DC Digital Ammeter Takes Accuracy To New Heights
Using a digital ammeter for accuracy: DC digital ammeters are rated for 99.99% full scale accuracy. A digital ammeter is an instrument that measures current flow in units such as amperes, milli-amperes (mA) or micro-amperes (A). It displays current levels on a digital display unlike analog meters, which use a moving iron or a moving coil to move a pointer across a scale. The digital ammeter provides full scale readout of +/-20,000 counts. It offers four full-scale ammeter direct reading current measurement ranges from 2.0000 mA with 0.1 A resolution to 5.000 A with 1 mA resolution, plus five voltage ranges from the 200.00 mV to 600.0V.
The voltage input ranges are significant in a digital ammeter for use with external current shunts. Since high DC currents in excess of 5 amperes would burn out most ammeters, such currents are normally determined from the voltage drop across an external low-resistance, precision resistor in series with the circuit. Such resistors are named "current shunts," since only a tiny fraction of the current to be measured passes through the meter while the rest of the current is "shunted" through the external resistor. According to Ohm's law V = I x R, so the voltage drop V across the shunt resistance R will be directly proportional to current I. By knowing the resistance R, current I can be determined from the voltage drop. For example, a 1 milli-ohm shunt having a current of 50 Amps flowing through it will produce a voltage of 0.001 ohms 50 A= 50 milli-volts. Typical voltages across a shunt at maximum rated current are 50 or 100 milli-volts.
A digital ammeter should be scalable in software so that the current through the shunt in amperes can be displayed based on the milli-volts across the shunt. Some digital ammeters go one step further by incorporating a current shunt and providing a special readout scale for that shunt. For example, the DC digital ammeter incorporates a 10 milli-ohm shunt, which is associated with that meter's 0-5.000A scale. Calibration factors for this scale, as well as other DC current scales down to 2 mA, are stored in EEPROM on the signal conditioner board in the meter.
Applications: The digital ammeter function is but one of many functions available in digital panel meters. Digital ammeters are used in automotive, telecom, marine, electrical utility and industrial applications. Read rates are available up to 50 or 60 conversions per second, while integrating the signal over a full power line cycle for high noise reduction. High read rates are of benefit for capturing peak or valley values, and for real-time computer interface and control. Peak and valley values can be displayed at the push of a front panel button or by grounding a rear connector. The push-button command in the front panel or a control signal at the back side connector is used to display peak and valley values that are captured automatically.
Things to look for in a DC ammeter: In the better digital ammeters, digital filtering is selectable for electrically noisy environments, including a batch averaging filter and an adaptive moving average filter which provides a choice of time constants. When a significant change in signal level occurs, that filter should adapt by briefly switching to the shortest time constant to follow the change, and then revert back to the user-selected time constant. There should also be a selectable auto-filter mode, where the filter time constant is automatically selected based on detected signal noise.
The digital display of a good digital DC digital ammeter should provide less than 0.01% full-scale error. In addition to providing a local display, digital panel meters should also offer a choice of plug-in boards for communications, control, and network use. For example, serial interface include Ethernet, USB, RS485 and RS232. Additional boards can provide a 16-bit, isolated analog output, and dual or quad relays for alarm or control. The LED display can be red or green. The display is also available with digits up to 8" high for a viewing distance up to 300 feet, for example across a factory floor.
About the Author
Barry Crowther has been working closely with Laurel Electronics Inc., who have been in the electronics business for many years. For more information about Laurels Electronic Inc. and Digital Ammeters please visit http://www.laurels.com/blog/
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