Question: Do digital 2-wire meters still function when their inputs go to zero?
No. All digital instruments require a minimal amount of power to drive their internal electronics. However,
the input level at which 2-wire instruments stop functioning is normally well below the level at which the system being
monitored has stopped functioning.
Question: What happens when the input goes below the minimum specified level?
The answer to this question is model dependent. LCD display models will normally
continue to operate well below their specified minimum input levels, but the display’s contrast will gradually diminish,
and more importantly, the readout accuracy is no longer guaranteed.
For LED display models, the intensity of the display will diminish, but the readout will remain fairly accurate down to the level at which the display becomes so dim that it
becomes unreadable. In applications in which the display is required to be totally off at times, be sure to drive the meter
with components which bring the input signal all the way to zero.
Question: Why aren’t absolute maximum input ratings specified?
Absolute maximum input levels are specified on our data sheets as the “Input Voltage
Range” or simply “Input Range”. This rating is the meter’s maximum continuous operating level at the highest-rated
operating temperature. In practical applications, momentarily exceeding the input range by 10% will not harm any of our
2-wire meters.
Question: What happens when a 2-wire meter’s input signal polarity is reversed?
Absolutely nothing – the display will not operate. Except for ac-mains powered devices
whose inputs are not polarity sensitive, all DATEL 2-wire meters are fully protected against reversed-polarity inputs
(i.e., “input signals that are hooked up backwards”).
When digital panel meters first appeared on the market the convention of using a
full-scale display range of “000” to “1999” for 3½ digit meters and “0000” to “19999” for 4½ digit meters became an
industry standard. And, for reasons still unknown to this writer, the leading left-most “1” (also referred to as the most
significant digit, or MSD) became known as the infamous “half-digit”. This half-digit represents the one-thousands digit
on a 3½ digit meter and the ten-thousands digit on a 4½ digit meter.
Question: How is the half-digit displayed?
The half-digit, or left-most ‘1’, is always OFF (not visible) with display readings
between “000” and “999” on 3½ digit meters and between “0000” and “9999” on 4½ digit meters. The half-digit is turned ON
with display readings greater than or equal to “1000” on 3½ digit meters and with display readings greater than or equal
to “10000” on 4½ digit meters. An enabled half-digit in combination with all other digits OFF is used to indicate an input
overrange condition which occurs when the applied input exceeds the meter’s measurement capability.
Question: What is a “count,” and how is it related to resolution?
First, let’s define the terms ‘display resolution’ and ‘counts’. A 3½ digit meter can
resolve an input signal into 1,999 parts, while a 4 ½ digit meter can resolve an input signal into 19,999 parts. A change
of 1 count is defined as the right-hand most digit going up or down by one (±1). For example, a display which is continuously
alternating between “1899” , “1900” , and “1901” is said to be indicating “1900” ±1 count (or ±1 digit).
A 3½ digit meter with a full-scale input range of ±2VDC resolves a 1.999V input
into 1999 counts, or 1999mV, with one count representing 1mV (0.001V resolution). If the first decimal point (DP1) is
enabled, 1999mV can be displayed as “1.999”. If the third decimal point is enabled (DP3), the same 1.999V input can also
represent “199.9”. The 0.001V resolution of the meter hasn’t changed, but the decimal point has made the 1999mV look like “199.9”.
The decimal points do not change the meter’s basic resolution or sensitivity, they are simply place holders used to suit
the user’s engineering scaling requirement. The “199.9” reading in the preceding paragraph could represent 199.9 pounds
(or, for the rest of the world, 199.9 Kilograms), 199.9 °C, etc.
Question: How is input polarity displayed?
DMS Series meters indicate negative input signals by turning ON a minus sign
display-segment (“-“) located to immediate left of the half- digit. Positive inputs are indicated by turning OFF the minus
sign segment. For example, “-1234” is displayed for negative inputs, and “1234” for positive inputs. The minus sign is ON
when IN HI (pin 11) is slightly more negative than IN LO (pin 12). Operation of the minus sign is automatic and can not be
externally disabled by the user.
Question: How do the decimal points function?
This is the second most frequently asked display-related question. The meter’s
decimal points are totally user selectable, that is, the meter’s analog-to-digital converter (ADC) has no control over
which decimal point is ON or which decimal point is OFF. This feature is due to the fact that most fixed-input range
digital panel meters are not intended to directly readout in volts, but instead are intended to display some other
physical parameter such as pounds, degrees, amperes, etc.
One of the few exceptions to the above are DATEL’s self-powered 2-wire meters.
These meters are designed to measure only one electrical parameter (AC volts, DC volts, etc.) and, depending on the
display resolution, a decimal point is pre-wired ON and cannot be disabled by the user.
Both the individual product data sheets and DMS Application Note 13 contain detailed information on how to enable the
meter’s decimal points.
Question: I don’t want any decimal points enabled, what do I do?
The answer is simple: absolutely nothing!-leave all the decimal-point control pins open
(i.e., not connected to anything). If not specifically enabled by the user, all decimal points on DMS Series panel meters
are OFF. Decimal points are ON only when the desired decimal-point input pin is externally connected by the user to the
meter’s power return (pin 3). However, unless otherwise indicated on the meter’s individual data sheet, unused decimal
point inputs can be connected to the meter’s positive power supply input (pin 1). See the applicable product data sheet
and DMS Application Note 13 (“Decimal Point Drivers”) for more information.
Question: Are low-power red LED meters dimmer than standard-intensity meters?
The answer is an emphatic no! DATEL’s low-power red LED meters, identified with an
‘-RL’ part number suffix, are actually brighter than their standard-intensity ,’-RS’ suffix counterparts! Red low-power
LED meters typically consume 10-15 times less power than ‘-RS’ meters. However, this dramatically lower power consumption
is achieved with no sacrifice in display intensity or electrical performance.
DATEL’s low-power red LED meters cost approximately 10% more than standard-intensity
meters. However, in many applications this extra cost is easily offset by using lower-cost power supplies. Low power LED
meters can be used in many applications where LCD display meters were once the only option-including battery powered
instrumentation. DMS Application Note 1, titled “The LED versus LCD Decision,” provides more information on the pros and
cons of LED and LCD display technologies.
Question: Do I need a 3½ or a 4½ digit meter?
In the course of answering literally thousands of application calls, we encounter
many customers who specify 4½ digit meters for applications in which a lower-cost 3½ digit meter would be adequate. For
example, monitoring the output of a 30VDC supply with a 200V full-scale input range 4½ digit meter will result in a
display reading of “30.00” with a resolution of 0.01V (1/100 of a volt). A 200V full-scale input range 3½ digit meter will
display the same 30V input as “30.1” (1/10 of Volt resolution).
Most voltage and/or current monitoring applications do not benefit greatly from
the extra resolution 4½ digit meters offer, in fact, in many applications, the extra digit may result in unwanted,
difficult-to-view, display bouncing of the last digit. In general, use a 4½ digit meter only when values over 1999 must be
displayed in their entirety for clarity or calibration purposes. If you are not sure which meter to use in your application,
don’t hesitate to contact DATEL-we are more than happy to discuss your requirements in greater detail! (800)230-6008(USA).
Question: Why does my process monitor’s display reading stay fixed at “000” as the 4-20mA signal is varied?
The most common cause for a process monitor not responding to changes in the loop
current occurrs when a single-ended transmitter is driving two single-ended input process monitors. The second process
monitor’s grounded negative (-) input effectively shunts the transmitter’s output current around the first monitor, that
is, no loop current passes through the DMS-30PC-4/20s-24RL’s input circuit. The solution is to either power the second
process monitor from an isolated DC power supply or use one of DATEL’s “-I” isolated-power process monitors.
Question: How do I hook up a +24V supply to a loop-powered DMS-20LCD-4/20S?
You don’t! The two input terminals on DMS-20LCD/4/20S can only be connected to the
modulated 4-20mA transmitter output current. The system power supply’s two “+” and “-” output leads should never be
connected directly to a loop-powered device’s “+” and “-” input terminals; doing so will place the full supply voltage
(sometimes in excess of 36Vdc!) across the monitor’s input resistors. These input resistors typically measure less than 30
Ohms, and can easily be damaged by the resulting excessive power dissipation.
Question: What do the ‘S’, ‘B’, ‘P’ and ‘I’ 4/20 suffixes in DATEL’s 4-20mA process monitor part numbers stand for?
The ‘S’, ‘B’, ‘P’, and ‘I’ suffixes in the 4/20 part number field are used to denote
the monitor’s display-readings capabilities. For example, the DMS-20PC-4/20S standard unipolar-reading model’s gain and
offset adjustments are optimized to display “000” with a 4mA input, and up to “1999” with a 20mA input. The DMS-20PC-4/2B
bipolar reading model is designed to display a negative number with 4mA, “000” with 12mA, and a positive number with 20mA.
The “4/20P” positive-reading model is optimized to display positive numbers with all inputs between 4 and 20mA.
The inverse-reading ‘4/20I’ model is designed to display inverse readings, that is,
a 4mA input will display a full-scale positive number, while a 20mA input will display “000”. The ‘4/20I’ model uses the
same DIP-switch settings tables as its ‘4/20S’ counterpart, the only difference is its full-scale and zero display-readings
are reversed. For more information see product data sheet.
Question: How does ambient light affect the selection of a process monitor?
Process monitors which must be readable in direct sunlight should use LCD displays,
while indoor applications can usually take advantage of an LED display’s superior legibility. Outdoor applications that
must also be viewable at night, or in dimly lit areas, should use locally-powered process monitors with backlit LCD
displays.
At the present time, high-intensity LED’s can not be illuminated adequately with
currents much below 60mA. Therefore, if display legibility is an important requirement, locally-powered LED display process
monitors are the better choice for indoor applications since their power consumption is not limited to the 4-20mA currents
circulating in the loop.
DC Ammeters FAQ’s
For more information please read the Product Data Sheets:
Simply put, a shunt is a resistor. In ammeter applications, a low-value resistor safely bypasses (or “shunts”) current flow around a precision voltmeter. The voltmeter measures the voltage drop—typically in the millivolt range—across the shunt resistor and displays a number corresponding to the current flowing through the shunt.
Question: What’s the block labeled “LOAD” on your DCA-20PC series data sheet’s connection diagrams?
All DCA-20PC DC ammeters are designed to measure the current flowing through an external ‘load’. This load could be something as simple as a DC motor, a light bulb, a heater, or it could be a complex piece of test equipment. The current drawn by the external load (i.e., the current the ammeter will measure and display) enters TB1-1, develops a small millivolt signal across an internal shunt, and then flows out of TB1-2 to the load.
Question: What do TB1’s ‘-SHUNT’ and ‘+SHUNT’ designations mean?
TB1-1 is connected to the negative side of DCA-20PC’s internal shunt (-SHUNT), while TB1-2 is connected to the negative side of the same internal shunt (+SHUNT). If the load current enters –SHUNT and leaves through +SHUNT, as indicated by the arrow labeled “LOAD CURENT,” the meter’s display will indicate positive current flow.
Question: What’s the function of the block labeled “DC SUPPLY”?
The primary purpose of the DC SUPPLY is to provide operating power for the DCA-20PC’s internal electronics. However, in many applications, this power supply is also used to power the external load.
Question: What’s the main difference between ‘–DC1’ and ‘-DC4’ models?
First, and foremost: ‘–DC1’ models do not provide electrical isolation between the DC SUPPLY that powers the ammeter and the power supply that powers the load circuit. This important point must be considered when using ‘–DC1’ models in dual-supply applications similar the one shown in the DCA-20PC data sheet’s Figure 2B diagram.
‘-DC4’ models include an internal DC/DC converter which provides a minimum of 500V of isolation between the DC SUPPLY and the load power supply in dual-supply applications. ‘–DC4’ models are also recommended when both positive and negative load currents must be displayed. Lastly, ‘-DC4’ models must be used in single-supply applications which have the shunt connected to the high-side (+) terminal of the DC SUPPLY.
Question: I have a DCA-20PC-X-DC1-RL ammeter which I believe is properly connected to a +12VDC power supply, but the LED display is totally off (dark). What’s the problem?
Check to see if jumper J3, located on the rear of the ammeter, is correctly installed for ‘-DC1’ models (i.e., it shorts out the two small pins). ‘-DC1’ models are shipped with J3 installed, which configures the ammeter to operate from +5 to 40VDC power supplies. When J3 is removed, the ammeter will only operate from +36 to +75VDC power supplies.
Question: Do all of the above questions also apply to your DCA5-20PC series ammeters?
Yes, especially the question pertaining to ‘–DC1’ and ‘–DC4’ models. For application purposes, the primary difference between these two ammeter families is the location of the shunt resistor: DCA-20PC ammeters have an internal low-loss shunt connected across TB1, while DCA5-20PC ammeters work in conjunction with user-supplied, external 50mV or 100mV shunts connected to TB1.
Question: I want to replace an old, 50A analog dc-ammeter with your digital DCA5-20PC-1-DC1-RL. How do I hook it up?
First, please read the DCA5-20PC data sheet in its entirety! If the analog ammeter contains an internal shunt, you’ll need to purchase a 50A/50mV external shunt (our part number 3020-01096-0). Disconnect the analog meter’s two large wires and reconnect them to the 50A/50mV shunt’s large threaded studs. Connect the shunt’s two side-mounted small screws to the DCA5-20PC’s TB1 ‘–IN’ and ‘+IN’ terminals. You’ll also need to connect an appropriate power source to TB2.
If the analog ammeter uses an external shunt, it’s typically marked with an amperage rating and output level (for example, 50A/50mV). Leave the larger wires in place and reconnect the shunt’s two small side-mounted screws to the DCA5-20PC’s TB1 ‘–IN’ and ‘+IN’ terminals, and apply power to TB2.
Question: I need to monitor the load current flowing in a 15A circuit. I plan on using your ACA-20PC-2-AC1-RL AC ammeter; do I need to buy anything else?
No. ACA-20PC-2-AC1-RL AC ammeters feature a built-in current transformer (CT) and auxiliary terminal blocks for connecting and measuring AC loads up to 20A.
Question: Can I use the same AC power source that’s powering my load to also supply operating power to your AC ammeters’ TB1 A and B terminals?
Yes, but only if the AC source complies with the ammeters’ input ratings printed on its label.
Question: Are your ACA5-20PC series ammeters supplied with external current transformers (CT)?
No, the external, 5A-output CTs are purchased separately. The ACA5-20PC data sheet’s ordering information section includes a full listing of available matching CTs.
Question: Can I use a combination of one ACA5-20PC ammeter, a 3-position selector switch, and three 5A CTs to monitor current in a 3-phase AC circuit?
Yes, but the selector switch must be a special type referred to as a “make-before-break” switch specifically designed for this type of application. A CT’s secondary output-winding can generate extremely high—and potentially lethal—voltages when the current flowing in its output leads is suddenly interrupted by disconnecting the CT’s output from the ACA5-20PC. Electro Switch Corporation’s www.electroswitch.com part number ‘2409C’ control switch is specifically recommended for this type of application.
Question: Can I remove the built-in CTs on your AC ammeters and then reconnect them remotely?
Our AC ammeters retain their UL-recognized component status when used in their factory-supplied configuration. Any modifications made to these ammeters voids their UL certification, including remotely mounting the built-in CTs, removing the terminal blocks, or any modification that involves disassembly of the ammeter or its components.
Question: I need to measure current on a 120 V / 400Hz AC Supply. Which one of your AC ammeters do you recommend?
All of our AC ammeters can be used to measure 120V/400Hz current up to their maximum full-scale rating. This is the current that passes through their built-in CT’s primary circuit (or the current passing through an external CT connected to our ammeters). However, the ac voltage that is used to supply operating power to the ammeter must be in the 50-60Hz range as specified in their product data sheet, under the ‘Power Supply Voltage’ section. Our AC ammeters can measure 400Hz currents, but they cannot be powered from 400Hz sources.
