Electronic-Speed-Controller (ESC) and the radio signal reciever on the model airplanes that these components were originally designed for.

Its my understanding that a servo is a device that takes an electrical signal and moves a device (such as a rudder, or a retracting landing gear). For the enthusiast who wishes to test and adjust model servos in his garage, demand has produced inexpensive servo signal adjusters. The industry standard seems to be a servo signal that usually operates somewhere between 0V and 5V.

It is useful to understand that some ESCs have a Battery-Eliminator-Circuit (BEC), which is a tiny DC/DC converter. This reduces the main pack voltage (18V and 22V are common, among others) down to 5V for the servos. The battery that is 'eliminated' is the need to have a separate 5V battery to power the models servos.

If you wish to use an ST as the basis for an RC-Ebike throttle, you should select an ESC that has a BEC, or...when using a BEC-less ESC, you either add a separate universal-BEC (UBEC), or a small 5V battery to power the servo supply wires. EVTodd has enjoyed success with 4 NiMH batteries of AAA-size in series for servo-supply 5V power, and Recumpence has employed UBECs with great performance reported.

I purchased a Turnigy ST, and also an E-Sky ST. The Turnigy is quite small, and the E-Sky unit is so tiny, the diameter of the knob in its pic is only 1/2"
http://www.hobbyking.com/hobbyking/stor ... oduct=8296
http://www.esky-heli.com/esky-esky-serv ... th=432_433

The Turnigy 85A ESC I had purchased has a 3-wire signal cable that had a confusing Orange/Red/Brown color scheme. This caused me some frustration, as many types of electronics can be immediately fried when plugged in wrong. I made sure the battery was dis-connected, and attempted to see if the plug was polarised in some way to ensure it could only be attached in the proper orientation. A fried component would take 2-3 weeks to replace from HK.

Much to my annoyance, the plug could be inserted in several ways. Also the STs had a square 9-pin arrangement that allowed the servo-plug to be attached either vertically or horizontally! With the assistance of several helpful ES posters, the issues were sorted out. There seem to be two fairly common plug and wire-color standards (among several other obscure standards):

http://www.servocity.com/html/connector_types.html
J_Futaba.jpg
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JR_HitecS.jpg
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It is my understanding that the Futaba/J female connector can accept both common male plugs, and the JR/Hitec-S male plugs can be used on either female plug. If you look closely at the graphic, you can see a rail alongside the Futaba male connector that attempts to ensure the plug is polarised, and can only be inserted one way. This ridge can easily be shaved off by a razor, if needed.

I'm told the JR/Hitec-S male connector is sometimes capable of being plugged in backwards. However, since the red/positive wire is the center wire on both standards, it has been reported that no damage will occur from a reverse plug-in. Since I will be purchasing raw connectors, I will use all Futaba/J's, since they are polarised, but can easly be modified if neccesary.

[in the pic above] The column of pins that the Futaba/J connector is attached to is "S2", the middle column is labeled "S1" (you can test two servos at once), and the nearest column is labeled "Batt" to allow adding a battery to test servos with.

Concerning the wire, there seems to be 3 common sizes. Ultra-light 32-AWG, the common medium weight 26-AWG, and the thicker 22-AWG. Long runs of wire may encounter a small amount of voltage drop. This would be where a 5V full-speed signal from the throttle ends up being only 4.5V when it reaches the ESC. I don't know how bad this issue will be, or if it is no issue at all for the common medium weight wire at a distance of 3-4 feet.

I have chosen to test the thicker 22-AWG wire, which is three individual wires that come in a "twisted" bundle rather than a 3-wire ribbon. One web-catalogue suggested that the twisted wire is somehow more resistant to radio static interference, plus it will eliminate voltage-drop as a possible issue at the beginning of my build. Rather than string several short pigtails together, I will buy a roll in order to have no extra connectors between the rear wheel drive and the handlebar ST-throttle.
FutabaPigtails.jpg
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Here is Hobby-Kings servo wire + connectors page:
http://www.hobbyking.com/hobbyking/stor ... tegory=376
Here is "Servo City's" wire and connectors page:
http://www.servocity.com/html/wire_conn ... cess_.html

I would like to take this opportunity to encourage everyone to AVOID the "Brown wire as Negative" standard. The Red/Black standard for Positive/Negative is one of the few standards that are consistently used around the world, regardless of language.

Since the Tech-Ref area is a "no discussion" section, please PM me with any helpful suggestions and links to additional suppliers. Corrections are also gratefully accepted.
Last edited by spinningmagnets on Dec 27, 2010 2:12 pm, edited 3 times in total.

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spinningmagnets
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Re: RC-systems, servo wire and connectors

Post by spinningmagnets » Dec 21, 2010 12:31 pm

I found the tiny wires and pins to be difficult to work with. For me, in the future,...I will buy a pigtail extender that is 6" long with a male Futaba on one end and a female on the other, cut it in half, and butt-splice the two pieces (of course, adding heat-shrink insulation) to the two ends of the throttle cable.

I have also recently read about shielded cable. It is a type of power cable found in hardware stores that has three individual wires encased inside an outer sheath. I will investigate the wire sizes that are available, and the prices per foot.

edit: The external cover is referred to as 'shielded' cable, and the style I am interested in is marketed as 22/3-shielded. The 22 refers to the gauge of the 3 individual wires, and the 3 refers to the number of bundled wires. I found this cable locally available for $0.30/foot, but I would have to buy 100-feet of it to get that price.

Image

In the second pic, the three Futaba housing pieces can only insert one way, so it was a relief to know I could NOT accidentally assemble them backwards. Once the pins and female plug are fully inserted and snapped-into their proper places, they cannot be removed without damaging the piece. The female socket-pins are used in the male housing, and the male pins are used in the 2-piece female housing.
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50-feet of 22AWG 3-wire twisted
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Last edited by spinningmagnets on Dec 27, 2010 2:15 pm, edited 5 times in total.

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spinningmagnets
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Re: RC-systems, servo wire and connectors

Post by spinningmagnets » Dec 22, 2010 12:07 pm

Since I am recommending a butt-splice to attach a pigtailed connector to the longer cable-run, I will include some pics of what I mean.

These pics are from when I attached a very long and robust cord from a trash-day vacuum cleaner onto a battery-charger cord, so that the charger would reach the entire area of my garage, however, the principles are the same.

ALWAYS slide-on sections of heat-shrink insulation before soldering, and I also recommend holding them back with a small piece of tape (because several times while soldering, the HS slid down, and then clamped onto the hot section). I had to de-solder, separate, and put another section of HS onto the wires.

I like to stagger the splices whenever possible. If they ever pull out, the powered parts will still not be touching. You can add an un-insulated crimpable splice-tube (or buy a common butt-splice and cut the plastic shell off). For that: tin the wire tips, insert, crimp, and re-heat to solidify solder bond, add HS. I prefer to twist the exposed wire together in some low pull-stress applications, as it is slimmer, and it still makes a fairly strong "anti-separation" bond, plus HS is cheap.
rce13 003.JPG
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Wires are soldered, and HS slid over to be shrunk.
rce13 004.JPG
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The multi-colored HS was all I had handy in that size. This bond will survive a lot of tugging, but I will not do that on purpose.
rce13 005.JPG
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Amberwolf PM'd me with a great suggestion. Based on his years of experience, he has had good luck butt-splicing wire by shoving the two stripped ends straight into each other, like fingers intertwining. This creates lots of contact surface between all of the wires, and also provides a slim profile, which is sometimes useful. Solder, heat-shrink, and its quite strong.
rce15 001.JPG
Amberwolf style...
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