Tragi 801 - Page 8 - RC Groups
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Mar 02, 2010, 03:37 PM
Registered User
nuevo's Avatar
very nice Marc!! Thanks for sharing.
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Mar 02, 2010, 07:23 PM
Registered User
"You cannot imagine how many times I have drawn the diagram of this set up to make sure that I did it right, cause I was really scared of doing it wrong. You would never know that I work with electric utilities would ya. I will do a drawing and have my wife scan it at work and then get it on here next week."


Did you get that wiring diagram drawn up? I think I understand it now (I was thinking originally that one of your 18Ga pair took the power out to the fuse saddle & the other was bringing it back from the flaps. Da,), but would sure like to see it for sure. Also if you can include all details & show what gauge wire you use in each section run would be very helpful. Thanks.
Mar 02, 2010, 07:32 PM
Registered User
R.M. Gellart's Avatar
SHV, I did not get it done, I am sorry for that but will, probably can get it done FRiday AM and on here Friday PM.

But you are correct, yes, the 18 gauge goes all the way to the wing saddle, but the taps for the RX come off where you see the plugs. The reason that there are two sets of 18 gauge leads is as was stated it gives redundancy and also gives redundancy to the RX.

Mar 02, 2010, 08:11 PM
Registered User
Thanks Marc,

Don't know why I pictured it the way I did, that was crazy of me. I'll look forward to your wiring diagram. I've thought about doing a similar one. Do you reduce the wire ga in the wing to 20 or 22ga, or carry the 18 ga out to the flaps & ail.?

edit: How do you terminate your harness at the flaps & ail.? With regular servo crimped connectors, deans soldered, etc.? Thanks.
Last edited by SHVentus; Mar 02, 2010 at 08:18 PM.
Mar 02, 2010, 08:23 PM
Team Futaba
Silent-AV8R's Avatar
I guess I need to see the wiring diagram too. It seems like this is essentially a complicated "Y" with one leg of the "Y" to the RX and the other leg to the wing servos.

Sounds neat, but the bad news is that this is really nothing more than a parallel wiring arrangement. According to Ohm's Law the voltage will be the same everywhere. Here's a photo illustrating how "Y" work. The 0.02 volt difference in the photo is within the error of the i4C meters.

The servos in the wing are not getting more juice or anything like that. Even in giant scale (40%) planes running 12 large digital servos the RX does not cause any significant voltage drop.

But like I said, maybe I am not understanding the actual wire arrangement you are using.
Mar 02, 2010, 08:59 PM
Hot Dawg Glider Pilot
schrederman's Avatar
We had a problem with some data radios out by the track. These are 65-watt radios that are fed by a 14VDC battery system. We had problems with the radios working correctly with the 12-gauge wiring harness supplied by the manufacturer. We found that the voltage dropped to less than 9 VDC at the input point when the radio was keyed and transmitting... or trying to. We fixed this problem by changing the wiring from the battery to the radio to 6-gauge. The voltage dropped less than 1 VDC when the radio was keyed, and the radios have been working as intended ever since... The smaller wire would have been fine if it was a lot shorter... but that wasn't possible.

A stalled servo is going to draw a lot more current than one at idle. The voltage will drop at the servo input when that happens. I believe increasing the wire size will increase the servo's performance under near-stall loads... by keeping the voltage from dropping so badly at the servo input, and may keep it from stalling prematurely. I believe to understand this, one needs to look up Kirshoff's laws of voltage dividers and current dividers. Remember that wire has some resistance, and there is a voltage drop across that resistance. The higher the resistance, and/or the higher the current, the more voltage that is dropped across that resistance...

Jack Womack
Mar 02, 2010, 09:27 PM
Team Futaba
Silent-AV8R's Avatar
Most of our servos will have about 1 amp at full stall. If you are stalling the servo you have an issue and that is not a condition I would plan for. Your radios at the RR pull way more load than do our servos. Which of course means a great voltage drop under load.

Keep in mind that full deflection under load is nowhere near the same thing as stalling the servo. I do agree however that using the largest wire you can certainly helps, up to the point of diminishing returns. That is why I always try to use the larger wire from the pack to the RX. Once it leaves the RX it is split to the 6 servos and each of those runs and loads is not that big a deal.

For instance, I took a battery and my Radio South load checker and did some tests. I used a 500 mA load, which is a bit more than a properly set up sailplane will pull on average.

Battery straight to load meter - No Load - 6.67v
Battery straight to load meter - 500 mA Load - 6.41v

4-inch extension = 6.66v no load - 6.38v 500 mA load

36-inch extension = 6.66v no load - 6.35v 500 mA load

So about 0.06 volts under load with a 36 inch extension. All I can say is that Honest to God my thumbs cannot feel the difference of 0.06 volts at the servo. It might, and that is a big might, make a 1% difference in torque, so say 65.6 versus 65 in-oz.

The more you load the circuit the more apparent the voltage drop becomes. But unless you really have some bad issues you should never be anywhere close to stalling one servo, let alone all 6.
Mar 02, 2010, 09:27 PM
fnnwizard's Avatar
Here's a quick drawing of my interpretation of Marc's wiring, I think I got it right, but Marc can verify... this way he can keep on working on finishing up the Cluster .

The yellow lines on the wing side connector is what I added is to add even more redundancy if for some reason one of the main 18g wire breaks. I did not see this on Marc's wing side connector. I believe this is what was mention earlier by one of the posters.
Last edited by fnnwizard; Mar 02, 2010 at 09:34 PM.
Mar 02, 2010, 09:46 PM
Team Futaba
Silent-AV8R's Avatar
Yep, that's what I saw too and as I said it is a Y. The RX and the DB-9 connector will get the same voltage to them. The DB-9 would also see the same voltage if it just went through the RX. There is not enough wire to make that big of a difference, especially if 22 or 20 ga wire is used instead of 24 ga.

For instance at 1 foot total run (6 inches per leg) the drop at 1 amp is only 0.02 volts on 20ga wire.

I use 18ga from the pack to the RX without a switch. Once at the RX is just does not make enough of a difference to matter. At least not one that I would ever feel.
Mar 02, 2010, 09:54 PM
Registered User

That's exactly the way that I had now envisioned the wiring diagram (as on the left in your diagram). I believe that another poster above said that to bridge the L & R as you have with the yellow (on the right) actually decreased the safety. I couldn't follow that logic though.

If this is how Marc wires the harness, my only other question then is what gauge wire he uses in the runs from the wing saddle to the flaps & from the saddle to the ailerons. Does he continue each leg with 18 ga, or does he use 20 or 22 ga for one or the other controls. I would think that the longer runs (ail.) would need a larger ga than the shorter run to the flaps. Could be wrong.

Also, does it matter what gauge the signal wire is. I realize it sees a lot less current, but is there any difference in going from 22 ga down to 30 ga?
Mar 02, 2010, 10:02 PM
Registered User

I've been lead to believe that the longer the run, the bigger the ga wire you need, just as when you buy extension cords - a 10' ext probably will be 18 ga lamp cord, a 25' maybe 16 ga, and a 100' probably a 12-14 ga (or there abouts, I'm clearly not an electrician). My understanding is that due to the resistance of the wire, you lose current & increase heat faster in long runs with smaller ga wire. Correct me if wrong.
Mar 02, 2010, 11:03 PM
Hot Dawg Glider Pilot
schrederman's Avatar
The current is the same anywhere in a simple circuit... But the voltage is dropped across the resistance of the wire, which was the point of my long-winded post...

Mar 02, 2010, 11:20 PM
Team Futaba
Silent-AV8R's Avatar
Yep. But for our loads and voltages the loss is negligible, which is the point of my long winded posts

Still fun to optimize to the max!!
Mar 02, 2010, 11:21 PM
fnnwizard's Avatar
SH, I think those posters got a little mixed up in that one was misinterpretting, thinking running 1 power wire to say Left Ail and Left Flap while the other meant to actually bridge the power pins.

I had posted this info in another thread talking aout voltage drop with respect to wire sizes in Xplorer 3.8:

Flap servo to battery is about 31" away.
The 378 and 761 will draw over 1A each in quick forward/reverse action... like in the dip before zoom etc, and stals at about 1.6 and 1.5 respectively.

So if using just the 1A draw as baseline and using copper wire,
the wire size used as shown below will result in less voltage to the servos. The amount of voltage run does not matter, 4.8, 6.0, or 6.6 etc. But usually the higher the voltage, the higher the amperage the servo will draw.

31" length of wire
22g = .086v
24g = .136v
26g = .217v
28g = .344v
30g = .545v

Ail Servo 59" away same assumptions as above:

22g = .139v
24g = .221v
26g = .352v
28g = .559v
30g = .885v

18g-20g for battery to switch or rx is plenty, at least for the current models.
22g for wings servos is plenty, even 24 is ok if running 6.6v (LiFePO4).
signal wire as small as you are comfortale with and still be able to withstand the jerk of launch and pounding of landing. Especially where it crimps to the pins. I run 24g for wing servos.
Mar 02, 2010, 11:21 PM
Hot Dawg Glider Pilot
schrederman's Avatar
It's all good stuff...


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