SPECIAL PRODUCT REPORT - Sinclair DM2 Digital Multimeter.

Not very many years have passed since the appearance in this country of the first digital multimeter selling at under £100.  Now a number are available at around £60 - quite a drop even if you don't take into account the intervening fall in the value of the pound.  As in the case of pocket calculators, the main factor behind this fall in price lies in the adoption of LSI techniques for the heart of the circuitry.

One of this new generation of digital multimeters is the DM2 by Sinclair Radionics - their second venture into the field.  In common with most of the lower priced digital meters it uses an integrating converter, in this case one based on the dual-slope technique which offers a number of advantages over the simple system.  A description of the basic priciples of the dual-slope integration d.v.m. would probably be of interest to those not already familiar with them...(diagram and description not shown in this excerpt).

Since integration by its very nature and definition takes time, integrating meters are not as fast in operation.  They are therefore unsuitable for applications where a rapidly fluctuating input is to be monitored, for instance where a control is to be set quickly to produce a certain voltage reading, or in data-logging systems where a successive sampling of a number of input levels is required.  To this cloud there is a silver lining, however, for this slow response will integrate out noise spikes which can upset faster reading instruments.

The Sinclair DM2 is a 3.5 digit instrument.  For the benefit of those readers unversed in d.v.m. terminology, this means it has a four-digit display of which the first (most significant) digit can only be 0 or 1.  A variation of this can occur where, as in the DM2, the leading (left hand) zero is suppressed to avoid displaying unnecessary information which might confuse the operator.

The readout in the DM2 uses four 8mm (0.3in.) 7-segment l.e.d. displays; the necessary decoder and driver circuits are included in the custom LSI chip.  Two sets of push-buttons select the five functions - a.c. voltage and current, d.c. voltage and current, and resistance - and the four range multipliers from 1 to 1000.  Automatic dual-polarity operation is incorporated, as is the automatic over-range and overload indication.  In each case the relative information appears in the l.e.d. display on the otherwise unused segments of the most significant digit.

POWER SUPPLY

For convenience and portability, the instrument is battery powered [RetroKit note:- interestingly the one in the picture has an internal mains supply], up to 60 hours of use being obtained with a single standard PP9 or equivalent.  Sockets are provided at the rear of the instrument for an external power supply.  This must provide a stable 9V with a current capability of 70mA average, 150mA surge.  One minor criticism is that these sockets are unlabelled as to function or input requirements [RetroKit note:- apparently this was rectified on later models].   An instrument which is so easily operated is likely to be quickly separated from its operating instruction leaflet, which clearly warns that no more than 10V must be applied.  This omission would seem to be a potential source of damage.  A separated a.c. mains power unit giving a suitable 9V supply is available from Sinclair.

The basic measurement range of the instrument is 1V d.c., giving a resolution of 1mV with over-range to +/-1.999V. A m.o.s.f.e.t. operational amplifier input stage achieves an input impedance of 100MOhms on this range; the voltage divider chain reduces this to 10MOhms on the other d.c. voltage ranges.  A mean averaging a.c. to d.c. converter is used for the a.c. voltage and current ranges.  Its frequency range is 20Hz-3kHz on voltage ranges up to 100V, 20Hz-1kHz on the 1000V and current ranges.

The DM2 can safely withstand a considerable overload on the lower voltage and current ranges, while fuse protection is incorporated for the current and resistance ranges.

The reviewer has been using a DM2 for about three months for servicing and experimentation and has found it reliable and easy to use.  The display is quite bright and is fitted with a violet filter to improve its contrast.  Even so, if your workbench is near a window or has a good bench-lamp you will find that you need to tuck the meter away in a shady corner for easy reading.

Complete screening of the instrument is provided by the case, based on robust aluminium extrusions.  The dual purpose handle/bench stand is useful, as is the optional carrying case which allows the DM2 to by used slung around the neck.

In the absence of any suitable instruments for comparison, I cannot really comment upon accuracy of readings obtained except to say that a selection of 1% resistors checked out well and other results were always "as expected".

Sinclair offer their usual year's guarantee on the DM2.  As they put it, "If it stops working, send it back and we repair or replace it free of charge."