Basic 4-20 mA Current Loops

The current loop is a widely used industrial control circuit topology. The basic current loop has three elements:

  • A transmitter that sends a dc current around the loop
  • A receiver that measures the current in the loop
  • DC voltage supply that powers the loop


Using DC Current Shunts

A DC current shunt is a specialized resistor used to measure high currents. The shunt is wired in series with a heavy load, such as a DC motor, battery charger, heater, plating bath or smelter pot line. A DC voltmeter is connected to the shunt in a 4-wire (Kelvin) configuration. The shunt has bolt holes or


Insulation Testing: Megohmmeter or Hipot Tester

There are two common methods for performing insulation tests on cables, wiring and electrical equipment. One uses a megohmmeter to measure insulation resistance. The other uses a hipot tester to check for insulation breakdown. Both apply a high voltage, either ac or dc, to the device under test (DUT) and measure the resulting current flow. (more…)

AC Measurement

AC Measurement Considerations

Alternating current (ac) measurements involve more factors than direct current (dc) measurements. Let’s look at how some of these affect the measurement. We’ll focus on signals with a fundamental frequency below 1kHz. This is where digital multimeters and panel meters are typically used. Higher fundamental frequencies require instrumentation with wider bandwidth, such as audio meters or oscilloscopes. (more…)