These days we think little about the test leads connected to our meter. Aside from the very important voltage ratings and the condition of the insulating materials, there is usually no need to worry. That’s because the input resistance of modern digital multimeters (DMMs) is so high that even one ohm of extra resistance in the leads will not make a measurable difference in our voltage measurement results. And in most cases that’s because we don’t have to worry about the loading effects of the meter’s ten megohm input circuit. But beware, that’s only true if the source impedance of the circuit we are measuring is low—less than a few thousand ohms.
It wasn’t always so easy
It turns out that at the dawn of the space age, in the late 1950s, the precision analog voltmeters used in development labs often required tens or even hundreds of milliamps to operate. I remember a 0.1% dc voltmeter with a 10inch-long mirrored 3 volt scale that was rated at 2 ohms/volt. That translated into an input resistance of only 6 ohms. So, when we were measuring low voltages, test lead resistance was an important factor.
We actually measured the resistance of the leads (typically 0.052 ohms each) and accounted for their contributions to the circuit for each measurement. With two of these leads in the circuit, the source had to be capable of handling the 500 mA loading effect of the meter, and meter readings had to be adjusted to reflect the voltage value of the source. In this case, that meant the source voltage was actually 3.052V when the meter indicated 3.0 V—a 1.7% error due only to lead resistance.
Back then the mainstay of the Fluke product line was the differential voltmeter. I saw and used my first Fluke 800 while testing Missile Ground Guidance Systems. The Model 800 dc differential voltmeter was an electronic marvel, using vacuum tubes to operate sensitive amplifiers and regulators, to measure up to 500 volts dc to an accuracy of 0.05% without loading the high impedance test circuit. And that was a good thing, because the sensitive guidance system circuits (also powered by vacuum tubes) had to be accurately tuned. There was one minor inconvenience. Although test lead resistance was no longer a factor, using the five-dial differential voltmeter required the operator to adjust the meter’s null detector to zero by twirling the dials.
Make your own differential voltmeter
So, can you measure a high impedance source (more than 1.0 megohm) accurately with your trusty DMM? Yes, provided that the test circuit has a solid ground reference for the measurement. You can emulate the operation of the differential voltmeter by twirling the dials of a suitable stable power supply to match the voltage you are trying to measure.
Just connect one lead of the floating power supply output to the ground reference and then connect your DMM between the point to be measured and the power supply output. Now, dial the power supply output until the DMM reads zero. When a stable null has been achieved, you can now remove the dmm lead from the test point and connect it to the power supply ground reference. You will now be reading the power supply voltage which you know to be equal to the voltage you were trying to measure.
Subtle measurement issues regarding test leads
There are still some characteristics of test leads that can create measurement errors with today’s high precision DMMs. One of the problems is a thermocouple effect that can occur when using normal test leads that are at a different temperature than the test point they are measuring, often due to heating from the technician’s hand. This is most likely to be a problem when measuring at millivolt and microvolt levels. The Fluke 180 series, and the newer 280 series, both offer 50 mV dc ranges with 1 uV resolution. For that reason, these meters are shipped with Fluke’s TL71 Premium Test Leads, which use alloys that minimize thermal problems. And it always helps if the test point and the common for the measurement are at the same temperature - that way the thermocouple effects will tend to cancel. Another aid is to clip the lead to a test point and allow its temperature to equalize before noting the reading.
Another recent Fluke advance in test leads for safe measurements
Recently, Fluke introduced a new family of test leads that take measurement safety to another level. The new TL175 TwistGuard test leads allow you to conveniently insulate the conductive probe so that only the sharp tip is visible. This ensures that you won’t accidentally create a short circuit by contacting a terminal near the one you want to measure. In addition, the multilayer silicone test lead insulation highlights any nicks or physical damage that might compromise their safe use.
Learn more about the leads mentioned in this column by looking here:
Finally! Watch this space for comments on some exciting new developments in current clamp accessories in the near future.