AHD RRampMeter Application Notes, Helpful
AHD RRampMeter Intro
As a result of my involvement in “Command
Control” Systems for 16 years, it became apparent that model
railroaders needed a simple to use, accurate tool to measure
volts and amps for their Railroads. The advent of DCC with its
unconventional waveform further amplified the need. I conveyed
the concept and format to Larry Maier, a model railroader, and
electrical engineer who is also a vital contributor to our DCC
development efforts. His resulting design speaks for itself. Jim
Scorse from NCE Corp also made some great suggestions.
The maximum DCC and DC voltage is approximately 23 volts
(covers the complete specified NMRA voltage range). The maximum
DCC and DC current is approximately 9.2 amps. The maximum AC
voltage is about 17 volts while the maximum AC current is about
6.5 amps. AC signals have a higher ratio of the maximum signal
value to the RMS signal value than does the DCC and DC signal.
These values may vary slightly from unit to unit due to
component tolerances. The accuracy is between 2%-3% full scale.
Two status indicator LED,S indicate either DCC or AC voltage, no
indication on LED,S means DC voltage is present. When measuring
DCC and AC voltages and current, any polarity will work. When
measuring DC voltage, proper polarity is necessary. If you
attempt to measure a DC voltage and there is no readout, simply
reverse the connection polarity and you will get the proper
readout. Units with battery or an external DC power supply will
display DC voltage and current irrespective of polarity.
Version IV RRampMeterHP is same as III but designed for large
scale high voltage and current applications with the following
specs: DCC: 38.6 v and 18-20 Amps; AC: 27.6 v and 18-20 Amps;
DC: 38.6 v and 18-20 Amps
Owners of the RRampMeter Version (1) can upgrade to Version
(2) or (3) by purchasing the following parts from Radio Shack.
Version (2) and (3) Upgrade
Item Part# Total Cost
Project Box 270-1802 $2.99
Banana Sockets Sets, 2 274-725 $2.98
Alligator Clips, 4 27-356 $3.98
Test Lead with Banana Plugs 278-704 $3.99
Version (3) Upgrade
9V Battery Snap Connectors 27-324 $1.99
Switch 275-409A $1.99
To install the battery connector (see Fig. 4), solder the red
or the positive lead of the battery connector at the J3 location
to the hole with the square pad. Solder the black wire to the
hole with the round pad. Solder these connections to the back
side of the board so that the battery connector will fit in the
enclosure. To install the switch (see Fig. 4), desolder the
jumper, and solder the switch into the three holes marked S1 on
the top side of the board.
Amperage must be measured in series by connecting the left
set of contacts or clip leads to the input power supply or power
source while the right set of contacts or clip leads are
connected to the load or isolated track section where current is
to be measured (Fig 1). The RRampMeter may be connected in the
reverse direction without damage, but the display will show the
current used by the RRampMeter in addition to the load current
(about 0.03 to 0.04 for no load) (Fig 4). The voltage display
also will not account for any voltage drop in the RRampMeter
Voltage can be measured from the left or right set of
contacts or clip leads. If measuring voltage only, then either
end of the RRampMeter may be used accurately.
We do not recommend soldering directly to the buss bars that
connect the adjustable contacts as this may interfere with the
operation of the adjustable contacts (Fig 2).
For Panel or Fascia mounting, Version (1) can be used. You
will have to cut out holes in your fascia for the LEDS and the
Indicator Lights. Four mounting holes are provided on the
circuit board for mounting. You can also use Version (2) and
mount the enclosure cover to your Fascia if you prefer a
dressier appearance (Fig 3). For mounting templates see, Fig’s 6
The RRampMeter is designed to read true RMS voltage and
current. The RMS values are proportional to the power being
supplied to the layout. An average reading meter (most
inexpensive meters found in electronic stores, hardware stores,
etc.) WILL NOT agree with the RRampMeter. The RRampMeter is
displaying the correct values.
Some DC power supplies use pulsed power for low speed. The
RRampMeter will read this signal at its correct RMS value, but
will display the AC PRESENT light. Once the supply transitions
to full DC, the RRampMeter will continue to display the correct
values, but the DCC PRESENT and AC PRESENT lights will both be
If the current display shows more than 0.00 amps when only
the booster is connected to the left hand input, then the
display value may be adjusted to 0.00 by turning R44 until the
display just transitions from 0.01 to 0.00.
When using a battery, the RRampMeter will not show 0.00 volts
with no signal connected. This is because the open contacts on
the input actually pick up some voltage from the surroundings
(power lines, DCC on the tracks, etc.). In addition, the
circuitry used cannot quite reach 0.00. The RRampMeter is
calibrated to read correctly above several tenths of a volt.
The RRampMeter will measure voltages down to approximately
7.00 volts without using the 9V battery option. For DCC, this is
more than adequate. To measure lower voltages, the battery
option must be used. If the battery is connected, one position
of the switch will turn the RRampMeter on using the battery. The
other position will disconnect the battery and allow the
RRampMeter to be powered from the input voltage. Either position
may be used with the battery connected.
If the RRampMeter is operated at currents in excess of 5 amps
on a continuous basis, then it must be mounted in such a way as
to allow free air circulation for cooling.
The RRampMeter is a great 9V battery tester. Just connect the
battery across the track inputs.
You may be surprised at how much booster voltage is lost in
track feeders, long stretches of track, and control switches.
The RRampMeter is telling the truth.
The RRampMeter will work with common rail systems. If you
want to measure the current in a single track block, connect the
common rail feed to J1-1 (J4-1 or J6) and the remaining side of
the booster to J1-2 (J4-2 or J7). A single output connection may
be run from J2-1 (J5-2 or J9) to the desired block. If you want
to measure the TOTAL current on the common rail feed, connect
the common rail to J1-2 (J4-2 or J7) and the remaining side of
the booster to J1-1 (J4-1 or J6). The common rail is then
connected to J2-1 (J5-2 or J9).
We have tried the RRampMeter in conjunction with the
programming track with mixed results. In some cases, the current
drawn by the RRampMeter to operate itself may be sufficient to
upset the programming sequence. If you want to operate the
RRampMeter with the programming track, it may be necessary to
use the battery option.
If you plan to use the RRampMeter without the case in a
situation where it will be handled, it may be wise to glue Y1
and C1 (located on the back of the board) to the printed wiring
board to prevent an accidental component removal. We find that a
touch of “Crazy Glue” or equivalent is ideal for this purpose.
J4 and J5 are optional and sized for a two terminal header
for use with a connector. The Digikey part number is ED1817-ND.
The mating plug is Digikey part number ED1717-ND. You may also
solder wires directly to these holes for a permanent
If you are installing the RRampMeter as a permanent fascia
display, a piece of red clear plastic or lighting gel in front
of the display will improve the contrast.
Why the RRampMeter
Maintaining and analyzing the electrical system of a layout
requires accurate measurements of the voltage and amps. When dc
was used a standard meter was all that was needed for these
measurements. With DCC use of a standard meter most of the time
will not give you an accurate measurement. Tests have show that
meters not designed to read the DCC wave forms can be off by as
much as ±50%. Even meters that are “true RMS” may not be
designed for the frequency range of DCC. The RRampMeter was
designed to fill the need for a highly accurate DCC meter to
measure of both voltage and amps. The RRampMeter is designed as
a flexible tool to monitor and analyze the electrical operation
of a layout. It is designed to work not only DCC power but to
make accurate measurements of ac and dc. The RRampMeter has an
amazing 2% accuracy. Because the original 10 amp range of the
RRampMeter was not adequate for large scales a 20 amp version
was added to the line.
A total of four models of the RRampMeter are available. There
are three models are available in the 10 amp range and one for
large scale with a 20 amp range. The standard meter is rated at
up to near 10 amps and up to 23 volts DCC or dc and 6 amp at up
to 16 volts on ac. The new Version VI RRampMeter for large scale
trains have a capacity of up to about 20 amps. The three basic
models are [A] a bare module design for panel mounting, [B]
mounted in a plastic case and [C] mounted in a plastic case with
the option of battery power. All of the meters are powered by
the input voltage. The voltage must be greater than 7 volts to
operate. Versions III and VI come with a backup 9 volt battery
to operate the meter when the input voltages of less than 7
volts. The meter can be used either as a portable meter or
mounted permanently in a panel. Screw terminals are supplied
with the meter that can be soldered to the back side of the
meter’s circuit board.
RRampMeter Circuit Modules
Version I - Bare Module RRampMeter Module; 7 to 23 volts 10 Amp
Version II - RRampMeter with enclosure and clip leads; 7 to 23
volts 10 Amp (DCC)
Version III - RRampMeter with enclosure, clip leads and 9 volt
battery backup; 0 to 23 volts 10 Amp (DCC)
Version IV - RRampMeter with enclosure, clip leads and 9 volt
battery backup; 0 to 23 volts 20 Amp (DCC)
A meter mounted near the system or booster will let you
monitoring the power supplied to the layout. This will let you
can determine how well your system or booster is regulating
voltage under load. You can also measure just how close you are
to the maximum power limit of the booster or system. This will
indicate the operation of the system/booster, but not the
voltage drop of the wiring and rails of the layout.
Voltage is read by connecting to the two terminals on the left
side of the meter. The end of the circuit board has an area that
allows you to put the meter directly on the rails to measure the
voltage. In order to measure amps, the current must flow through
the meter by connecting a load to the two terminals on the right
side of the meter.
Most common meters can read both ac and dc, but can not
accurately read DCC power. In order to accurately read DCC power
a “true RMS” meter, like the RRampMeter is needed. This is due
to the shape and frequency of the DCC signal. Even many “true
RMS” are not designed for the high frequency of the DCC wave
form. The RRampMeter automatically detects and switches to the
type of power it is measuring.
Two LEDs indicate DCC or ac, no LED on indicates dc.
Layout Voltage Loss
When the rail voltage to a decoder drops the train speed can
also drop along with lights dimming. There are many places in
the path from the booster to the decoder where voltage can be
lost. The voltage from the booster or system may have a small
drop as more current is drawn. The wiring from the booster to
the rail will also lose some voltage. Devices like circuit
breakers and block detector can add to the voltage loss. Nickel
Silver rail is not as good a conductor electricity as copper
wire and can be a significant part of the voltage loss. Rail
joiners can also cause a loss in voltage. To determine the
layout voltage loss the voltage must be measured at the rails
when current is flowing. Without a current flow there is little
to no voltage loss. It is almost impossible to get a good stable
voltage reading using a train running as a current load. The
best way to measure the loss is with some type of steady load.
An automotive lamp turns out to be a good device to use as a
steady load. They are cheap and easily available. A couple of
pieces of wire with clips can be soldered the lamp. (See photo)
Depending on your scale and booster rating one of the following
automotive lamps should work. The #912 draws about 1 amp the
#1141 about 1.5 amps and the #1156 about 2.25 amps. (Due to the
low cold resistance of a lamp, the 1156 lamp can cause low
powered systems like the Zephyr to shut down [overload]. The 912
should be OK for this test.) Choose a lamp that is near the
maximum current used in a block, not the current used by the
The first test should be to determine the voltage loss of the
system or booster. [A] Measure the output voltage of the booster
at a point close to the booster with no trains running. If you
have an RRampMeter connected as a panel meter close to the
booster this reading should work. [B] Next connect the load to
the rails load (lamp) to the rails with the meter still next to
the booster. The difference between the two readings will give
you the voltage loss of the booster at this current. [C] Read
the voltage at the rails with out a load. [D] Read the voltage
at the rails with the load. The lamp can be connected the
terminals of the RRampMeter so a number of reading can be made
in the same block. You may be surprised at the voltage loss at
different points of the same block. This can be due to the poor
conductivity of Nickel Silver rail. Poor connections of rail
joiners is another thing to look for. Wire that is under size is
also a cause of voltage loss. When making measurements of loss
across things like rail joiners and connections the voltage is
so low that the RRampMeter with the battery option make be
needed. It is best to keep the voltage loss due to wiring and
rails under 1 volt. More than a couple of volts can cause
slowing of locomotives and in extreme cases even cause the
decoder to drop out. There is a wire chart that shows the length
of wire for a ½ volt drop due to wire resistance. The chart
shows the voltage drop for 1, 2, 5 and 10 Amps. This is a chart
for one way resistance. If you wire out to the rails and back
(double the length ) this chart becomes a 1 volt chart.
Which Size Wire?
The 20 to 18 gauge wire should be used only for Z and N scales.
This size can be used for short track feeders in larger scales.
The 16 gauge works for most small layouts with short runs. The
14 to 12 gauge for larger layouts in most scales. The 8 to 10
should be reserved for older O scale and G scale layouts. This
larger size wire becomes a bit cumbersome to work with.
Stranded wire can be used anywhere, but solid should only be
used where it will not be flexed or moved.
Voltage loss for ½ volt for different currents and wire
If your layout uses common rail wiring and you have more than
one booster you can monitor the current from both boosters. Run
the common of both booster through the meter and this will get
you an indication of total layout current. NOTE this will only
work with common rail wiring.
Used with permission of AHD. Copyright ©
2004-2005 by AHD. All Rights Reserved.