Micro Series True RMS meters offer a high accuracy 4-1/2 digit display for five voltage ranges and four current ranges. It is suited for AC applications where there is considerable distortion of current waveforms from nonlinear loading. The input is AC coupled to read only the AC component, such as ripple on a power supply.
The True RMS meter uses precision circuitry to compute the root-mean-square of complex waveforms from 10 Hz to 10 kHz. Spikes up to 2.4 times the maximum of each range are accurately measured. This provides a crest factor (Vp / Vrms) of 240 for a signal amplitude of 1% of full scale, decreasing to 2.4 at 100% of full scale.
Use with Current Transformers
High common mode rejection allows stable readings with current shunts located on the high side of the line. Five amp input capability allows the output of 5 A current transformers to be applied directly to the meter, with no need for a stepdown transformer. The current reading can easily be scaled to display the input to the current transformer. Digital scaling and calibration eliminate zero and span drift associated with potentiometers in non-microcomputer based meters.
The typical accuracy is 99.9% of full scale, decreasing to 99.8% for low frequency at high amplitude, or high frequency at low amplitude. The above graph shows that maximum error is 0.1% of full scale over all frequency ranges of interest to AC power measurement. Error increases to 0.2%, represented by the two shaded areas, at low frequency and high amplitude, or high frequency and low amplitude.
Crest factor increases in inverse proportion to signal amplitude. As shown on the same graph, the maximum crest factor (Vp / Vrms) is 2.4x at full-scale amplitude, increasing to 240 at 1% of full-scale amplitude.
The Micro RMS meter utilizes the same high-speed analog-to-digital conversion method as all other Micro Series panel meters. However, the meter settling time and control response to a step change in AC amplitude are limited by the RMS converter circuit to approximately 1 second.
Maximum error as a function of frequency and signal amplitude
Micro Series meters can be powered from 95-240 Vac ±10% and 95-370 Vdc, or optionally from 12-34 Vac and 10-48 Vdc. Any setup functions and front panel keys may be locked out for simplified usage and security. The popular 1/8 DIN version with 0.56″ high digits is environmentally sealed to NEMA-4X (IP65) from the front when panel mounted, and is available with red or green LEDs.
AC Voltage (1% to 100% of full scale)
AC Voltage Range
10 Hz to 10 kHz
AC Current (1% to 100% of full scale)
AC Current Range
10 Hz to 10 kHz
AC Voltage & Current
5 digits, 7-segment
Available with 0.56", 1.0", 4.0" or 5.0" digits.
High brightness red. Green available with 0.56" digits.
-99999 to +99999
-99990 to +99990 (selectable count by 10 with rounding)
Minus sign, 2 red LED lamps
0.005% of reading/°C
2.4x at 100% of full scale
RMS to DC
1 s conversion settling time
60/s at 60 Hz, 50/s at 50 Hz
56/s at 60 Hz, 47/s at 50 Hz
3.5/s at 60 Hz, 3/s at 50 Hz
Max applied voltage
660 Vac for 20, 200 and 600 V ranges, 125 Vac for other ranges
25x for 2 mA, 8x for 20 mA, 2.5x for 200 mA, 1x for 5 A
95-240 Vac ±10% and 95-370 Vdc
12-34 Vac and 10-48 Vdc
DC or 49-440 Hz
Safety-rated to 250 Vac, meter ground to earth ground, DC to 60 Hz, 4.2 kVp per High Voltage Test
5 Vdc ±5%, 100 mA max
10 Vdc ±5%, 120 mA max
24 Vdc ±5%, 50 mA max
50 Vdc to meter ground
Connector Pin Assignments
0°C to 60°C
-40°C to 85°C
95% at 40°C, non-condensing
NEMA-4X (IP-65) when panel mounted
Why Measure AC Power?
Many AC loads, such as electrical motors, will only operate reliably if the AC line voltage and frequency are within specified tolerances. Otherwise permanent damage to expensive plant equipment may occur. Motor generators and UPS systems will only operate reliably with the load for which they are rated. Drops in line voltage or frequency may indicate an excessive load and the possibility of equipment damage.
Micro Series meters and counters are low-cost means to instrument AC power lines with great accuracy:
AC RMS Voltmeter. Full-scale ranges of interest are 200.00 V with 10 mV resolution and 660.0 V with 100 mV resolution. True RMS capability allows the display of RMS voltage for non-sinusoidal waveshapes, such as square waves from a UPS.
AC RMS Ammeter. A built-in 5 A range can be used to display currents up to 5.000 A with 10 mA resolution, or to accept the output of 5 A current transformers. The 200.00 mV range can be used with external current shunts. With either transformers or shunts, scaling to the input current is easily accomplished via the meter’s front panel pushbutton switches.
Frequency Meter. Inverse period is used to determine AC line frequency to six-figure accuracy (60.0000 or 50.0000) in a few line cycles plus 30 ms.
Phase Angle Meter. Two signals with identical periods are applied to Channels A and B of the Micro phase angle meter. A display from -180º to +180º is obtained by timing the rising edge of one channel and the falling edge of the other channel. A resolution of 1º, 0.1º or 0.01º is selectable. Accuracy is .01% up to 100 Hz, .1% at 1 kHz, and 1% at 10 kHz.
Using Micro Series Meters and Counters to Instrument an AC Line
Successful synchronization of two motor generators requires that the voltage outputs of the generators be close to each other, that the two frequencies be identical, and that the voltage waveshapes be in phase.
The two voltages can be measured by two Micro AC RMS Voltmeters, which offer 200.00 V and 660.0 V ranges. Or a single meter can be multiplexed by using an external toggle switch.
The two frequencies can be measured to six-figure accuracy by a single Micro Dual Channel Frequency Meter, where each channel monitors a generator. The two AC neutrals must be tied to meter ground. Pressing a front-panel key toggles the reading between the channels.