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CHECKING YOUR
PLANE'S AMMETER
BY WILLIAM RYNONE, PH.D., P.E.
Ever wonder if your plane's ammeter was "doing its job?" I recently
noticed that the meter needle in my
plane was hardly moving. Suspecting
that there was a problem, I mailed the
unit to a repair shop. Unfortunately,
the repair cost was $195. This "rude
awakening" prompted me to investigate what could be done by the average
plane owner to verify the operation of
his ammeter. It's worth noting that ammeters are used in two modes:
Sometimes they are used to monitor
the current flow from the generator (or
alternator). In this instance, chances
are that the meter will have the "zero
mark" on the extreme left hand side of
the meter face. For a battery monitor,
it would likely have the zero hashmark in the center so that it may show
when the battery is being charged or
discharged.
are more commonly rated by maximum current handling capabilities and
the corresponding voltage developed
Before discussing meter movement across them. Jerry Bishop of Sigmatesting options, it is appropriate to ex- Tek, a large manufacturer of aircraft
plain the mechanics of a plane instrumentation, stated that the most
ammeter. The current to be monitored, common (perhaps 95%) ammeter
whether battery or generator, is re- shunt used in light aircraft is a 60 amquired to flow through a resistor of pere, 50 milivolt assembly. At 60
very low, accurately known, resistance amperes, the power dissipated is three
value. When the current flows through watts, quite hot to the touch. The test
the resistance, called an ammeter set discussed in this article applies the
shunt, or simply a shunt, the current same voltages to the meter movement
generates a low voltage value across as would be developed when carrying
the shunt. The ammeter meter move- full, 2/3 or 1/3 maximum current in
ment is connected across the shunt the plane. Some 40 ampere units were
(see Figure 1).
used in Cessna 150s. A few shunts are
rated at 100 milivolts at full current
and a few are rated at 33-1/3 milivolts
AMMETER SHUNT
at full current.
BASIC AMMETER
OPERATION
A SIMPLE TEST CIRCUIT
A QUICK CHECK
An easy method of validating ammeter calibration can be accomplished
by correlating the predicted needle position versus its observed indication
caused by a known load. The load must
be capable of drawing an appreciable
fraction of the meter's maximum rating.
The only single load that I can think of
that meets this requirement is the landing light. For my plane, the General
Electric #4522 landing light is rated at
250 watts. Dividing this value by 12.5
volts yields a current of 20 amperes.
Therefore, with the engine not running
and only the landing light energized,
the ammeter should deflect one-third of
the way, one hash-mark (3 hash marks
between 0 and 60), for my plane.
92 JUNE 1999
FIGURE 1
The term "ammeter" is a generic
one. It is sometimes applied to just the
meter movement or sometimes the meter movement with its corresponding
shunt, connected as a pair.
The reason why a low resistance is
used for the shunt is to minimize the
amount of power that is dissipated in
the shunt. This power is wasted, accomplishing no other useful function
than enabling the meter movement to
display system current. Shunts could
be rated by their resistance values, but
Two electrical definitions that are
useful for the testing of analog meters
(meters with a moving needle) are current and voltage sensitivity. Current
sensitivity (Ics) is the required amount
of current flowing through the meter to
cause the needle to completely deflect.
Correspondingly for the voltage sensitivity (V sen ). For aircraft ammeter
meter movements, this value is predominately 50 milivolts (50 mv = 0.05 volt).
For other aircraft meter movements,
e.g. temperature gauges, etc., this value
may typically be 100 mv.
In the instance where the movement
current sensitivity is known or can be
calculated, a constant current source
could be used to check the meter. Let's
define a "constant current source." A
constant current source is a source of
current (battery, power supply, etc.)
whose current output is independent of
the load, in this instance the load would
be ammeter meter movements of varying resistances. If the meter movement
to be investigated happens to be a
plane ammeter, an assumption of a 50
mv voltage sensitivity may be appropriate. With the aid of a digital
Volt-Ohm-Miliammeter (VOM), the
meter resistance may be measured and
the current sensitivity may then be calculated: I c s = V s e n /R m . Professor
Ralph Santoro suggested that a simple,
low-cost, constant-current source may
then be constructed with a nine volt
transistor radio battery (new) and a series connected resistor. The resistor
value may be calculated as follows:
First calculate the movement current
sensitivity, e.g. for a 50 milivolt, 1,000
ohms meter movement:
Ics = v sen/Rm=0-05/1000=50 microamps. Then
Rseries = ^source^cs =
6
9/50 x 10' = 180,000 ohms. To check
the half-scale reading, double the resistor value, i.e. RSeries = 2 x 180,000
= 360,000 ohms.
The circuit would be as shown in
Figure 2.
Rseries
METER
MOVEMENT
9 VOLTS
BATTERY
(NINEJ/LT.KCF)
FIGURE 2
A METER MOVEMENT
TEST SET
If you have an experimenter's
bent, and enjoy building gadgets,
you may wish to accurately check
your plane's ammeter (or other meter
movements) by building this "home
brewed," $25 calibration test-set. Although the following meter
movement test set was intended to
test plane ammeters, it may be used
w i t h any low-power, basic meter
movement. These would i n c l u d e
voltmeter, fuel gauge, oil pressure
(electric), engine temperature, etc.
gauges. The test set simulates a constant voltage source and is
particularly useful if the meter movements being checked are designed
with standard voltage sensitivities. I
designed the unit "after the fact" but
intend to use it to check my ammeter
when it has been returned.
TEST SET
OPERATIONAL DETAILS
The test set consists of electrical
and electronic components that use
120 volts AC utility power. The meter movement to be tested is
connected to the test set via flea
clips mounted on the ends of insulated test leads with banana plugs on
the other ends. The test set power
supply converts the utility voltage to
a low DC voltage. This DC voltage
is then applied to a three position rotary switch e n a b l i n g the user to
select full scale, 2/3rds or l/3rd scale
output voltage. This output is then
applied to a buffer consisting of an
operational a m p l i f i e r (OP-AMP)
and then to the meter movement.
Without the buffer, variations in meter resistance would cause
corresponding variations in the voltage applied to the meter under test.
expand the hole and make the hole
rectangular in shape. The electronic
parts are mounted on a perf board (a
bakelite board of 1/16" thickness that
has perforated holes located in a
square pattern).
TEST SET USE
If the meter movement to be tested
is connected to system wiring, mark
and disconnect the system wires.
Optimum test conditions exist when
the meter movement is bench tested.
Connect the meter movement to the
Meter Test Set
TEST SET
CONSTRUCTION
The components are housed in an
aluminum or plastic cabinet, readily
available in most electronic parts retail stores. Holes should be drilled as
specified in the accompanying drawing. Where the 120 volts AC
connector is mounted, a hole is
drilled and a nibbling tool is used to
CONSTANT CURRENT TFST FIX!l *F
FOR METER MOVEMENTS
Simple Test Circuit - 9 volt transistor radio
battery shown on the left.
FIGURE 3
5.0000
1/3
9/0
FULL
6
METER MOVEMENT TEST SET
FRONT PANEL
(MM_FRPNLKCF)
UP SCALE
(THREE POSITION ROTARY SWITCH)
1-2500
1.2500
DOWN SALE
(DPDT TOGGLE SWITCH)
o
CO
—
ON
OUTPUT _|_
VOLTAGE
'
1.7500
OFF
a
h»
(LIGHTED POWER SWITCH) °,,
(BLACK
BANANA JACK)
(RED
BANANA JACK)
SPORT AVIATION 93
5.0000
METER MOVEMENT TEST SET
BACK PANEL
(MM_BKPNLKCF)
O
o
O
q
CO
(IEC POWER
PLUG)
~
Y
\
FIGURE 4
test set with the test leads and flea
clips. Set the selector switch to the
l/3rd full scale position. Should an
incorrect analysis of the meter sensitivity have occurred, the l/3rd scale
setting will apply the least voltage to
the movement, minimizing the possib i l i t y of damage. If a d i g i t a l
volt-ohm-miliammeter (VOM) is
available, it would be desirable to
measure the meter movement resistance. Why a digital VOM, you ask?
These units are usually very accurate
and also apply little power to the circuit
whose resistance is being measured.
The meter movement resistance
multiplied by the current required to
drive the needle to full-scale deflection (meter current s e n s i t i v i t y )
yields the value of the applied voltage to move the needle to full scale
deflection (voltage sensitivity).
Knowing this value will enable the
user to refer to Table 1 and thus
change the resistors Rl, R2 and R3
(to accommodate various movements) that are "plugged-in" the
perf board. Table 1 enables the user
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94 JUNE 1999
METER MOVEMENT TEST FIXTURE (MMTF-PS. KCF)
F1
0.5 AMPS
PAGE 1 - POWER SUPPLY
T1
120 VAC: 12 VAC c.t. D1
+ 5 VOLTS
REGULATOR
REC1
RYNONE ENGINEERING INC.
P.O. BOX 4445
ANNAPOLIS, MD21403
P/F: 410-263-0794
O
FI6URE5
to insert resistors that correlate the
movement voltage sensitivity with
developed test set voltages. Occasionally, current sensitivity and/or
meter resistance values are printed
on the meter face or case.
CONSTRUCTION OPTIONS
The following may be implemented
to reduce the construction cost:
•Omit the 1EC bulkhead power
plug. Result — The line cord will be
permanently attached to the test set.
• Omit the fuse holder and use a
fuse with pig tails. Result — Fuse replacement will require soldering.
•Replace the rotary switch with a
single pole, double throw, center-off
(SPOT) toggle switch. Result —
Switch settings will be an unconventional l/3rd scale, full scale, 2/3rd
scale sequence.
• E l i m i n a t e the reversing single
pole, double throw (SPOT) toggle
switch. Result — To check both upscale and downscale meter calibration,
the test lead connections to the meter
movement must be reversed by the
user.
•Eliminate the banana jacks and
plugs. Result — The test leads will be
permanently connected to the test set.
For information, use SPORT AVIATION'S Reader Service Cord
SPORT AVIATION 95
METER MOVEMENT TEST FIXTURE
AMPLIFIER CIRCUIT
UPSCALE
RYNONE ENGINEERING INC.
P.O. BOX 4445
ANNAPOLIS, MD21403
P/F: 410-263-0794
METER MOVEMENT
SW2
SP3T
SWITCH
NOTE:
SWITCH
POSITION "0"
<
IS OPTIONAL
REQUIRES
SP4T SWITCH
(MMTF-AMP.KCF
FI6URE 6
Each of these options will result in
a cost saving of approximately $2-3.
ACKNOWLEDGMENTS
Over 2000 RVs have been completed and
flown in 22 countries... probablv more
than any other homebuilt aircraft design
in the world.
Why?
You'll understand when vou fly one!
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P.O. Box 160 North Plains, Oregon USA 97133 503-647-5117
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www.vansaircraft.com
For informotion, use SPORT AVIATION'S Reader Service Card
96 JUNE 1999
My thanks to Professors (Ret.)
Ralph Santoro and Stephen Burns,
U.S. Naval Academy and Don A.
Jones, Engineer, Northrop Grumman
for their suggestions for improvement
of this article. Jerry Bishop, Tech Specialist of Sigma-Tek and Bill Wheat,
Engineer of Mooney Aircraft, kindly
offered information concerning industry standards for analog meters. Dick
Wilkinson provided labels, D. A. Lurz
took photos and Jim Pryne of ARINC
did test set construction.
ABOUT THE AUTHOR
W i l l i a m Rynone has owned a
Mooney for many years and done most
of the maintenance on the plane. He
has an A&P, ATP and Professional Engineers licenses and has worked as a
project engineer and taught electrical
engineering. Bill has published a 600
page Electrical Engineering textbook
and 35 engineering and aviation articles. He can be contacted at P. O. Box
445, Annapolis, MD 21403, phone/Fax
410/263-0794.
METER MOVEMENT TEST SET
PARTS LIST
Power Supply
• Cabinet (CAB1) (RS #270-1807)
• Pcrf Board (l'Bl)(RS #276147 A)
• IF.C power receptacle (RliCl) (RS
//27S-1251
• Fuse holder (Fl)
• 0.? Ampere fuse
• SPST power switch (SW1) (RS
TABLE 1
Full Scale
2/3 Scale
R1 (ohms)
R2 (ohms)
33
62
1/3 Scale
Shunt Voltage
R3 (ohms)
33.3 mv
16
50 mv
47
91
24
100 mv
100
180
47
Note: Resistors are nearest 5% (or better) carbon composition
value. Resisters R1 ,R2 and R3 are not soldered into the pert
board, but rather plugged into a integrated circuit (I.C.) socket.
•275-692)
• 120 v.a.c. to 12 v.a.c. (center
tapped) (o> 50 m.a. transformer (Tl)
(RS#273-1365A)
• Full wave bridge (FWB) or
Diodes (4) 100 p.i.v. e.g. 1N4001
(DI-D4(RS#276-1H)2)
• (Capacitors (2) 470 mierofarads at
10 volts (Cl, C2) (RS #272-1018)
• Capacitors (2) 0.1 microfarads
disk ceramic ((M, C4)
• 15 volts regulator 78L05 (REG1)
• -5 volts regulator (79L05 (RF.G2)
Amplifier
• Resistors (all 1/8 or 1/4 watt, five
percent or better)
-4.7K(R4)
-33, 47 and 100(R1)
-62,91 and!80(R2)
-16,24and47(R3)
- 10K(R5)
• Operational amplifier, low offset
voltage (OA1) (RS #276-1715)
• Operational amplifier socket
(SOCM)(RS #276-1988)
• SP3T, break-before-make rotary
switch (SW2) (RS //275-1386A)
• SPDT, immature toggle switch
(SW3) (RS #275-635)
• One red and one black banana
jack(Jl,J2)(RS«274-725B)
• One red and one black banana
plug(Pl,P2)
•Flea clips (2) (CL1.CL2)
• Socket to mount resistors (SOC2)
Note: With the exception of the
operational amplifier, none of the
parts are critical. The op-amp
should have an off-set voltage that
is superior to a 741. The author purchased many of the above parts
from the local Radio Shack store.
Where the wrappers were not discarded prior to writing this article,
the Radio Shack part number is indicated (e.g. RS #123-456).
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For information, use SPORT AVIATION'S Reader Service Cord
SPORT AVIATION 97