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By Richard W. Lay
Beckman Instruments' first attempt at the consumer electronic test gear market is through a digital multimeter, the Tech 310, which has several features heretofore unavailable in a unit in its $130 price range-including a basic DC accuracy of .25% in addition to the attributes which have come to be standard on DMMs in this age of microelectronics, that is compact size, light weight, and portability for field as well as bench use, the Tech 310, is a breed apart in that a Beckman designed and manufactured low current drain CMOS integrated circuit contains all of the active circuitry associated with this instrument. The result is a completely portable test instrument capable-the manufacturer says-of up to 2000 hours of continuous operation from one alkaline, 9-volt battery.
The Tech 310 is a 3 1/2 digit unit, with liquid crystal display, that carries seven functions in 29 measurement ranges.
Special features and capabilities of this unit are the low current ohms setting for in circuit measurements on all ohms ranges, a special diode test setting, and the patented "Insta-Ohms" feature for instantaneous continuity tests.
The latter, until now not available on DMMs due to relatively long response times, permits quick identification when checking for shorts. When a short circuit is encountered, a small ohms symbol on the LCD is displayed instantly.
Four front panel jacks serve as all of the inputs and ground connectors for the 310. Testimony to the ruggedness of the meter is the fact one is for use in measuring AC or DC currents up to 10 amps.
However, factory tests indicate the 310 is capable of accepting up to 20 amps for 3 seconds or less without damage, the manufacturer reports.
Basic overload protection is for spikes up to 6kV on voltage and resistance ranges. The unit is rated at 1500VDC or 1000AC (rms). The LCD is capable of a maximum reading of 1999 with an over load indication of "OL" for all out of range measurements.
In the AC mode the 310 provides a basic accuracy on all sine waves of .75% from 45Hz to 2KHz; 1.5% from 2KHz to 5KHz, and 2.5% for frequencies up to 10KHz. For AC current the maximum frequency response is 400Hz.
Beckman's radial dial provides ac cess to all function and range settings.
There are seven ohms ranges; six AC amp and six DC amp ranges from 20 ma. to 10 amp; five DCV ranges from 200mV to 1500 VDC; and five AC volts ranges from 200mV to 1000VDC.
A single nine volt alkaline battery is easily accessible by removing four back cover screws. A blinking decimal point serves as the unit's low battery indicator during the last 200 hours of life, approximately. However, Beckman claims the battery should remain operative for about two years under normal usage conditions.
The heart of the unit, as mentioned before, is the LSI chip, which Beckman refers to as the multiprocessor. This chip contains an active input filter, an averaging ac-to-dc converter, the Insta-Ohms detection and resistance measurement circuitry, an analog to digital converter, and all of the necessary digital and LCD display drive circuitry.
The number of discrete components in the 310, due to the use of this "multi processor," have been reduced to under 40. Included as part of the discrete circuitry are the function/range switch, three resistor networks, a voltage reference diode, several circuit protection components, the battery and the display.
Beckman says it has designed into the multiprocessor chip a second order Bessel filter to assure broadband normal-mode rejection of 60dB when combined with the 6dB per octave rejection of the chip's integrating converter.
The low current mode for all resistance range settings permits use of the Tech 310 for in circuit measurements without forward biasing transistors or di odes. The maximum voltage that can be developed across an unknown resistance is .25 volts, Beckman says. On the other hand, the special diode test function operates on the ±2 volt full scale DC measurement range with a 5mA constant current source. This is scaled to ±200mV and converted in the multiprocessor chip and displayed.
The Tech 310 is available with a number of accessories at additional cost. Included are the high voltage probe allowing DC measurements up to 50Kv; a special RF probe for working in high frequency circuits up to 200MHz; and an AC current clamp, which permits its use in circuits with up to 200 amps without breaking the circuit.
The basic DC input impedance is 22 Megohms. For AC volts it is 2.2 meg-ohms shunted by 75pF.
Also see: Fluke DMM
(source: Electronic Technician/Dealer, Nov. 1979)