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Protection for volt-ohmmilliammeters

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Many of us who make frequent use of a general-purpose test meter will, at one time or another, inadvertently apply voltage to the terminals of an instrument having the function or selector switch set at the ohms position. This act of negligence may result in a burnt-out multiplier shunt, a new configuration for the pointer or even more serious damage to the meter movement.

The installation of a fuse as a preventive measure against this mistreatment of a meter is not always desirable or completely effective. The resistance of low-current fuses may not be too uniform and, in some cases, the resistance - whatever it happens to be - is great enough to affect the calibration of the ohms ranges of the instrument.

A more positive protection for a meter may be obtained by connecting a No. 14 flashlight lamp in series with the internal shunts. It has been determined that the resistance of these 2.5-volt 0.3-amp. bulbs checks consistently at very close to one ohm. Therefore, once the meter circuit has been modified to include a bulb, it is possible to make replacements in event of failure without concern over variations in calibration.

Fig. 1 shows how a No. 14 bulb has been connected into a typical general-purpose test meter.

Fig 1
Fig. 1. Protective arrangement for volt-ohm-milliammeters suggested by W9AFT.

The circuit and constants shown are for the popular Simpson type 260 meter. The 1-ohm resistance of the bulb is compensated for by removing approximately 2 inches of resistance wire from the 11.5-ohm shunt (R16 of the Simpson circuit). Usually, it is possible to find mounting space within the meter case for a socket for the bulb. This method of installation simplifies the changing of a burnt-out lamp.

Any voltage applied to the ohms terminals that is lower than the burn-out rating of the bulb will not cause damage to the meter shunts. At higher values of voltage, the bulb will burn out even before the pointer can deflect to full scale. It should be pointed out that all resistance ranges of a circuit of the type shown are protected by this simple installation.

Harlon Wright, W9AFT.