With TJ Byers
ARRL has published a circuit to test ground resistance. However, it involves the use of 110 volts AC. Not wanting to run around the yard with an extension cord, would you please suggest a battery-powered circuit that would economically measure ground resistance (in a three-electrode configuration) at 60 Hz to 200 Hz?
Dwight Holtzen CQN3ARU
Yes, I have the circuit you request. But let me give the reader some background information on earth ground resistance before I spring it on them with no explanation of what in the world you're talking about. First, earth ground is used as a conductor in power distribution systems. Its purpose is to minimize the hazard of a lightning strike by grounding one leg of the AC line in soil. Second, radio waves use it too, as a ground plane to increase the effective radiation power of the antenna. In both cases, it's important to know the resistance of the soil and treat it with chemicals if the resistance is too low.
There are three ways to test soil resistance using two-, three-, or four-point measurment. The most popular is the three-point configuration (shown below). Its parameters are very well-documented, and it's easily implemented. You can think of it as a four-wire ohmmeter with two electrodes in common. The two outer electrodes establish a current in what can be considered an "earth" resistor.
The voltage test point (P2) for three-point soil measurements is at 62 percent of the distance from the common probe (P1). The resistance is determined by using Ohm's Law (R = E/I), where I is the current flowing between P1 and P3, and E is the voltage between P1 and P2. When driving the stakes, it's essential that they be in a straight line — with P3 as far from P1 as practical (within limits — not the next county!). Unfortunately, I don't have room to discuss the effects of effective resistance zone overlap, but suffice it to say that the three-point method fairly compensates for it.
As for the circuit above, it delivers an output equal to that found in most portable ground resistance testers — 26VAC (open) at 40 mA (shorted). A single 555 astable oscillator provides a 60-Hz squarewave that drives a 6VAC wall-wart transformer. The timing capacitor (0.1 µF, pin 2) determines the frequency — reduce it to .05 µF and the frequency increases to about 130 Hz. The rest is up to you and your Fluke (DMM).