Some hams have questioned the accuracy of the DVM DMMs and VTVMs shown
in the videos to include the input impedance in DC and AC voltage
measurements and the percentage of accuracy of the current measurements. I
used 3 different make and models of DVM DMMs and 2 different makes and
models of VTVMs during actual voltage, current, and resistance troubleshooting
and repairs plus the same measurements shown in the videos. Therefore, this
document provides the DC/AC voltage input impedance plus resistance
specifications for the DVM DMM and VTVMs I use and which are shown in the
videos.
Rolls MU-118
10meg input impedance in all DC and AC volt ranges
DC current accuracy
+/- .8% in the 20ma range
+/- 1.2% in the 200ma range
+/- 2% in the 10amp range
Resistance accuracy
+/- .8% in all ranges up to and including 2meg
+/- 1% in the 20meg range
The MU-118 manual shows a "200meg" range. However, my MU-118 seen in the
videos does not have a 200meg range so that range specification doesn't apply.
BK Model 66B7000:
10meg input impedance in all DC and AC volt ranges
DC current accuracy
+/- .8% in the 20ma Range
+/- 1% in the 200ma Range
+/- 2% in the 20amp range
Resistance accuracy
+/- .8% in all ranges up to and including 2Meg
+/- 1% in the 20Meg range
Excel DT9205A
10meg input impedance in all DC and AC volt ranges
DC current accuracy
+/- .8% in from 20ua to 20ma
+/- 1.2% at 200ma
+/- 5% at 20amps
Resistance accuracy
+/- .8% up to and including 2Meg
+/- 1% at 20Meg
The Excell DT9205A manual shows a 200Meg ohm resistance setting which has a
+/-5%. However, my DT9205A doesn't have a 200Meg setting so that 200Meg
specifications doesn't apply.
The typical DVM DMM and VTVM meter lead resistance has been found to
range from .7 to .9 ohms. You should know that this lead resistance does exist
and you should consider it when performing voltage, resistance, and current
measurements.
The VTVM's I use are either Heathkit or Eico. The typical input resistance for the
VTVMs is 1meg ohm shunted by a 40pf in ALL AC ranges. The typical input
resistance for the VTVMs is 10meg in all DC ranges but increases to 11meg with
the DC probe which has a 1 meg resistor in the RED positive probe's tip.
The VTVM's typical "analog meter" AC and DC accuracy percentage is 5% of full
scale for all AC ranges and 3% of full scale in all DC ranges. These percentages
should be taken into consideration when performing AC and DC voltage
measurements.
The RF probe I use is a Heathkit PK-3 with 3 diodes in series in the handle. This
RF probe is used with the Heathkit IM-13 and IM-18 VTVMs.
Any basic DVM/DMM and/or VTVM you use, regardless of the make and model,
will have an affect on the circuit under test. Therefore, you MUST take this
affect into consideration when performing voltage, current, and resistance
measurements. When measuring high voltage, make connections to the circuit
under test with circuit power OFF "first" then apply power. Once the
measurement has been taken, turn circuit power OFF and allow capacitors to
bleed down to zero then remove the meter probe connections.
73
Mike W5RKL
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