MISCONCEPTIONS: We have many
misconceptions in Radio Control (RC)! Some
are new, since there have been and will
continue to be major changes in equipment
and techniques. Some are old and never seem
to go away, but that is to be expected since
there is a new crop of RCers born every week.
One of these misconceptions came to light in
a letter from a reader in Stilwell, Kansas, who
writes:
“You mentioned ‘servo reversers,’ which
brings me to the point. If I need a Futaba S148
to run opposite to its normal direction (in a
left and right aileron application) can the red
and black wires simply be reversed at the
connector? I have asked several people whom
I thought might know if the wire switch will
work, but I haven’t gotten an answer. I
haven’t tried it for fear of lousing something
up.”
You did well in not trying this oft-given
advice, especially if you are allergic to smoke.
Not very long ago I saw this procedure
suggested on the Internet, and I was once told
by a local flier that a person at one of the
largest hobby shops in my area told him that it
was the easy way to reverse a servo’s rotation.
I would guess that this belief exists
because it is a known fact that you can reverse
the rotation of a common direct current (DC)
motor by reversing the polarity of the drive
voltage. Although it is true that the same will
happen, and has to happen to a servo motor to
reverse the direction of its output, the voltage
is applied by the servo amplifier and not
directly from the servo plug. And the
amplifier, like any other electronic circuitry,
will not be happy with reverse voltage.
It can be a costly mistake because
amplifiers, especially those in the more costly
servos, are costly themselves. Don’t think you
can return them to their maker (importer) and
claim normal failure. Reverse voltage damage
is readily apparent to a trained technician, and
the same type of damage will almost never
otherwise occur.
So how do you reverse servo rotation?
Some you can’t, for purely mechanical
reasons. Please refer to the drawing; the
requirements are simple and only require
changing four connections—two at the motor
and two at the feedback potentiometer (pot),
which is located under the amplifier.
You should have the proper (small)
soldering iron and skills before attempting this
change. The reason some servos cannot be
reversed is that the pot or motor, and in some
cases both, are connected directly to the
printed circuit (PC) board without wire
connections, and it is necessary to cut lands
(the metallic conductive areas on the PC
board to which the components are attached)
and run jumper wires. Most RC companies
will not reverse those servos for you!
There are some exceptions, such as the
Airtronics 94102 servo and others that use the
same motor and are attached directly to the
board. It has three connections: a ground in
addition to the two voltage points. The ground
connection is the center one in a triangle
configuration, and it is a push-on slip ring on
the end cap of the motor.
It is necessary to remove the motor
completely from the board, rotate the slip ring
180°, reinstall it on the board, and resolder the
connections. The power connections are
automatically reversed when this is done, but
don’t forget that you still have to reverse the
pot leads, which in the case of the 94102 are
readily accessible wires.
Many of the available Electronic Speed
Controls (ESCs) have reverse voltage
protection built in, which will prevent any
damage; the unit will not operate the
companion motor. Why not servos, you ask?
It is common for ESC users to wire battery
packs and often all the motor components in
electric-powered models, but it is uncommon
for users to switch the wiring in servo plugs—
except as mislead! I doubt if a servo exists
with the necessary space for any additional
reverse voltage protection circuitry.
(Editor’s note: AMA and Model Aviation
do not advise making electronic modifications
to RC equipment. The author’s comments and
opinions are his alone, and AMA takes no
responsibility for any damage to equipment or
accidents resulting from such modifications.
Also, any modification to a manufacturer’s
product typically voids the warranty.)
Mail: Except for batteries and chargers,
servos are easily the most questioned part of
RC systems. Cyril Bauer of Golden, Missouri,
writes:
“I have a special device with plugs that I
use in series with my multi-meter between the
servo and the radio (receiver) to check the
current demand, and assume that I have a
problem if the current demand stays high.
Sometimes I get a high reading that does not
recede, each servo will react differently. Is
this an accurate check of current demand or
does the meter give a false reading? I had a
problem of short power duration when I
decided to check current usage.”
Let’s look at current-reading
instruments—ammeters or those that read
Eloy Marez
E l e c t r o n i c s
2626 W. Northwood, Santa Ana CA 92704
Anatomy of a servo. On the left is the pot and on the right is the motor. To reverse
rotation, the outer wires on both must be reversed. See text for details.
Face of a common 0- to 15-volt analog
meter, which can be electronically
modified to read 9-12 volts, then is known
as an ESV.
94 MODEL AVIATION
03sig3.QXD 12.20.02 8:19 am Page
submultiples, such as milliammeters or
microammeters (thousands and millionths of
an ampere respectively). We have two types
of instruments: analog and digital. Analog
meters are the older needle type and digital
are the ones that display the value being
measured in numbers. Both have unique
features and provide better data in specific
circumstances.
In the case of a varying value, such as
servo current in Cyril’s case, the analog
instrument is the best by far. Because of
mechanical limitations, the needle is unable
to follow the rapid changes and averages out
the peaks and valleys, resulting in average
but still useful values. Readability is a factor,
on a meter with the average 21⁄2- or 3-inch
scale, you have to interpolate; accuracy is not
its greatest feature.
Digital meters provide us with readings in
at least hundredths and sometimes
thousandths of a unit. They are apparently
more accurate, but note that word
“apparently”! As in most measuring devices,
accuracy is a matter of quality—read “cost.”
Do not expect your $19.95 Wonder-Meter to
provide you with 100%-accurate information;
it won’t. Even the better class of instruments
come in different grades, with different
guaranteed accuracies, and, yes, with
different prices.
For certain applications, such as MilSpec
(Military Specifications), the calibration has
to be checked periodically.
The digital instruments have another
serious drawback; in reading unsteady values,
they will flicker so much that even the
computer- and video-game-whiz teenager in
your house will have trouble reading them.
In the current-reading application, all
multimeters, analog and digital, in addition to
the inaccuracy inherent because of poor
quality add another problem. To read current,
most such instruments insert a resistance in
series in the circuit and actually read the
voltage drop across it as current flows. These
resistances are small fractions of an ohm, but
they are resistances and will actually result in
a higher total resistance within the circuit,
thus a lowering of the actual current.
The manual with professional instruments
will have a discussion about this, described as
“burden voltage,” and will provide a formula
that will compensate for the increased
resistance and provide a true current reading.
There is an easy way to confirm this
meter-imposed resistance. The next time you
are making a current reading, note the value
and switch the meter to the next highest scale.
You will find that the displayed readings are
not exactly the same—the result of different
resistance values being inserted into the
circuit and actually different amounts of
current flowing.
Now for Cyril’s questions. As stated, the
results are not going to be 110% accurate, but
in this case that is not super important. The
system will definitely point to discrepancies
if any exist.
Hopefully using an analog meter,
establish an average for each unloaded servo
that is without a pushrod connected. To do
so, cycle the transmitter stick back and forth
at a rate that the servo will rotate end to end
constantly. Get a feel for the servo speed
under those conditions, then quickly
disconnect the meter and cycle the servo
again; any noticeable increase in speed is an
indication that you have a poor meter that is
causing enough voltage drop to reduce the
servo speed.
If under this unloaded test “the high
reading that does not recede” occurs, you are
back to that quality business I discussed
before because, like meters, all servos are not
created equal! Or it may be that such a servo
has a dirty pot or damaged gears.
The next step is to connect the pushrods.
First, operate them manually to be sure that
there is no binding or excessive friction in the
rods or hinges. One or the other could easily
be the cause of the short power duration
mentioned because it will put more of a load
on the servo and increase its current draw.
There will be an increase in the reading
with the pushrods connected because the
servos will be working harder and the
increase will differ for each control; the loads
will not be the same throughout. It is
important to watch for high steady readings at
extremes of rotation, which indicate that the
servo is bottoming; that is, whatever it is
connected to has reached the end of its travel
before the servo does. Such occurrences
happen most often on the throttle and nose
gear channels than on any of the others.
ESV Misconceptions: Since we have
touched on the subject of meters, it seems to
be a good time to discuss Expanded Scale
Voltmeters, or ESVs. The term can be
applied correctly only to analog meters;
digitals are something else entirely.
An expanded scale meter is one to which
electronic circuitry has been added, tailoring
it having to display a narrow range of values.
Refer to the sketch of the meter face shown;
the basic meter is intended to read 0.0-15.0
volts. If we use such a meter to read the 9.6
nominal volts of an RC transmitter battery, it
will be difficult to read an exact value and
impossible to discern small differences.
To increase the readability, with the
circuitry mentioned, we can change it to read,
say, from 9.0 to 12.0 volts; a 9.6-volt Ni-Cd
battery will read considerably higher when
first charged. With such a change, there will
be no movement with any voltage less than
9.0 and will peg at the right-hand side with a
voltage greater than 12.0, but it is possible to
read 9.6 and variations with greater accuracy!
A commercially manufactured instrument
will have the proper markings on the meter
face; home-brewers will have to change them
or just remember the new values. That is an
ESV! Period! But in RC, all such instruments
made especially for us place a calculated load
on the battery, which will read different with
and without such a load. Although the load
feature is a good idea, we had an ESV
without it.
What’s the load? A simple resistor that
will draw current from the battery—50% of
the capacity in milliamperes (mA)
recommended. Using Ohm’s law (resistance
in ohms equals the voltage divided by the
current in amperes), we can arrive at the
required resistor value for any given battery.
Take the common 600 milliampere-hours
(mAh) 9.6-volt transmitter battery, which we
wish to load at 300 mA. The voltage (9.6)
divided by .3 amp (300 mA) equals 32 ohms.
The nearest standard values are 30 and 33
ohms; either will do the job.
In purchasing resistors, one has to specify
the wattage of the unit. Another form of
Ohm’s law (power in watts is equal to the
current in amperes multiplied by the voltage)
is used to calculate that. In this case, .3 amp
multiplied by 9.6 volts equals 2.88 watts;
standard values are 3.00 and 5.00 watts;
either one is usable. The latter is actually a
better choice because it will generate heat
while it is in use.
There is no such thing as a digital ESV,
except maybe in the RC marketplace. The
inherent greater accuracy of the digital
instruments does not require any expanding
of the display—it will automatically read the
small variances in voltage—with the
limitations of their basic quality as already
mentioned. Digital voltmeters can and are
provided with loads as explained, and a
common non-RC meter can have such a load
added simply by connecting the proper
resistor across the meter leads while the
battery voltage is being read.
Receiver Crystals: Cyril also had some
receiver questions. He wrote:
“When I order a radio, do they check it
out or do they have lots of radios on all
frequencies? Some distributors mention high
and low bands. If I order a radio and request
high band, what do I get?”
The first part of the question will vary
according to brand. Some seem to work well
across the relatively small spread of the 72
MHz band, and the crystal set specified will
arrive separately. Others will arrive with the
proper label on the box and on the
equipment. We’ve come a long way since we
used to have to pick crystals that matched and
tuned the receiver to its companion
transmitter.
Some makers split the 72 MHz band in
half—a low-band system being one that
operates on the 11 to 35 channel frequencies,
and 36 to 60 designated as their high band.
This does not necessarily mean greater
frequency accuracy; it can be a requirement
of the circuitry and/or crystals being used.
Go fly! The stuff works well in spite of the
misconceptions! MA
Edition: Model Aviation - 2003/03
Page Numbers: 94,95,96
Edition: Model Aviation - 2003/03
Page Numbers: 94,95,96
MISCONCEPTIONS: We have many
misconceptions in Radio Control (RC)! Some
are new, since there have been and will
continue to be major changes in equipment
and techniques. Some are old and never seem
to go away, but that is to be expected since
there is a new crop of RCers born every week.
One of these misconceptions came to light in
a letter from a reader in Stilwell, Kansas, who
writes:
“You mentioned ‘servo reversers,’ which
brings me to the point. If I need a Futaba S148
to run opposite to its normal direction (in a
left and right aileron application) can the red
and black wires simply be reversed at the
connector? I have asked several people whom
I thought might know if the wire switch will
work, but I haven’t gotten an answer. I
haven’t tried it for fear of lousing something
up.”
You did well in not trying this oft-given
advice, especially if you are allergic to smoke.
Not very long ago I saw this procedure
suggested on the Internet, and I was once told
by a local flier that a person at one of the
largest hobby shops in my area told him that it
was the easy way to reverse a servo’s rotation.
I would guess that this belief exists
because it is a known fact that you can reverse
the rotation of a common direct current (DC)
motor by reversing the polarity of the drive
voltage. Although it is true that the same will
happen, and has to happen to a servo motor to
reverse the direction of its output, the voltage
is applied by the servo amplifier and not
directly from the servo plug. And the
amplifier, like any other electronic circuitry,
will not be happy with reverse voltage.
It can be a costly mistake because
amplifiers, especially those in the more costly
servos, are costly themselves. Don’t think you
can return them to their maker (importer) and
claim normal failure. Reverse voltage damage
is readily apparent to a trained technician, and
the same type of damage will almost never
otherwise occur.
So how do you reverse servo rotation?
Some you can’t, for purely mechanical
reasons. Please refer to the drawing; the
requirements are simple and only require
changing four connections—two at the motor
and two at the feedback potentiometer (pot),
which is located under the amplifier.
You should have the proper (small)
soldering iron and skills before attempting this
change. The reason some servos cannot be
reversed is that the pot or motor, and in some
cases both, are connected directly to the
printed circuit (PC) board without wire
connections, and it is necessary to cut lands
(the metallic conductive areas on the PC
board to which the components are attached)
and run jumper wires. Most RC companies
will not reverse those servos for you!
There are some exceptions, such as the
Airtronics 94102 servo and others that use the
same motor and are attached directly to the
board. It has three connections: a ground in
addition to the two voltage points. The ground
connection is the center one in a triangle
configuration, and it is a push-on slip ring on
the end cap of the motor.
It is necessary to remove the motor
completely from the board, rotate the slip ring
180°, reinstall it on the board, and resolder the
connections. The power connections are
automatically reversed when this is done, but
don’t forget that you still have to reverse the
pot leads, which in the case of the 94102 are
readily accessible wires.
Many of the available Electronic Speed
Controls (ESCs) have reverse voltage
protection built in, which will prevent any
damage; the unit will not operate the
companion motor. Why not servos, you ask?
It is common for ESC users to wire battery
packs and often all the motor components in
electric-powered models, but it is uncommon
for users to switch the wiring in servo plugs—
except as mislead! I doubt if a servo exists
with the necessary space for any additional
reverse voltage protection circuitry.
(Editor’s note: AMA and Model Aviation
do not advise making electronic modifications
to RC equipment. The author’s comments and
opinions are his alone, and AMA takes no
responsibility for any damage to equipment or
accidents resulting from such modifications.
Also, any modification to a manufacturer’s
product typically voids the warranty.)
Mail: Except for batteries and chargers,
servos are easily the most questioned part of
RC systems. Cyril Bauer of Golden, Missouri,
writes:
“I have a special device with plugs that I
use in series with my multi-meter between the
servo and the radio (receiver) to check the
current demand, and assume that I have a
problem if the current demand stays high.
Sometimes I get a high reading that does not
recede, each servo will react differently. Is
this an accurate check of current demand or
does the meter give a false reading? I had a
problem of short power duration when I
decided to check current usage.”
Let’s look at current-reading
instruments—ammeters or those that read
Eloy Marez
E l e c t r o n i c s
2626 W. Northwood, Santa Ana CA 92704
Anatomy of a servo. On the left is the pot and on the right is the motor. To reverse
rotation, the outer wires on both must be reversed. See text for details.
Face of a common 0- to 15-volt analog
meter, which can be electronically
modified to read 9-12 volts, then is known
as an ESV.
94 MODEL AVIATION
03sig3.QXD 12.20.02 8:19 am Page
submultiples, such as milliammeters or
microammeters (thousands and millionths of
an ampere respectively). We have two types
of instruments: analog and digital. Analog
meters are the older needle type and digital
are the ones that display the value being
measured in numbers. Both have unique
features and provide better data in specific
circumstances.
In the case of a varying value, such as
servo current in Cyril’s case, the analog
instrument is the best by far. Because of
mechanical limitations, the needle is unable
to follow the rapid changes and averages out
the peaks and valleys, resulting in average
but still useful values. Readability is a factor,
on a meter with the average 21⁄2- or 3-inch
scale, you have to interpolate; accuracy is not
its greatest feature.
Digital meters provide us with readings in
at least hundredths and sometimes
thousandths of a unit. They are apparently
more accurate, but note that word
“apparently”! As in most measuring devices,
accuracy is a matter of quality—read “cost.”
Do not expect your $19.95 Wonder-Meter to
provide you with 100%-accurate information;
it won’t. Even the better class of instruments
come in different grades, with different
guaranteed accuracies, and, yes, with
different prices.
For certain applications, such as MilSpec
(Military Specifications), the calibration has
to be checked periodically.
The digital instruments have another
serious drawback; in reading unsteady values,
they will flicker so much that even the
computer- and video-game-whiz teenager in
your house will have trouble reading them.
In the current-reading application, all
multimeters, analog and digital, in addition to
the inaccuracy inherent because of poor
quality add another problem. To read current,
most such instruments insert a resistance in
series in the circuit and actually read the
voltage drop across it as current flows. These
resistances are small fractions of an ohm, but
they are resistances and will actually result in
a higher total resistance within the circuit,
thus a lowering of the actual current.
The manual with professional instruments
will have a discussion about this, described as
“burden voltage,” and will provide a formula
that will compensate for the increased
resistance and provide a true current reading.
There is an easy way to confirm this
meter-imposed resistance. The next time you
are making a current reading, note the value
and switch the meter to the next highest scale.
You will find that the displayed readings are
not exactly the same—the result of different
resistance values being inserted into the
circuit and actually different amounts of
current flowing.
Now for Cyril’s questions. As stated, the
results are not going to be 110% accurate, but
in this case that is not super important. The
system will definitely point to discrepancies
if any exist.
Hopefully using an analog meter,
establish an average for each unloaded servo
that is without a pushrod connected. To do
so, cycle the transmitter stick back and forth
at a rate that the servo will rotate end to end
constantly. Get a feel for the servo speed
under those conditions, then quickly
disconnect the meter and cycle the servo
again; any noticeable increase in speed is an
indication that you have a poor meter that is
causing enough voltage drop to reduce the
servo speed.
If under this unloaded test “the high
reading that does not recede” occurs, you are
back to that quality business I discussed
before because, like meters, all servos are not
created equal! Or it may be that such a servo
has a dirty pot or damaged gears.
The next step is to connect the pushrods.
First, operate them manually to be sure that
there is no binding or excessive friction in the
rods or hinges. One or the other could easily
be the cause of the short power duration
mentioned because it will put more of a load
on the servo and increase its current draw.
There will be an increase in the reading
with the pushrods connected because the
servos will be working harder and the
increase will differ for each control; the loads
will not be the same throughout. It is
important to watch for high steady readings at
extremes of rotation, which indicate that the
servo is bottoming; that is, whatever it is
connected to has reached the end of its travel
before the servo does. Such occurrences
happen most often on the throttle and nose
gear channels than on any of the others.
ESV Misconceptions: Since we have
touched on the subject of meters, it seems to
be a good time to discuss Expanded Scale
Voltmeters, or ESVs. The term can be
applied correctly only to analog meters;
digitals are something else entirely.
An expanded scale meter is one to which
electronic circuitry has been added, tailoring
it having to display a narrow range of values.
Refer to the sketch of the meter face shown;
the basic meter is intended to read 0.0-15.0
volts. If we use such a meter to read the 9.6
nominal volts of an RC transmitter battery, it
will be difficult to read an exact value and
impossible to discern small differences.
To increase the readability, with the
circuitry mentioned, we can change it to read,
say, from 9.0 to 12.0 volts; a 9.6-volt Ni-Cd
battery will read considerably higher when
first charged. With such a change, there will
be no movement with any voltage less than
9.0 and will peg at the right-hand side with a
voltage greater than 12.0, but it is possible to
read 9.6 and variations with greater accuracy!
A commercially manufactured instrument
will have the proper markings on the meter
face; home-brewers will have to change them
or just remember the new values. That is an
ESV! Period! But in RC, all such instruments
made especially for us place a calculated load
on the battery, which will read different with
and without such a load. Although the load
feature is a good idea, we had an ESV
without it.
What’s the load? A simple resistor that
will draw current from the battery—50% of
the capacity in milliamperes (mA)
recommended. Using Ohm’s law (resistance
in ohms equals the voltage divided by the
current in amperes), we can arrive at the
required resistor value for any given battery.
Take the common 600 milliampere-hours
(mAh) 9.6-volt transmitter battery, which we
wish to load at 300 mA. The voltage (9.6)
divided by .3 amp (300 mA) equals 32 ohms.
The nearest standard values are 30 and 33
ohms; either will do the job.
In purchasing resistors, one has to specify
the wattage of the unit. Another form of
Ohm’s law (power in watts is equal to the
current in amperes multiplied by the voltage)
is used to calculate that. In this case, .3 amp
multiplied by 9.6 volts equals 2.88 watts;
standard values are 3.00 and 5.00 watts;
either one is usable. The latter is actually a
better choice because it will generate heat
while it is in use.
There is no such thing as a digital ESV,
except maybe in the RC marketplace. The
inherent greater accuracy of the digital
instruments does not require any expanding
of the display—it will automatically read the
small variances in voltage—with the
limitations of their basic quality as already
mentioned. Digital voltmeters can and are
provided with loads as explained, and a
common non-RC meter can have such a load
added simply by connecting the proper
resistor across the meter leads while the
battery voltage is being read.
Receiver Crystals: Cyril also had some
receiver questions. He wrote:
“When I order a radio, do they check it
out or do they have lots of radios on all
frequencies? Some distributors mention high
and low bands. If I order a radio and request
high band, what do I get?”
The first part of the question will vary
according to brand. Some seem to work well
across the relatively small spread of the 72
MHz band, and the crystal set specified will
arrive separately. Others will arrive with the
proper label on the box and on the
equipment. We’ve come a long way since we
used to have to pick crystals that matched and
tuned the receiver to its companion
transmitter.
Some makers split the 72 MHz band in
half—a low-band system being one that
operates on the 11 to 35 channel frequencies,
and 36 to 60 designated as their high band.
This does not necessarily mean greater
frequency accuracy; it can be a requirement
of the circuitry and/or crystals being used.
Go fly! The stuff works well in spite of the
misconceptions! MA
Edition: Model Aviation - 2003/03
Page Numbers: 94,95,96
MISCONCEPTIONS: We have many
misconceptions in Radio Control (RC)! Some
are new, since there have been and will
continue to be major changes in equipment
and techniques. Some are old and never seem
to go away, but that is to be expected since
there is a new crop of RCers born every week.
One of these misconceptions came to light in
a letter from a reader in Stilwell, Kansas, who
writes:
“You mentioned ‘servo reversers,’ which
brings me to the point. If I need a Futaba S148
to run opposite to its normal direction (in a
left and right aileron application) can the red
and black wires simply be reversed at the
connector? I have asked several people whom
I thought might know if the wire switch will
work, but I haven’t gotten an answer. I
haven’t tried it for fear of lousing something
up.”
You did well in not trying this oft-given
advice, especially if you are allergic to smoke.
Not very long ago I saw this procedure
suggested on the Internet, and I was once told
by a local flier that a person at one of the
largest hobby shops in my area told him that it
was the easy way to reverse a servo’s rotation.
I would guess that this belief exists
because it is a known fact that you can reverse
the rotation of a common direct current (DC)
motor by reversing the polarity of the drive
voltage. Although it is true that the same will
happen, and has to happen to a servo motor to
reverse the direction of its output, the voltage
is applied by the servo amplifier and not
directly from the servo plug. And the
amplifier, like any other electronic circuitry,
will not be happy with reverse voltage.
It can be a costly mistake because
amplifiers, especially those in the more costly
servos, are costly themselves. Don’t think you
can return them to their maker (importer) and
claim normal failure. Reverse voltage damage
is readily apparent to a trained technician, and
the same type of damage will almost never
otherwise occur.
So how do you reverse servo rotation?
Some you can’t, for purely mechanical
reasons. Please refer to the drawing; the
requirements are simple and only require
changing four connections—two at the motor
and two at the feedback potentiometer (pot),
which is located under the amplifier.
You should have the proper (small)
soldering iron and skills before attempting this
change. The reason some servos cannot be
reversed is that the pot or motor, and in some
cases both, are connected directly to the
printed circuit (PC) board without wire
connections, and it is necessary to cut lands
(the metallic conductive areas on the PC
board to which the components are attached)
and run jumper wires. Most RC companies
will not reverse those servos for you!
There are some exceptions, such as the
Airtronics 94102 servo and others that use the
same motor and are attached directly to the
board. It has three connections: a ground in
addition to the two voltage points. The ground
connection is the center one in a triangle
configuration, and it is a push-on slip ring on
the end cap of the motor.
It is necessary to remove the motor
completely from the board, rotate the slip ring
180°, reinstall it on the board, and resolder the
connections. The power connections are
automatically reversed when this is done, but
don’t forget that you still have to reverse the
pot leads, which in the case of the 94102 are
readily accessible wires.
Many of the available Electronic Speed
Controls (ESCs) have reverse voltage
protection built in, which will prevent any
damage; the unit will not operate the
companion motor. Why not servos, you ask?
It is common for ESC users to wire battery
packs and often all the motor components in
electric-powered models, but it is uncommon
for users to switch the wiring in servo plugs—
except as mislead! I doubt if a servo exists
with the necessary space for any additional
reverse voltage protection circuitry.
(Editor’s note: AMA and Model Aviation
do not advise making electronic modifications
to RC equipment. The author’s comments and
opinions are his alone, and AMA takes no
responsibility for any damage to equipment or
accidents resulting from such modifications.
Also, any modification to a manufacturer’s
product typically voids the warranty.)
Mail: Except for batteries and chargers,
servos are easily the most questioned part of
RC systems. Cyril Bauer of Golden, Missouri,
writes:
“I have a special device with plugs that I
use in series with my multi-meter between the
servo and the radio (receiver) to check the
current demand, and assume that I have a
problem if the current demand stays high.
Sometimes I get a high reading that does not
recede, each servo will react differently. Is
this an accurate check of current demand or
does the meter give a false reading? I had a
problem of short power duration when I
decided to check current usage.”
Let’s look at current-reading
instruments—ammeters or those that read
Eloy Marez
E l e c t r o n i c s
2626 W. Northwood, Santa Ana CA 92704
Anatomy of a servo. On the left is the pot and on the right is the motor. To reverse
rotation, the outer wires on both must be reversed. See text for details.
Face of a common 0- to 15-volt analog
meter, which can be electronically
modified to read 9-12 volts, then is known
as an ESV.
94 MODEL AVIATION
03sig3.QXD 12.20.02 8:19 am Page
submultiples, such as milliammeters or
microammeters (thousands and millionths of
an ampere respectively). We have two types
of instruments: analog and digital. Analog
meters are the older needle type and digital
are the ones that display the value being
measured in numbers. Both have unique
features and provide better data in specific
circumstances.
In the case of a varying value, such as
servo current in Cyril’s case, the analog
instrument is the best by far. Because of
mechanical limitations, the needle is unable
to follow the rapid changes and averages out
the peaks and valleys, resulting in average
but still useful values. Readability is a factor,
on a meter with the average 21⁄2- or 3-inch
scale, you have to interpolate; accuracy is not
its greatest feature.
Digital meters provide us with readings in
at least hundredths and sometimes
thousandths of a unit. They are apparently
more accurate, but note that word
“apparently”! As in most measuring devices,
accuracy is a matter of quality—read “cost.”
Do not expect your $19.95 Wonder-Meter to
provide you with 100%-accurate information;
it won’t. Even the better class of instruments
come in different grades, with different
guaranteed accuracies, and, yes, with
different prices.
For certain applications, such as MilSpec
(Military Specifications), the calibration has
to be checked periodically.
The digital instruments have another
serious drawback; in reading unsteady values,
they will flicker so much that even the
computer- and video-game-whiz teenager in
your house will have trouble reading them.
In the current-reading application, all
multimeters, analog and digital, in addition to
the inaccuracy inherent because of poor
quality add another problem. To read current,
most such instruments insert a resistance in
series in the circuit and actually read the
voltage drop across it as current flows. These
resistances are small fractions of an ohm, but
they are resistances and will actually result in
a higher total resistance within the circuit,
thus a lowering of the actual current.
The manual with professional instruments
will have a discussion about this, described as
“burden voltage,” and will provide a formula
that will compensate for the increased
resistance and provide a true current reading.
There is an easy way to confirm this
meter-imposed resistance. The next time you
are making a current reading, note the value
and switch the meter to the next highest scale.
You will find that the displayed readings are
not exactly the same—the result of different
resistance values being inserted into the
circuit and actually different amounts of
current flowing.
Now for Cyril’s questions. As stated, the
results are not going to be 110% accurate, but
in this case that is not super important. The
system will definitely point to discrepancies
if any exist.
Hopefully using an analog meter,
establish an average for each unloaded servo
that is without a pushrod connected. To do
so, cycle the transmitter stick back and forth
at a rate that the servo will rotate end to end
constantly. Get a feel for the servo speed
under those conditions, then quickly
disconnect the meter and cycle the servo
again; any noticeable increase in speed is an
indication that you have a poor meter that is
causing enough voltage drop to reduce the
servo speed.
If under this unloaded test “the high
reading that does not recede” occurs, you are
back to that quality business I discussed
before because, like meters, all servos are not
created equal! Or it may be that such a servo
has a dirty pot or damaged gears.
The next step is to connect the pushrods.
First, operate them manually to be sure that
there is no binding or excessive friction in the
rods or hinges. One or the other could easily
be the cause of the short power duration
mentioned because it will put more of a load
on the servo and increase its current draw.
There will be an increase in the reading
with the pushrods connected because the
servos will be working harder and the
increase will differ for each control; the loads
will not be the same throughout. It is
important to watch for high steady readings at
extremes of rotation, which indicate that the
servo is bottoming; that is, whatever it is
connected to has reached the end of its travel
before the servo does. Such occurrences
happen most often on the throttle and nose
gear channels than on any of the others.
ESV Misconceptions: Since we have
touched on the subject of meters, it seems to
be a good time to discuss Expanded Scale
Voltmeters, or ESVs. The term can be
applied correctly only to analog meters;
digitals are something else entirely.
An expanded scale meter is one to which
electronic circuitry has been added, tailoring
it having to display a narrow range of values.
Refer to the sketch of the meter face shown;
the basic meter is intended to read 0.0-15.0
volts. If we use such a meter to read the 9.6
nominal volts of an RC transmitter battery, it
will be difficult to read an exact value and
impossible to discern small differences.
To increase the readability, with the
circuitry mentioned, we can change it to read,
say, from 9.0 to 12.0 volts; a 9.6-volt Ni-Cd
battery will read considerably higher when
first charged. With such a change, there will
be no movement with any voltage less than
9.0 and will peg at the right-hand side with a
voltage greater than 12.0, but it is possible to
read 9.6 and variations with greater accuracy!
A commercially manufactured instrument
will have the proper markings on the meter
face; home-brewers will have to change them
or just remember the new values. That is an
ESV! Period! But in RC, all such instruments
made especially for us place a calculated load
on the battery, which will read different with
and without such a load. Although the load
feature is a good idea, we had an ESV
without it.
What’s the load? A simple resistor that
will draw current from the battery—50% of
the capacity in milliamperes (mA)
recommended. Using Ohm’s law (resistance
in ohms equals the voltage divided by the
current in amperes), we can arrive at the
required resistor value for any given battery.
Take the common 600 milliampere-hours
(mAh) 9.6-volt transmitter battery, which we
wish to load at 300 mA. The voltage (9.6)
divided by .3 amp (300 mA) equals 32 ohms.
The nearest standard values are 30 and 33
ohms; either will do the job.
In purchasing resistors, one has to specify
the wattage of the unit. Another form of
Ohm’s law (power in watts is equal to the
current in amperes multiplied by the voltage)
is used to calculate that. In this case, .3 amp
multiplied by 9.6 volts equals 2.88 watts;
standard values are 3.00 and 5.00 watts;
either one is usable. The latter is actually a
better choice because it will generate heat
while it is in use.
There is no such thing as a digital ESV,
except maybe in the RC marketplace. The
inherent greater accuracy of the digital
instruments does not require any expanding
of the display—it will automatically read the
small variances in voltage—with the
limitations of their basic quality as already
mentioned. Digital voltmeters can and are
provided with loads as explained, and a
common non-RC meter can have such a load
added simply by connecting the proper
resistor across the meter leads while the
battery voltage is being read.
Receiver Crystals: Cyril also had some
receiver questions. He wrote:
“When I order a radio, do they check it
out or do they have lots of radios on all
frequencies? Some distributors mention high
and low bands. If I order a radio and request
high band, what do I get?”
The first part of the question will vary
according to brand. Some seem to work well
across the relatively small spread of the 72
MHz band, and the crystal set specified will
arrive separately. Others will arrive with the
proper label on the box and on the
equipment. We’ve come a long way since we
used to have to pick crystals that matched and
tuned the receiver to its companion
transmitter.
Some makers split the 72 MHz band in
half—a low-band system being one that
operates on the 11 to 35 channel frequencies,
and 36 to 60 designated as their high band.
This does not necessarily mean greater
frequency accuracy; it can be a requirement
of the circuitry and/or crystals being used.
Go fly! The stuff works well in spite of the
misconceptions! MA
