Current is measured by placing an ammeter in series with the branch circuit of interest. Solderless banana plugs such as the Pomona 1825 provide a convenient and robust way to connect a multimeter to your circuit as shown in Figures 1 and 2. The technique is summarized in Video 1.
Scope
This engineering brief is written for students working within the confines of a 24 VDC industrial control environment. It should not be used as a general procedure for working on energized high-voltage control panels. Follow the Pomona 1825 datasheet which states:
“For CE compliance and for personal safety, do not hold in hand when voltages exceed 30 Vrms/60 Vdc.”
Also, follow all applicable local, state, and federal regulations when working on electrical systems. This includes de-energizing the circuit, lock out and tag out all energy sources, discharging the circuit, and testing for residual voltage.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Teach It → Shop Practices
Learning Pathway: Industrial Automation Technician → Fundamentals
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update:11 Sep 2026
Figure 1: Meter connection using the solderless banana plugs.
Video 1: Demonstration of the Pomona 1825 to connect to the meter and power supply.
Why is current difficult to measure?
Most meters such as the pictured Fluke 87V include a pair of test leads. These work well for measuring voltage as the probes are placed across two nodes.
The voltage measurement does not disturb the circuit. That is, assuming there is no circuit loading by the meter. Technically, we state that the circuit’s Thévenin impedance is significantly lower than the meter’s input impedance.
Current measurement requires circuit modification. It deliberately disturbs the circuit so that the meter may be inserted in series with the branch of interest. In this case, the ammeter acts as a piece of wire with a fuse and a small internal resistance.
This causes a variety of problems when conventional probes are used.
The 120 VAC source examples are used to illustrate a hazard that is not present in the 24 VDC classroom trainer. As stated in the scope section, this article does not contain procedures for working on high-voltage circuits.
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Safety: The probes are effectively connected together inside the meter via a fuse and a low-resistance shunt. A slip of the probe could expose the operator to high voltage. For example, if one probe is connected to a 120 VAC source, the exposed tip of the second probe becomes an accident waiting to happen. This is especially dangerous for the inexperienced technician troubleshooting at 3 AM.
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Damaged Equipment: Since the probes are effectively connected together, it becomes easy to cross power supplies inside the industrial control panel. For example, suppose one probe has an alligator clip to a 120 VAC source. If the second probe accidentally touches a 24 VDC control line, there could be damage. If we assume a grounded 24 VDC system, the meter’s fuse should open. However, there’s a good chance that the +24 VDC power supply and associated sensors will pop in the process. You will certainly have a hard conversation with your supervisor.
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Startled User: Some circuits contain input capacitors that make a snapping sound (spark) when the meter probes are initially connected. Likewise, highly inductive circuits can cause a hiss (arc) when a probe is removed. This is unsettling and can cause the technician to jump. It leads to probe slippage and the results described in the previous two bullets.
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Erratic Operation: Some circuits require sustained power. This is challenging as the quality of the electrical connection depends on the technician’s application of steady pressure to the probes.
The bottom line is that measuring current with handheld conventional probes is unstable and hazardous. This is especially true inside an industrial control panel. There are better alternatives.
Tech Tip: We can measure current without breaking the circuit using an instrument such as the open-fork T6-1000. However, each instrument has its place. The featured Fluke 87V has better resolution for measuring the small DC currents associated with the Figure 2 lab setup. Stated another way, match the meter to the measurement. We will save non-electrical-contact current measurements for another day.
Banana Plug Connection for Measuring Current
Figures 1 and 2 show how the solderless banana plugs are used to facilitate current measurements. Those of you familiar with terminal blocks will immediately recognize the wire connection pattern. The wire is inserted from the side and a small screw is used to hold the wire. In fact, the pre-cut and pre-ferruled wires used with the industrial trainer fit perfectly with the plugs.
The featured banana plugs are:
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Black: Pomona 1825-0
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Red: Pomona 1825-2
Tech Tip: The banana plugs provide stable connections that allow the circuit to operate for extended periods of time. As an interesting sidebar, a relay coil’s current drops as it heats up. As shown in Figure 2, the Phoenix Contact 2903334 relay coil had an initial current of 18.8 mA. The measured current dropped to 17.2 mA after about 15 minutes of continuous operation. Characterizing the copper wire’s temperature coefficient of resistance makes a good exercise for advanced students. Video 2 presents the datasheet values and demonstrates the hot vs cold current change.
Figure 2: Measuring the current of a Phoenix Contact relay.
Video 2: Connection of the relay with compassion of datasheet and measured values for coil current.
Parting Thoughts
Abide by the hand-held and hands-free safety warning contained in the Pomona datasheet. It aligns with the best practice of setting up the meter and then not touching the meter or the connections for the duration of the measurement.
Finally, Figure 3 shows the connections to the bench power supply. The banana plugs provide a clean and reliable connection that can save classroom time.
Figure 3: Power supply connection using the Pomona 1825 plugs.
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.