CGRetired

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Some questions have come up about using extensions on servos, batteries, and so on for our RC aircraft. I got a PM from a forum member asking me about using extensions on digitgal servos and it got me thinking about it.

I found several charts defining various numbers for current for various gauge wires and came up with this one. I hope it helps.

These measurements were taken with standard copper wire at 72 degrees F or 22.2 degrees C.

Notice that the higher the AWG the smaller the wire is and the higher the internal resistance is and, of course, the less current it can carry.

CGr.

CGRetired

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Well, let me try that again..

CGr.

goirish

CG, Happy Holiday! and the same to all RCU members.

CGRetired

My Feedback: (1)

Thanks, Irish!


CGr.

CGRetired

My Feedback: (1)

Irish: Can you read the attachment in my first post?

CGr.

faulknej

My Feedback: (9)

Ican.

CGRetired

My Feedback: (1)

Ok. Thanks. I hope it helps those that wonder about current carrying of wires to and from servos.

The numbers are for DC ( + and - ) leads from the receiver that would supply power for the servos.

CGr.

LSF2298

My Feedback: (4)

Nice chart CG, since servos have three wires, would we double the ohms per foot for the + and -, then calculate the signal wire at the value listed on the chart?

Rodney

The current on the signal wire is so small that you can get by with anything physically strong enough to not break under use. The important wires are the positive and negative leads and even those pale in comparison the the problems a bad connector creates. If you have good gold plated connectors, you should be able to get by with 22 gauge wire in all cases. Be especially wary of connectors in servo extensions (avoid if at all possible). If you must use extensions solder in the necessary extra length by cutting the original cable in two and splicing in the extra length of wire.

CGRetired

My Feedback: (1)

Well, two wires are the power, black and red for some, black and white for others, and Spektrum/JR have their own color scheme, red and brown I believe.

The signal wire is not a current carrier, but I believe if you do get the HD version of the Hobbico servo extensions, all three wires are the same size or gauge. The main point would be the power or current carriers. They are equal in size and you don't have to figure the two-way current draw as the black - brown - whatever the neutral or ground wire is is the common for the power bus.

The original question was what size wires would be used for an extension for a digital servo that can draw as much as 3200 ma (3.2 Amps). Of course, that's not constant current, but worst case current draw and the Hobbico HD would probably be just fine.

CGr.

ByLoudDesign

I do not have a lot of the electronics reference books I use to, but I am thinking the numbers are for solid single strand wire, as a #10 multi-strand wire can handle a lot more current!

EloyM

My Feedback: (194)

ByLoudDesign is correct, wire tables generally cover only solid wire. Stranded wire would have to be tested individually,
as all do not have the same number of strands or the same conductor diameter.

ByLoudDesign

Voltage is present in the whole conductor! Current flows on the surface and therefore when you get a #10 wire the surfase is much less then say on with 19 strands. Like sa a 105/30 which has 105 strands of #30 wire in is, it is flexable and easy to rout through the wing to the motor!

Rodney

Skin effect (the electons flowing only on the outer surface of the wire) does not occur at DC or at the low frequencies used in RC. You have to get well up into the higher radio frequencies before that effect can even be detected with the best of equipment. Also, a 22 gage solid wire will have the same resistance as a 22 gage multistrand wire as the gage is determined by the total cross section, not of individual wires.i.e., if you summed up all the cross sections in each strand of a multistranded 22 gage wire it will be the same as the cross section of a single solid 22 gage wire and the resistance will be identical if the copper alloy is the same in both wires.

CGRetired

My Feedback: (1)

This thread is a basic answer to a basic question. I gave some numbers to use as a reference and are by no means accurate to a calibrated reference. I found them on the internet so the numbers are what they are. In other words, if you are looking for a servo extension for a digital servo, the point here is that the current carrying capability of the HD wires is probably just fine.

Please don't make this into something it isn't. This is an RC Flight Beginners forum not an electrical engineers classroom.

CGr.

ByLoudDesign

Thank you for your Moderation!

blikseme300

My Feedback: (118)

The gotcha with higher current draw servos is that the real limiting factor is the servo plugs. These "standard" plugs are typically not suited for higher current use due to their potential for higher resistance. The quality of servo plugs vary widely and you often get what you pay for. Good quality gold-plated plugs are best.

Bliksem

HighPlains

My Feedback: (1)

Litz wire need not apply.

Campgems

CGr, when you put up the chart, it became a electrical engineers classroom. There has been a "LOT" of good information on the subject as a result of your post. All of it valid from my experience. Most all of it is practical information, in that it applys to the topic in hand. Thanks for starting this topic. I hope most of the Beginners take the time to read it and under stand it. It will save their plane at some point.

Don

Campgems

One of the issues with plugs is compatibility. About 10 years back, the PCindustry discovered the gold/tin plated contacts gave different signal strength than gold/gold, or tin/tin. The problem is that we use the same contacts for both the voltage and the signal lines. I've got to agree that gold to gold is probably your best bet.. A mix of contact metals probably isn't. Seeing as a lot of the servo contacts start out a Gold on the receiver end, it seem a good idea to carry that out to the servo. The trouble is that all servos don't use gold contact pins.

The short runs are not a problem, but when you get into the 20" extensions, the problem starts to surface, along with the wire gage and individual wire count. It is something to consider before adding a bunch of exertions in your plane. There is a system you have to worry about , not just the length of the wire.

Don

HighPlains

My Feedback: (1)

Even gold plating of copper is worthless unless there is a nickle barrier layer.

Srewinkel

Wouldn't you be limited to the current the battery, or the leads from battery can carry.  Why worry about going to 18 AWG to the servo, when your battery is 22?  I would think you would be more concerned with signal loss on longer runs.

CGRetired

My Feedback: (1)

Hi Don.

Yeah, you're right, but that was just an example that had some very basic figures because of some of the basic questions about using extensions. I just didn't want it to get boring, for those that didn't understand or didn't care about the technical stuff.

Thanks.

CGr.

CGRetired

My Feedback: (1)

Srewinkel, I guess one main point here is that perhaps some don't understand how much current these wires can carry. When they start to get warm is when there begins to be a problem.. and the wires will get warm if there is to much current flow. Most of our applications will work just fine with what we have as basic, off the shelf components, with only the extreme examples, 3D, pattern, jets, large scale, & etc, planes that put heavier than standard load demands on the control surfaces, therefore, the servos and of course, the battery pack.

HighPlains

My Feedback: (1)

The "fusing" current of wire is much much higher than what your chart shows for normal operation and minimal voltage drops. Any amount of current "warms" the wire, just a matter of degree.

The problem for RC systems is that some receivers can lose the information in the RF processing section when current changes start modulating the voltage rail. I'm not sure if this is totally due to resistance or some inductance of the longer wires. Good reasons for heavier wire, higher voltage systems, and even an isolated battery pack for the receiver with a common grounding point.




(sorry.. I hit edit for your message rather than reply.. no harm done. my reply is below.)