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Old Jan 19, 2013, 12:06 AM
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Throttle servos frying - cause unknown

It appears that I have fried a couple of throttle servos - the one that came with the plane when I bought it, and a spare I used for testing. Here's the background:
  • When I first connected the receiver (Futaba 6000HS) and transmitter (Futaba 8FG), all of the servos were performing properly
  • Over the last couple of days I've done some work with the wiring - rerouting, shortening leads and labeling
  • Today after putting everything back together, the rudder and elevator servos responded but throttle servo (Futabe S3152) did not.
  • I noticed as I advanced the throttle past the halfway point the ignition LED goes out. It comes back on again as the throttle is lowered
  • I plugged the throttle servo into another channel on the receiver but it still did not respond - this is when I suspected it was fried
  • I plugged another servo into the throttle channel (3) to test the receiver and the servo responded. This also leads me to believe the problem is with the throttle servo
  • I plugged a spare servo (JR DS821) into the throttle channel (3) to test it and it worked (at first)
  • A second test of the spare throttle servo saw the servo not responding to transmitter input. However, when the receiver is turned on the throttle servo rotates 1/8 turn counter-clockwise and emits a low-volume, high pitched hum - and it gets hot fast. Thus I suspect this servo is now also fried.
  • The wiring of the ignition cutoff (42 percent products Opto Gas Engine Kill Switch) has been verified as correct
  • The wiring of the voltage regulator (Smart Fly 2S Lithium, 5.2V output) has been verified as correct

Here's the wiring diagram:



The lead between the ignition cutoff and the voltage regulator is especially long. While I was cleaning up the wiring I folded the wire back on itself several times and secured it with a zip tie, like so:



I have no idea if this could be the source of the problem but I have since removed the zip tie and unfolded the wire again.

I bought a new throttle servo but I'm reluctant to try it before I can figure out what the problem is. Help would be much appreciated!
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Old Jan 19, 2013, 07:20 AM
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United States, TX, Leander
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I would check the voltage with a meter at the receiver to make sure the regulator is working. I'm confused by your diagram. It shows the ignition kill powering your receiver. Shouldnt the receiver power be tapped off the input to the ign sw?
Edwin
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Old Jan 19, 2013, 07:46 AM
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If that diagram is correct and your battery voltage is being regulated prior to the kill, the kill feeding the reciever power, you are fortunate the one servo is all that is being effected. Irregardless of load the servos still require a certain amount of power to fuction, without that power it is overloading by underpowering, overheating by trying to do its job with insufficient power, and frying.
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Old Jan 19, 2013, 07:53 AM
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Put another way, that diagram is correct in the use of the regulator and kill for the engine. The lead to the reciever is for the radio signal to kill the engine not to power the radio.So the reciever requires a seperate battery or lead from that battery for power. The above diagram is not for the reciever power but for the ignition power.
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Old Jan 19, 2013, 08:14 AM
probro 2567
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Midlothian,VA
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If this is correct this is just an ignition kill. It is not used to supply the current needed to run the rx. You need a second rx battery. To run only one battery for the whole system you need one of these. It will run the ignition at the desired (chosen by you) voltage and run the rx at full battery voltage.

http://www.tech-aero.net/ultra-ibec



Ace beat me to it while I was searching for the IBEC.
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Last edited by kwj48; Jan 19, 2013 at 08:28 AM.
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Old Jan 19, 2013, 08:35 AM
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Make sure you have no binding or resistance on the throttle linkage. I would suspect the servo is getting jammed up at the end of its throws with the carburetor. If you have a spring on the carb throttle arm, you need to disconnect it as the spring forces the servo to constantly fight to hold its position against the spring trying to close the throttle down. That constant fight overheats the electronics inside.

With the high performance digital servos that can cause the electronics (driver transistors) to overheat and subsequently fail. The servo is rated for 6.0v operation and normally pushing them over that with 6.6v (but it is 7.2v when charged) and when fully charged it can be way over the max servo voltage rating.

I normally use a standard servo for throttle, nothing fancy, as it doesn't need high performance nor does it usually need high torque either. In some cases with the larger gas engines a metal gear servo may be needed due to the engine and air frame vibration causing a subtle movement that can stress the throttle servo. Using a flex cable or nyrod instead of a straight length of piano wire works well too.
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Old Jan 19, 2013, 10:33 AM
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Apologies - here is an updated diagram showing the Rx power.



Note that there is no voltage regulator between the Rx battery and the Rx.

Here are the wiring changes I made:

ELEVATOR
- shortened both servo leads
- replaced connecter between Y-harness and extension with a solder joint

RUDDER
- Shortened servo lead: wires cut and new connector installed

IGNITION
- shortened ignition LED lead: wires cut and new connector installed

Note that no changes were made to the throttle wiring (except to disconnect it from the receiver, re-route and reconnect). When the original servo stopped working I checked and noticed that its connector into the receiver was reversed. However, with the power connection in the centre of the plug it's my understanding that this can't cause any damage. Could this have damaged the servo or receiver?

When I tried the spare servo in the Rx, it was connected neither to linkages nor extension - I held it in my hand and plugged it directly into channel 3 on the Rx.
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Old Jan 19, 2013, 02:45 PM
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Quote:
Originally Posted by earlwb View Post
With the high performance digital servos that can cause the electronics (driver transistors) to overheat and subsequently fail. The servo is rated for 6.0v operation and normally pushing them over that with 6.6v (but it is 7.2v when charged) and when fully charged it can be way over the max servo voltage rating.
Would you run 6v servos on an unregulated 5 cell NMXX battery?
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Old Jan 19, 2013, 02:52 PM
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Neither of the servos you mention are rated for over 6.0v. Since you say the JR servo was not connected to anything when it fried, I guess you've demonstrated their voltage limitation.

Reversed servo connection shouldn't have caused any damage, but the servo would not work at all when reverse-connected because it wouldn't be getting any signal from the receiver.
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Old Jan 19, 2013, 03:11 PM
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Quote:
Originally Posted by abenn View Post
Neither of the servos you mention are rated for over 6.0v. Since you say the JR servo was not connected to anything when it fried, I guess you've demonstrated their voltage limitation.

Reversed servo connection shouldn't have caused any damage, but the servo would not work at all when reverse-connected because it wouldn't be getting any signal from the receiver.
Do you not think that the servo manufactures do not know what nominal voltage and charged voltage of a 6.0v battery is? Both servos give a spec for 6.0v. If it wasn't rated for 6.0 volt they wouldn't have specs for it. A 5 cell nimh/nicad go over 7 volts when fully charged. A123's do the same thing. People have been doing this for years.

ds821=Speed: .19 sec/60 @ 4.8v .15 sec/60 @ 6v ,Torque: 72 oz-in @ 4.8v 88 oz-in @6v


Futaba 3152 = Speed: .22 sec/60 @ 4.8V.,,18 sec/60 @ 6V Torque: 70 oz-in @ 4.8V , 87 oz-in @ 6V

I would look for a crossed up wire somewhere.
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Old Jan 19, 2013, 07:54 PM
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always go where you worked last...if everything worked before you're wiring changes then you have inadvertently done something
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Old Jan 19, 2013, 11:32 PM
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I found evidence that channels 3 and 7 are linked in the transmitter:

Futaba T8FG ch3 and ch7 linked (0 min 19 sec)


I can't for the life of me figure out why. I've checked and there are no mixes in place. I created a brand new model in the transmitter and it has channels 3 and 8 linked!
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Old Jan 20, 2013, 01:26 AM
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Okay, it's quarter after two in the morning and I found it.



I Googled 8FG delete entire memory and found an old thread where a guy was having the same problem with channels 3 and 7 linked. Turns out that the Function menu within the Linkage menu is where you assign functions to channels. Channel 7 had a VPP (I'm assuming variable pitch propeller) function linked to J3 - the throttle stick. When I removed the VPP function and went back to the servo menu, hey presto! Now channel 3 is no longer linked to channel 7!
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Old Jan 20, 2013, 02:55 AM
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Quote:
Originally Posted by kwj48 View Post
Do you not think that the servo manufactures do not know what nominal voltage and charged voltage of a 6.0v battery is? Both servos give a spec for 6.0v. If it wasn't rated for 6.0 volt they wouldn't have specs for it. ....
I know the spec But a 2S A123 nominal voltage is 10% over 6v, and mine can be as high as 3.72v per cell straight off the charger. Personally I wouldn't do it, especially with digital servos which work more of the time trying to hold their position.

It'll be interesting to see the solution. The only source of any voltage higher than 6.6v nominal is the ignition unit itself. I agree that a crossed wire (externally, or within the unit) could be the culprit, but how can it burn out the throttle servo without affecting the receiver and other servos as well?
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Old Jan 20, 2013, 07:26 AM
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Quote:
Originally Posted by abenn View Post
I know the spec But a 2S A123 nominal voltage is 10% over 6v, and mine can be as high as 3.72v per cell straight off the charger. Personally I wouldn't do it, especially with digital servos which work more of the time trying to hold their position.

It'll be interesting to see the solution. The only source of any voltage higher than 6.6v nominal is the ignition unit itself. I agree that a crossed wire (externally, or within the unit) could be the culprit, but how can it burn out the throttle servo without affecting the receiver and other servos as well?
Yo, OP here... see posts 12 and 13. Channels 3 and 7 were linked in the Tx.
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