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› could any electric heli be made to "tethered"
11-28-2005 08:48 PM  12 years agoPost 1
patriot21

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Byron,MN

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Just wondering if any heli could be coverted to be plugged into a wall
because when i have flown in my house i just basically do small patterns and hovering anyway......

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11-28-2005 09:55 PM  12 years agoPost 2
hyperopt

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SanDiego, CA

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Tether the heli to 12V automotive battery is simple to achieve if you are running 3S.

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11-28-2005 11:12 PM  12 years agoPost 3
ShellDude

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East Coventry, PA

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Its also simple to fry the mosfets on your ESC if you're not careful. The nature of a tether is such that its long length allows for voltage to build-up on the wire and when it dumps it overloads the speed controller.

Its easily corrected by use of a diode (I think its a diode) on the line which acts as a filter.

Sorry I'm not more specific... I've only read about it. I'm sure someone with the right details will chime in.

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11-29-2005 02:56 PM  12 years agoPost 4
patriot21

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Byron,MN

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Would this be to much for a speed control to handle for making a tethered heli.....
http://www.radioshack.com/product/i...rentPage=search

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11-30-2005 01:54 AM  12 years agoPost 5
jollsnj

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Alfred, New York

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that transformer is pumping out 12.6v AC that will not work. you could try a comp power supply with an output pot

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11-30-2005 05:13 AM  12 years agoPost 6
ShellDude

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East Coventry, PA

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a bit of quoting...
The leads from the battery to the ESC are length critical. Why?
Because the inductance of the leads in combination with the pulsed
current flowing to the motor leads to voltage spikes. These spikes occur
at the FET on and off transitions. Long leads increase the magnitude of
the spike leading to FET failures. I have scope traces to prove it and
some junk FETS :-(
It doesnt take much inductance at these currents to get a voltage high
enough to reach the breakdown voltage of the power MOSFETs. I have not
looked at the recent parts but for years people used the SMP60N03 part.
This part with proper heatsinking could sustain 60 amps and had a 30 volt
breakdown voltage. In theory you could run 22 cells with it because that
would give a resting voltage of less than 30 volts with a fully charged
pack. I destroyed a few parts on purpose to find out what happens when
you exceed the 30 volts. On most of the parts the device would start to
conduct if the voltage went just a volt over 30 volts. And the FET would
turn into a little ball of metal very quickly conducting equally to all
three pins. I am certain that all FETs have this characteristic in
common. The FETs regluate the voltage going into the motor by turning on
and off at a high rate of speed. When the FETs are turned off the current
flow combined with the inductance in the wire will cause the voltage to
spike above the battery voltage by a rather large amound for a very short
time. I have seen a 7 cell pack spike to over 150 volts with just a
couple of feet of wire when the current was in the 30 amp range. This was
a very short spike (a few tens of nanoseconds above the breakdown voltage)
but it doesnt take much to destroy the FET in this manner. So if you are
running the controller at near its rated voltage and at high currents when
the FETs switch off the voltage at the input to the FETs can easily exceed
the FET breakdown voltage.

I hope that is clear enough. If not you can get a book on basic
electrical engineering and work through the calculations if it interests
you that much.

Doug Ingraham
Rapid City, SD USA
Just a few more quotes... answer's coming....
> Set me straight on something if you will.... to suppress that spike,
> we use a Schotky from motor lead to battery plus. Roughly what kind
> of average current does the Schotky have to handle? I ask because I
> always heard it was about 50% of the output current. But on the
> Piccoboards I repair, they use a 1A Schotky for an output that they
> rate at 12A. ??

Its a good question. And no real good answer because there are too many
variables. Switching rate of the controller, inductance of the motor, and
current. 50% seems high for an average. What you are talking about is
the average current because the instantaneous current is going to be the
motor current and it will quickly decay from that point. The question is,
for the device they are using, can it handle the impulse current of the
full motor current. I am guessing that for the average arrangement it is
closer to 10%. Its a little tricky to measure and it is one of those
things that the parts are expected to do but not rated that way.

If it gets hot then you are overstressing it.

Doug Ingraham
Rapid City, SD USA
So there ya have it. You need a Schotky ... heh

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11-30-2005 07:12 AM  12 years agoPost 7
Rob_T

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I know all about using schotky diodes to supress back EMF from motors and long wires to the motor, but I cant for the life of me see where to fit one to ovcercome the issues caused by long wires between the battery and ESC. I would think you need a high ripple current capacitor at the ESC. The same issue has been discussed in the Joker forum about a year ago as the longer wires in lipo packs compared to nicad packs was thought to be causing an issue with some ESCs. Most brushless ESCs do have a capacitor across the battery but they usually are inadequate for doing the whole job and rely on having relatively short wires to the battery.

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12-08-2005 02:06 AM  12 years agoPost 8
ShellDude

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East Coventry, PA

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I'm curious to know what would happen if you put the schotky on the negative and positive posts for the wires coming from the battery.

Not curious enough to try it, but I'll ask around.

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12-09-2005 12:48 AM  12 years agoPost 9
OT45

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Kingston, NY

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I recall seeing a tethered electric shuttle at the WRAM show in the early 90's. It was an electric conversion kit with a battery box and a pretty long line.
Personally I'm not a big fan of dangling wires on a model helicopter.

scratch building is not just for planks

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12-09-2005 01:47 AM  12 years agoPost 10
ShellDude

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East Coventry, PA

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Do not try to put a schotky across the positive and negative terminals leading to your ESC from your battery. Bad things will happen.

It sounds like it will cause a short that could potentially fry a LiPo pack then the schotky will fail.

A fellow ezone member, one of the individuals quoted above, mentioned the use of a capacitor in addition to the one that all ESCs already have.

I asked him for some additional details. When I hear from him I'll be sure to post them here.

I'll probably try this myself with my car battery once the "experts" have fully weighed in.

Shell

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12-09-2005 05:22 AM  12 years agoPost 11
Rob_T

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I would think a "computer grade" capacitor around 1000uF or larger would do the job. A larger capacitor wont harm. You'll get some arcing when you connect the heli to the battery because of the big capacitor, so croc-clips rather than the normal RC type connectors might be a good idea! You could use a resistor (about 10 ohms) to do a 2 stage hookup- first connect via the resistor then short out the resistor to avoid the spark, but I wouldn't bother myself...

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12-10-2005 03:14 AM  12 years agoPost 12
ShellDude

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East Coventry, PA

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I was told this would do it too but if you look at its dimensions its pretty big... and you still have to solve the ever changing balancing problem.

http://www.filterconcepts.com/dc/ds_series.html

Also Doug Ingraham was kind enough to pass this on to me. Notice his comments about weight issues.
The ideal setup is to eliminate the tether . How long is the
tether? At those currents you will need some pretty heavy wire so it is
not out of the question that a LiPo battery pack might be less weight than
the weight of the wire plus the weight of the capacitor. 13 gauge wire if
you are talking maybe 10 feet of tether. Up to 20 feet or so use 12 gauge
and beyond that use 10 gauge. Of course the weight of 20 feet of 12 gauge
wire is going to be more than the pack weight. My gut feeling is that if
you need more than a 10 foot tether it is not worth doing from a weight
saving standpoint.

You will probably want to use a cap rated 16V low ESR electrolytic type
with a lot of microfarads mounted right on the input of the speed control.
I won't be trying this. Its much easier to just plug in my battery and go.

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