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Old May 07, 2012, 10:28 PM
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USA, FL, Milton
Joined Jul 2008
120 Posts
Thanks for the good suggestions. I've already switched the new airplane back to a 4 cell 1500 mah Elite pack with JR switch as used in my #2 Shadow.

I'll check each servo individually as installed for current draw, but since there was no evidence of damage on the regulator output wiring or connector I kinda doubt I'll find anything - but I'll look anyway.

The thought of an internal short or high resistance path within the connectors had occurred to me. I have not tried to open up the connectors or pull the pins because everything is nicely melted together, but I may have another go. Sure can't do more damage than is already done.

I have some Deans 3 pin connectors and will look at those or something similar if I ever get up the nerve to try this again. Gotta be careful because as Target noted I think I've used up all my luck with this airplane in one shot on this event!

lc
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Old May 07, 2012, 10:31 PM
War is over (if you want it)
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Thousand Oaks, CA
Joined Mar 2004
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You know far more about EE than I Paul, but I tend to think that people grossly oversize their wiring. 28AWG with the cheapest crap insulation will carry 3A continuous all day long in the hottest Death Valley sun. That's a lot of power, certainly more than an Eneloop pack can produce, which is so popular in F3X models. So while people may think that each of their 6 servos can draw 1.5A, really most modern digitals like MKS6125 and ATX761 pull well under 1A when nearly stalled and if you leave them like that for long enough to melt a wire, chances are the servo will melt as well. So, no, we don't have all 6 servos drawing 0.7A in a F3J launch and even if we did, the wiring would take many minutes to heat.

All that said, I wouldn't use 28AWG for a battery lead either, but only because it is less durable than 22/24 and the weight is never a concern here. For wings though, 30AWG is overkill even for long runs out to ailerons, but it's too small to crimp well at the RX end and the wing plugs need to be mounted PnP style to avoid handling the fragile wire so often 28AWG is a compromise.
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Old May 07, 2012, 10:48 PM
PLD
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Australia, QLD, Charters Towers
Joined Dec 2003
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Depends on what you're willing to accept for/as a voltage drop.

The trouble isn't handling the nominal current (1A) or power dissipation as you say, 28~30AWG is just fine with that, about 0.07~0.16V drop across a half-foot pairing at 1A; it's dealing with the surges. With digital servos you'll see 5~10A surges on unloaded servo transients, and even worse under load. If you have a 10A surge, that suddenly turns into a 0.7~1.6V drop as opposed to a ~0.25V drop with 24AWG.

If there was only one pair of wires I'd strongly recommend 'overdoing', it's the batter/regulator -> RX ones, purely to minimise voltage sags, you can go thin on the servos lines since you obviously won't get your RX browning out (in a way, thinner servo lines prevents browning out a bit more because of the extra resistance acting as a current limiter).

Paul.
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Old May 07, 2012, 10:51 PM
War is over (if you want it)
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Thousand Oaks, CA
Joined Mar 2004
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Liftchaser, if your plug has been corroded, pitted, stretched open, or the wiring has been partially broken the result will be a little bit of heat and a large voltage drop. I really think there would need to be a short somewhere for the plug to melt like that. Could the regulator itself have shorted against the fuselage or battery?

Attached is a picture of a nice F3J model I recently bought second hand. Note the wiring damage I found. Not a big concern in itself as it is pretty unlikely to end up pressed hard against the wing skin and would require similar contact of the positive bus to the airframe somewhere else to complete the circuit, but it shows what might happen.
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Old May 08, 2012, 08:54 AM
mostly gliders
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Skellefteċ, Sweden
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Quote:
Originally Posted by vespa View Post
That's pretty crazy! I'm guessing this was a standard servo connector? I strongly advocate switches and PnP wing wiring and every means necessary to avoid touching any wires at any time (other than charging), but still your melted connector is surprising. Could you have wiring somewhere else that shorted?
Quote:
Originally Posted by vespa View Post
...
... For wings though, 30AWG is overkill even for long runs out to ailerons, but it's too small to crimp well at the RX end and the wing plugs need to be mounted PnP style to avoid handling the fragile wire so often 28AWG is a compromise.
Hello vespa
What is this "PnP " you talking about?

/Ville
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Old May 08, 2012, 11:01 AM
War is over (if you want it)
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PnP: "Plug and Play" - all connectors mounted such that wires are never seen nor touched.
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Old May 08, 2012, 09:34 PM
mostly gliders
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Skellefteċ, Sweden
Joined Sep 2003
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Quote:
Originally Posted by vespa View Post
PnP: "Plug and Play" - all connectors mounted such that wires are never seen nor touched.
Thanks for the reply vespa, but I still do not understand what connectors you mean? Can you show any example, picture or link on a PnP: "Plug and Play" connector?

/Ville (still confused)
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Old May 09, 2012, 10:33 AM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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A glued in (both ends) MPX 6 pin connector is "Plug and Play".

R,
target
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Old May 09, 2012, 11:09 AM
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Any connector you want, DB9, MPX, Deans, Futaba, etc. just glued/screwed to both parts such that you never see or touch any wires. One or both ends of course must be soft mounted (5200, Goop, silicone, etc) to avoid stressing the connector with airframe flex or misalignment.

For example most people setup 3 piece wings with connectors hard-glued to the center panel and loose pigtails on the tips that they yank, scrape, mash and mangle every time they transport or assemble the plane. It's no big deal if the wire breaks and you lose an aileron, but it sure is a problem if the power wires short together because it will kill the whole system.
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Old May 09, 2012, 11:18 AM
mostly gliders
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Skellefteċ, Sweden
Joined Sep 2003
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Quote:
Originally Posted by target View Post
A glued in (both ends) MPX 6 pin connector is "Plug and Play".

R,
target
Thanks Target, I suspected it was so. Without scientific proof, I believe these UNI (JR / Hitec) connectors to be almost the worst thing you can choose. Should I use similar connectors, as I do, I prefer Multiplex three or four pole servo connectors. They feel atleast robust and I have never had any problems with them, never, and it's absolutely true, PnP or not, (I usually glue one part and the other is loose). If possible, I use the MPX 6 or DSUB(DB9), both very good connectors.

/Ville
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Old May 09, 2012, 11:27 AM
mostly gliders
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Skellefteċ, Sweden
Joined Sep 2003
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Quote:
Originally Posted by vespa View Post
Any connector you want, DB9, MPX, Deans, Futaba, etc. just glued/screwed to both parts such that you never see or touch any wires. One or both ends of course must be soft mounted (5200, Goop, silicone, etc) to avoid stressing the connector with airframe flex or misalignment.

For example most people setup 3 piece wings with connectors hard-glued to the center panel and loose pigtails on the tips that they yank, scrape, mash and mangle every time they transport or assemble the plane. It's no big deal if the wire breaks and you lose an aileron, but it sure is a problem if the power wires short together because it will kill the whole system.
Thank you vespa for the reply. I wrote my post while you posted yours, I'm not that fast and always try to write somewhat understandable. Very difficult at times. I use Google Translate, and then it can take a very long time sometimes.
Now I follow.

/Ville
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Old May 09, 2012, 12:28 PM
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USA, FL, Milton
Joined Jul 2008
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[QUOTE=vespa;21549342]Liftchaser, if your plug has been corroded, pitted, stretched open, or the wiring has been partially broken the result will be a little bit of heat and a large voltage drop. I really think there would need to be a short somewhere for the plug to melt like that. Could the regulator itself have shorted against the fuselage or battery?

Hi Vespa,

I think you're onto something there. I recently switched suppliers for the pins and sockets I use to make end connections and I suspect there's something fundamentally wrong with the new items.

I think I mentioned I had switched the new Shadow back to a 4 cell pack and JR switch. In the process I shortened the main power lead to the switch and crimped on a new connector. Didn't change the battery side. Hooked everything up and ran the airplane for probably 10 -15 min. to check it out. Everything worked OK so I thought I'd put the airplane on charge to bring the pack back up to full charge. The charger indicated an open circuit and when I tried to switch the power back on I had nothing. I pulled everything back out of the fuselage and started checking from the battery outward. I powered the RX with a separate battery and digital VOM in the circuit to check current draw. Nothing out of the ordinary there, max current draw was no more than 350 ma with all the servos and RX running to their extremes.

The battery, including the connector, looked perfectly normal. The switch harness looked normal at first but as I examined each connector I could see the same melting beginning in the power connector I had shortened. I need to do some more digging, but I believe there is something in the base material or plating of the pins, or damage in the crimping process, that is setting up a high resistance path for the current across the connector. That, in turn, is generating lots of heat and melting the plastic, probably eventually burning through the wire.

In both cases so far the damage has been at the first connector downstream of the battery, the point where current and voltage is highest. Switching to a soldered 3 pin Deans or similar connector at that point may be a better solution than any crimped connection.

Needless to say, I'm going back to my original supplier for pins and sockets. Saving a buck vs saving an airplane is false economy at its worst.

lc
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Old May 09, 2012, 03:45 PM
Registered User
Crestview, Florida
Joined Dec 2005
633 Posts
I'm the guy who launched the Shadow for Liftchaser and what he didn't tell you was that the Shadow was headed straight down at Mach 2......
....INTO AN ASPHALT RUNWAY!!!

Tap
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Old May 09, 2012, 06:20 PM
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United States, CA, San Luis Obispo
Joined Aug 2006
1,627 Posts
Liftchaser, what crimped connector/pin/sockets are you using? I was just about to order the EMS set so I could clean up some wiring.
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Old May 09, 2012, 06:32 PM
Build A Rocket Boys!
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Portland, Oregon
Joined May 2002
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I use the connectors and pins from Hansen Hobbies. Do they have a good track record with everyone?
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