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Voltage drop and the dreaded "Brownout" , What's really at fault ?

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Voltage drop and the dreaded "Brownout" , What's really at fault ?

Old 09-28-2015, 12:03 PM
  #101  
SkidMan
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Originally Posted by John_M_
Paul, just curious what servos are you using, they must be using HV servos... I would run A123's on the servo buss, but the servos I'm running are rated for 6 volts and I wouldn't push them at 6.6 volts... I won't use a regulator, so I'm stuck with NiMh. ...
John, I originally had the same concern. After some research, and what Baracuda points out, I am very comfortable using A123 packs on servos that have a 6 volt rating. I think it is safe to say that everyone using A123-based packs are using them with servos that have a 6 volt rating. If they were using HV servos they would be using 2-cell Lipos to take advantage spending the extra money.

In the end, especially since we are only talking about a fraction of a volt, AND we are typically driving servos at only a fraction of their rated output, even flying 3D, IMO and for many others, the issue is negligible. The slightly higher voltage will result in faster servo motion thereby consuming more power (producing more heat and wear) than with a lower voltage supplied to the servo. Splitting hairs, we can assume that this will cause some slight reduction in servo life (too many variables to quantify).

However, some people will include a forward biased diode on one power lead which results in a 0.6-0.7 volt drop. They are very cheap and very reliable.

Just to throw it out there, slightly off topic, additional advantages include the very high charge rates you can use on these batteries (10 amps on the most popular cells), safety, and very low self-discharge rates.

Paul

Last edited by SkidMan; 09-28-2015 at 12:20 PM.
Old 09-28-2015, 12:20 PM
  #102  
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Right but a freshly charged 2S A123 pack can be anywhere from 7.2 to 7.4 volts; sure the voltage drops as soon as the battery is under load, but its still above 6.6 volts for a good portion of its charge...with NiMh, the surface charge drops considerably faster; fresh off the charger, 6.8 - 7.2 volts depending on the rate of charge, but under load it quickly drops to 6 -6.2 volts... not saying the servos couldn't handle the s2 A123 voltage under normal flying conditions, but the voltage is the push behind the current and the higher the initial voltage is, the higher the current draw will be by the device... would be nice to go all A123 though, maybe I'll experiment and see how a couple servos handle it... or I can switch to HV servos, but to put HV servos in a sport aircraft, is just a waste of money for me personally,


John M,

Last edited by John_M_; 09-28-2015 at 01:00 PM.
Old 09-28-2015, 12:48 PM
  #103  
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Originally Posted by John_M_
... to put HV servos in a sport aircraft, is just a waste of money for me personally, ...
For me too.

Originally Posted by John_M_
Right but a freshly charged S2 A123 pack can be anywhere from 7.2 to 7.4 volts; sure the voltage drops as soon as the battery is under load, but its still above 6.6 volts for a good portion of its charge..

... not saying the servos couldn't handle the s2 A123 voltage under normal flying conditions, but the voltage is the push behind the current and the higher the initial voltage is, the higher the current draw will be by the device.
Here are my thoughts on that. Like you bring up, LiFePO4 packs will quickly drop to 6.6 volts. The chances are that the aircraft is still on the ground during that period with very little load on the servos, hence very little power required by the servos (power, not voltage is the problem). After that point, I am like you not driving the servos at their full rated capacity.

Do some searching on A123 and servos. I do not think I've come across any situations where people complained about problems using servos with 6 volt ratings, although some have had problems with older servos rated at 4.8 volts. Like you I started slowly but I'm in up to my neck now.

Paul
Old 09-28-2015, 12:53 PM
  #104  
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I do way more installations than the casual modeler and I use Hitec, Futaba, MKS, and Savox 6v rated servos with A123's a lot.

No issues burning up any servos
Old 09-28-2015, 01:05 PM
  #105  
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Originally Posted by BarracudaHockey
I do way more installations than the casual modeler and I use Hitec, Futaba, MKS, and Savox 6v rated servos with A123's a lot.

No issues burning up any servos
I'll bet you don't get brown outs either :-)
Old 09-28-2015, 01:20 PM
  #106  
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Originally Posted by SkidMan
(power, not voltage is the problem)

Paul
Exactly, the servos will be worked harder, especially under load while the initial surface charge drops off... you're working the device at its upper voltage limit, shorting its life, so there is a small risk factor there...... I definitely would not go with the forward biased diode, I would upgrade to HV servos before playing around with stuff like that... would be fine for surface vehicles, but not with an aircraft.

Originally Posted by BarracudaHockey
I do way more installations than the casual modeler and I use Hitec, Futaba, MKS, and Savox 6v rated servos with A123's a lot.

No issues burning up any servos
I'll look into it... I could get my super cub under 20 pounds if I could go all A123, right now it weighs 20.6 lb's

***EDIT***

Just a quick note: a quote from the GF40 ignition module spec's... so again I go by the mfg's instructions, and I won't use a regulator on the ignition system either, so I use NiMh.

Ignition Battery: 4.8-6.0V NiCd or NiMH, 6.6V LiFe or 7.4V LiPo pack
LiPo battery requires voltage regulator
John M,

Last edited by John_M_; 09-28-2015 at 01:29 PM.
Old 09-28-2015, 01:29 PM
  #107  
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Originally Posted by SkidMan
I'll bet you don't get brown outs either :-)
Um....no
Old 09-28-2015, 01:55 PM
  #108  
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Originally Posted by dirtybird
I forgot to mention that if whatever receiver you are using has a circuit board with the etched "traces" made too small they can not carry the Amps ,
I personally witnessed a test to destruction of a 8-channel RX a few years back. We applied a steady 6 volts to the RX and increased the current load in steps. We finally got the RX to fail at something north of 65 amps. Unless you do something really wrong there just is no way you are ever going to draw that much in a plane. I don't care how big it is or how many servos you are running.

EMCOTEC wired up a 14-servo 40% planes a few years back with on board data recording. They measured an average load of 4 to 5 amps total with transient spikes to 30 amps during snap rolls. This is consistent with my own experience with large gas planes. About 4 amps average with short lived transient spikes.

And finally, I've asked both Horizon and Hobbico if they have ever seen a receiver that failed due to current overload on the servo bus. The answer was no.

Bottom line is this is something you really do not have to worry about unless you have somehow managed to set your radio up so every servo can be simultaneously mechanically stalled.
Old 09-28-2015, 01:55 PM
  #109  
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Originally Posted by John_M_
Exactly, the servos will be worked harder, especially under load while the initial surface charge drops off... you're working the device at its upper voltage limit, shorting its life, so there is a small risk factor there.
Yes. But here's the interesting point, the servos aren't doing a whole lot of moving when you power up the plane for the first flight of the day. Other than the weight of the control surfaces and any spring on the throttle, very little power is required. Here is what I am thinking about: you assemble the plane, turn on the power switch, wiggle the control surfaces to make sure they are operating properly, start the engine and run it up and down a few times, and only then do you take it to the taxi way where you use the throttle and rudder (at least), and pull out onto the active runway where you double check all the control surfaces. All these operations require very little power (not over driving the servos) but go a long ways to burning off the initial surface charge - this would almost certain be true with digital servos that draw a small but greater amount of power than analog servos even when they are just standing still. Personally, I'm also not likely to accelerate to full speed and throw all the control surfaces to full defection performing a snap roll at the very beginning of my first flight of the day. I'm assuming that either my battery pack is down to 6.6 volts before I get to the runway or perhaps the first lap or two while I make sure everything is functioning properly before do something demanding. So far my servos have put up with the abuse I dish out to them.

Here is a key premise to my thinking: The danger of higher voltages is greatest when the servo requires greater power to move to or hold its commanded position. Also, the danger of damage is lower when less power is required.

But hey, the most important thing is that you only do what you are comfortable doing.

BTW - have you ever noticed servos listing their min-max operating voltages? It seems to me that they just list rotational rates and torque at different voltages rather than saying that 6.0 volts (or whatever) is the maximum the servo is capable of safely operating at.


Paul
Old 09-28-2015, 01:56 PM
  #110  
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Originally Posted by jmiles1941
dirtybird the one I saw was a klein tools and it has a pouch and my mistake the price is 79.95 my bad but my fluke is around 300 even the cheaper ones will do what you are looking for , or the 19 dollar volt amp from hangar 9 just unplug the servo from the rx plug in the tester hook the servo to the other end set it to amps and it will let you know the current draw thanks
I have a Sears Craftsman that I bought several years ago.I think I paid around $50 for it. I was interested in the Amprobe because it will also measure the voltage with no connection. That would be handy. But I am not going to pay $200 just for that.
Old 09-28-2015, 02:09 PM
  #111  
dirtybird
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Originally Posted by Silent-AV8R
I personally witnessed a test to destruction of a 8-channel RX a few years back. We applied a steady 6 volts to the RX and increased the current load in steps. We finally got the RX to fail at something north of 65 amps. Unless you do something really wrong there just is no way you are ever going to draw that much in a plane. I don't care how big it is or how many servos you are running.

EMCOTEC wired up a 14-servo 40% planes a few years back with on board data recording. They measured an average load of 4 to 5 amps total with transient spikes to 30 amps during snap rolls. This is consistent with my own experience with large gas planes. About 4 amps average with short lived transient spikes.

And finally, I've asked both Horizon and Hobbico if they have ever seen a receiver that failed due to current overload on the servo bus. The answer was no.

Bottom line is this is something you really do not have to worry about unless you have somehow managed to set your radio up so every servo can be simultaneously mechanically stalled.
Don't attribute that small trace vs amps to me. I copied that from TGWR's post.
Eloy Marez (sp?) did that test many years ago and published it in RCM. He got similar results.
Spektrum rates their receiver at 30 amps.
Old 09-28-2015, 02:29 PM
  #112  
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Originally Posted by SkidMan
Yes. But here's the interesting point, the servos aren't doing a whole lot of moving when you power up the plane for the first flight of the day. Other than the weight of the control surfaces and any spring on the throttle, very little power is required. Here is what I am thinking about: you assemble the plane, turn on the power switch, wiggle the control surfaces to make sure they are operating properly, start the engine and run it up and down a few times, and only then do you take it to the taxi way where you use the throttle and rudder (at least), and pull out onto the active runway where you double check all the control surfaces. All these operations require very little power (not over driving the servos) but go a long ways to burning off the initial surface charge - this would almost certain be true with digital servos that draw a small but greater amount of power than analog servos even when they are just standing still. Personally, I'm also not likely to accelerate to full speed and throw all the control surfaces to full defection performing a snap roll at the very beginning of my first flight of the day. I'm assuming that either my battery pack is down to 6.6 volts before I get to the runway or perhaps the first lap or two while I make sure everything is functioning properly before do something demanding. So far my servos have put up with the abuse I dish out to them.

Here is a key premise to my thinking: The danger of higher voltages is greatest when the servo requires greater power to move to or hold its commanded position. Also, the danger of damage is lower when less power is required.

But hey, the most important thing is that you only do what you are comfortable doing.

BTW - have you ever noticed servos listing their min-max operating voltages? It seems to me that they just list rotational rates and torque at different voltages rather than saying that 6.0 volts (or whatever) is the maximum the servo is capable of safely operating at.


Paul

I agree with your thinking, it sounds rational, lol... it is myself that I need to convince, lol.

I'm sure the servo operating spec's are underrated purposely for longevity and reliability reasons; not to mention for liability reasons as well, protecting themselves.

I've just discovered, that the GF40 ignition voltage requirements is 4.8v to 8.4v unregulated, and will take a 4 to 6 cell NiCad / NiMh, or 2S 6.6v LiFe, 7.4v Lipo... so If I change out the two NiMh packs, servo / ignition to 2S A123 LiFe packs, I'll have to move all the battery packs forward to rebalanced the CG, but I gain much better performance form the A123 battery packs, I'll shed close to a pound in weight... looks like I got me a new project for this weekend, thanks guys... if I end up with a lawn dart, no worries, I won't blame anyone but myself




John M,

Last edited by John_M_; 09-28-2015 at 02:52 PM.
Old 09-28-2015, 06:13 PM
  #113  
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I have been looking at Sony VTC4 and 5 and the equivalent Panasonic batteries These cells are rated at 2.1 to 2.6 AH and have a maximum current rating of 20-30 amps. Each cell is about the size of a pencell and weighs 1.5 ounces. They are actually the same chemistry as our LIPOLy's that are not lipoly but LIFE. I made up an 8cell pack that weighs 12 oz producing 33 V. I plan to use this pack to power an aircraft.
I am using a two cell pack to power a late model ignition that is rated for 8V.
Since they are in an aluminum can they are safer than a LIPOLY. .
I got twelve off of ebay for $4 ea.
The MFG will not sell them individually. They are intended to be made up into laptop batteries so you have to rely on ebay. Beware of fakes!
Old 09-28-2015, 06:49 PM
  #114  
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I've read about those... 3.8 volt cells, full charge 4.2V... there are "protected" versions as well, where there is a small circuit at one end of the cell that controls the charge / discharge cut off voltage... not sure if I would use the protected ones in an airborne system... but at 2.6 Ah, 2 cells would be great for a HV servo buss setup, super light weight to boot... max continuous discharge of 30A, that's impressive for such a small cell.


John M,
Old 09-28-2015, 10:40 PM
  #115  
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The usual current probe and DMM will not measure the peak currents accurately.
It takes an oscilloscope probe or a current shunt and an oscilloscope to measure the
transient currents generated by a digital servo.
Old 09-28-2015, 11:49 PM
  #116  
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Originally Posted by dirtybird
If a battery shorts out it will definitely take the other one down.
I agree with you Dirtybird. some time ago a freind decided to put a second 4 cell NiMh battery in his first jet. My advice was to replace the single battery with twin, new batteries and isolate them with diodes. However, he had read in a UK mag that this was unecessary, because everything works just fine if you plug two batteries into your RX. The writer startted his article by pointing out that he had no knowledge of electronics! And of course the writer was quite correct, when everything is working, everthing is working. All is well with the world.
But we do not build redundant systems for when everything is working but rather for when something goes wrong, and in order to test if your redundant system actually works, you need to put the system into the failure mode it was designed to overcome.
My friends jet burst into flames when he switched on the second battery. Of course we had a fire extinguisher on hand and the fire was out in a few moments. It was not the battery that had shorted out but the switched that it was wired to. The leads on that switch were blackend, and the terminals of the switch were welded together. This had discharged battery 1 whist it was switched off, as it had a negative lead running between the terminals. II suspect the the insulation on this wire thad been mechanical worn away. In all probability this had been a "slow burn" that had turned the switch into a lump of carbon, but then when he switched on the second battery, that battery was presented with a short which quickly heated the wires to the point that the insulation melted, creating an even better short, and a fire. It is for this failure mode that we use diodes. A little bit of knowledge can be a dangerous thing.
However, I would caution you against using two batteries in the way you have suggested. If you use one for the Rx and a different battery for the servos, you are now relying on two batteries, switches and harnesses to be good. That is a worse situation than having a single battery. Better to have 2 batteries in parrallel, either through some propietery power distribution hub, or through two diodes, one in each switch lead. In this way you have double the capacity of battery on tap, and double the current availibility too. Best of all should one system fail, you have second. Bigger capacity and better suited batteries are they way to go, and always two of them IMO.
Finally, NiMh are for TV remotes!
John

Last edited by JohnMac; 09-28-2015 at 11:58 PM.
Old 09-29-2015, 09:43 AM
  #117  
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Might add that whether analogue or digital servos are used they are still only as good as hookup or slop in the mechanical part of the system. If the system has no slop then the digitals , as said many times, minimizes the amount of trimming that has to be done. My preference... digitals; you just have to put up with the buzzing.
Old 09-29-2015, 11:34 AM
  #118  
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Originally Posted by JohnMac
A little bit of knowledge can be a dangerous thing.
However, I would caution you against using two batteries in the way you have suggested. If you use one for the Rx and a different battery for the servos, you are now relying on two batteries, switches and harnesses to be good. That is a worse situation than having a single battery. Better to have 2 batteries in parrallel, either through some propietery power distribution hub, or through two diodes, one in each switch lead. In this way you have double the capacity of battery on tap, and double the current availibility too. Best of all should one system fail, you have second. Bigger capacity and better suited batteries are they way to go, and always two of them IMO.
Finally, NiMh are for TV remotes!
John
John Mac (another John M )


The idea of using two separate batteries, one for the servos, and one for the receiver is not for redundancy protection... its to get the current hungry hi-voltage, digital / coreless servos off the receiver's pin connector buss... putting the servos on their own high current buss, and powering the receiver with its own battery, done properly is just as reliable as a single battery arrangement... actually its more reliable in the terms of protecting the receiver from a brown-out... and with the batteries we have today that were designed for repeated hi-discharge / re-charge cycles, used to power these minimally current hungry devices in retrospect, the batteries will never see their maximum designed thermal / discharge rates in these two particular locations (receiver and servo power), which further reduces the chance of a battery failure.

A properly assembled battery pack (one each for the receiver & servo buss), with a high quality soft switch, or even mechanical switch; properly isolated from vibration, has proven to be extremely reliable... in over the 50 years I've been in this hobby, since I was a young kid, and especially in the last 25 years, I have never seen a battery pack, short out internally or go open from a bad cell... all the failures I seen related to batteries, are caused by the operator; poor installation, poor assembly of the battery pack / cold solder joints... broken battery or servo wires from allowing them to dangle and not properly restrained.. using damage batteries recovered from crashes, etc.... granted mechanical switches can fail under vibration, but this is preventable with proper installation and maintenance... and as previously mentioned, the use of good quality components.




John M,
Old 09-30-2015, 07:38 AM
  #119  
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Originally Posted by John_M_
Right, all the signal wires and at least one common ground should be carried through ... the other wires do add some strain relief to the connections to the board.




John M,
JOHN: Those Servo City Servo Boards are only $18.00 and I love them !!! That was the cure for my receivers...it works great. It is a lot cheaper than a jr Receiver.

I only use A-123 for the receiver power...6.6V I don't like regulators so what are you using for your servos?

Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.
Old 09-30-2015, 08:05 AM
  #120  
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Originally Posted by TOPGUN WINNER
JOHN:

Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.
Look up the specification for the servo.
I usually just do a google search for XXXservo specification.

Here is one for the Futaba 3003.

https://www.servocity.com/html/s3003...l#.VgwIkVRViko

Last edited by dirtybird; 09-30-2015 at 08:09 AM. Reason: add link
Old 09-30-2015, 08:06 AM
  #121  
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Originally Posted by TOPGUN WINNER
JOHN: Those Servo City Servo Boards are only $18.00 and I love them !!! That was the cure for my receivers...it works great. It is a lot cheaper than a jr Receiver.

I only use A-123 for the receiver power...6.6V I don't like regulators so what are you using for your servos?

Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.
if you aren't sure you take a digital volt meter read the resistance across the red and black wire that will give you R (resistance) then you divide the A (amps ) by R to get E(voltage) remember any voltage that is in excess of the rated voltage turns to heat energy can never be destroyed , it goes from one form to another
Old 09-30-2015, 08:20 AM
  #122  
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Originally Posted by TOPGUN WINNER
JOHN: Those Servo City Servo Boards are only $18.00 and I love them !!! That was the cure for my receivers...it works great. It is a lot cheaper than a jr Receiver.

I only use A-123 for the receiver power...6.6V I don't like regulators so what are you using for your servos?

Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.
Servos that show a 6V rating are rated for 6V

Look up tail rotor servos like the 9253 and 9254, both are damaged by excessive heat caused by the high frame rate from a gyro combined with higher than a 4 cell pack, you'll see they only rate the torque and speed for 4.8v

Last edited by BarracudaHockey; 09-30-2015 at 08:22 AM.
Old 09-30-2015, 09:04 AM
  #123  
dirtybird
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Current is what destroys a servo,not voltage. A servo rated for 4.8 volts can be used on 8.4v if it is not in a heavy use situation. The higher voltage will draw higher current if it is worked hard and that is what burns it out. The tail rotor servo in a helicopter is working hard all the time and is subject to hard use. The same servo can be used on say the ailerons of a foamy and not be bothered.
In any case I would never exceed its specified rating.
.If you use a stabilizer in your aircraft,keep in mind the control servos will likely be working all of the time like a hellie tail servo does
Old 09-30-2015, 10:12 AM
  #124  
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Originally Posted by TOPGUN WINNER
JOHN: Those Servo City Servo Boards are only $18.00 and I love them !!! That was the cure for my receivers...it works great. It is a lot cheaper than a jr Receiver.

I only use A-123 for the receiver power...6.6V I don't like regulators so what are you using for your servos?

Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.

Yes I know, thank you ... if you buy the un-assembled kit, you can use the heavy duty servo extension wires and cut them to length from where you placed your receiver and servos to the power board and solder them into the board... that way they lay in nicely, and can be dressed in tidy.

Right now I'm using a 5 cub "C" cell 3000 mAh NiMh pack for the servo buss... but I'm going to swap that out for another 2500mAh 2S A123 pack... as it turns out, the 0.6 volt difference isn't going to hurt anything... and have found out that the surface charge on a NiMh battery pack takes longer to normalize than the surface charge of the A123 batteries, so in actual fact the 5 cell 6 volt NiMh batteries are harder on 6 volt rated devices, than the 6.6 volt A123 batteries are... so out goes the two 3000 mAh NiMh packs (servo buss & ignition) and in goes two more 2S A123 packs.


Originally Posted by TOPGUN WINNER
Question: How do you know if a servo is rated for 4.8V or 6.0V ??? Just because they show the Torque for 6v input for a particular Servo does not mean it is rated for 6v use...So how do we know? I find that very misleading.

They wouldn't state the speed and torque values for 6 volts, if the servos were not designed to handle 6 volts.

Originally Posted by BarracudaHockey
Servos that show a 6V rating are rated for 6V

Look up tail rotor servos like the 9253 and 9254, both are damaged by excessive heat caused by the high frame rate from a gyro combined with higher than a 4 cell pack, you'll see they only rate the torque and speed for 4.8v
+1





John M,

Last edited by John_M_; 09-30-2015 at 11:03 AM. Reason: Clarification
Old 09-30-2015, 01:12 PM
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Originally Posted by BarracudaHockey
Servos that show a 6V rating are rated for 6V

Look up tail rotor servos like the 9253 and 9254, both are damaged by excessive heat caused by the high frame rate from a gyro combined with higher than a 4 cell pack, you'll see they only rate the torque and speed for 4.8v
I have Futaba servos 9253 and you are correct...they are rated at 4.8V and cost around $130...I use them on "Pull-Pull" Rudder Systems.

However, as you might imagine I have spent a lot of time talking to the Spektrum Tech guys on the phone. All very nice and very intelligent. Dan Smith is the Manager of that Group and a very nice man. We have had several discussions on this subject and he told me the following: " Because everyone is using more 6V batteries, we try to show the Torque and speed for all our Servos if you are using 6Volts...However, that does not necessary mean we are rating them at 6V"

Check it out, I was totally surprised...If it showed the Torque for 6v I naturally thought it was rated for 6 volts the same as Futaba and the 9253. I wonder if anyone makes an accurate chart that shows what servos are rated for what Voltage?

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