Learn Automation with MINATA #01: Voltage, Current and Resistance — Understand Before You Touch the Circuit

A sensor rated 24 VDC does not mean it will surely work the moment you connect it to a 24 V supply. A PLC input that does not light up does not necessarily mean the PLC is broken. A solenoid coil that measures voltage but does not pull in is not necessarily short of voltage.

The first three questions are always:

How much potential difference is there between the two points?
Does the closed circuit actually let current flow?
What is the total impedance along the path the current takes?

That is exactly voltage, current and resistance. Understanding the relationship between these three quantities helps a machine person read drawings more correctly, measure the right points, and avoid swapping parts on a hunch.

1. Voltage is the difference between two points

Voltage is denoted V or U, with the unit volt (V). It does not "sit on one wire" in an absolute sense; voltage is always defined between two points.

When we say the control supply is 24 VDC, we usually have:

Positive terminal: +24V.
Reference point: 0V.
The meter between +24V and 0V: about 24 V.

If you put both probes on the same +24V wire, the meter reads almost 0 V because the two points have no significant potential difference.

This is a very common measurement mistake: saying "the wire end has power" without stating measured against which point.

2. Current only flows when the circuit is closed

Current is denoted I, with the unit ampere (A). In control circuits we often deal with milliamperes (mA), where:

1 A = 1000 mA

Current flows when there is a complete loop:

Source → load → return path to the source

For example, a 24 VDC solenoid coil pulls in only when:

There is suitable voltage across the two ends of the coil.
A contact or control transistor has created the conducting path.
The 0V return wire to the source is not broken.
The coil itself is not open internally.

If only +24V reaches one end while the other end is open, the meter can still give a misleading reading depending on how the probes are placed, but the load current does not flow and the valve does not act.

3. Resistance opposes current

Resistance is denoted R, with the unit ohm (Ω). For a simple DC circuit, Ohm's law states:

V = I × R

From which:

I = V / R
R = V / I

NIST describes this as the proportional relationship between the volt, the ampere and the ohm; the unit is 1 Ω = 1 V/A.

Example 1: A 24 VDC indicator lamp

Assume the equivalent circuit has a resistance of 2.4 kΩ:

I = 24 / 2400 = 0.01 A = 10 mA

Example 2: A solenoid coil with increased resistance

A solenoid coil rated 24 V, 6 W draws approximately:

I = P / V = 6 / 24 = 0.25 A

Equivalent resistance while working:

R = V / I = 24 / 0.25 = 96 Ω

If a connector becomes oxidized and adds series resistance, part of the voltage drops across the connector. The coil receives a lower voltage, the pull-in force weakens, and the connector heats up again.

4. A quick reference table for beginners

Quantity	Symbol	Unit	Basic measurement	Common mistake
Voltage	V or U	volt (V)	Measure in parallel between two points	Measuring without defining the reference point
Current	I	ampere (A)	Connect the meter in series or use a suitable clamp meter	Leaving probes in the A jack and shorting the supply directly
Resistance	R	ohm (Ω)	Cut the power, isolate the part to be measured	Measuring Ω while the circuit is still energized

Do not use the resistance or continuity mode on an energized circuit. Before measuring, isolate the energy and confirm the de-energized state according to your workplace safety procedure.

5. Voltage drop is what the load actually receives

On a drawing, conductors are usually treated as ideal. On a real machine, wires, terminals, contacts and lugs all have resistance.

When current flows:

V_drop = I × R_wiring

Suppose a valve draws 0.25 A and the abnormal total resistance on the supply/return path is 8 Ω:

V_drop = 0.25 × 8 = 2 V

The solenoid coil is left with only about 22 V. When the machine is cold it may run, but when it is hot or the air pressure changes, the valve begins to act intermittently.

Therefore, when troubleshooting a 24 VDC load:

Measure the source voltage with no load.
Command the load to operate.
Measure immediately across the two ends of the load while current is flowing.
Measure the voltage drop of each segment: contact, terminal, supply wire and return wire.

Measuring with no load sometimes hides a bad joint.

6. Distinguishing an open circuit from a short circuit

State	Equivalent resistance	Current	Common symptom
Normal circuit	As designed	Within limits	Load works correctly
Open circuit	Very large	Near 0	Load does not run
Short circuit	Very small	Rises very high	Fuse/breaker trips or the wire heats up
Increased contact resistance	Locally increased	May drop	Weak load, intermittent, hot connector

An open circuit and a short circuit both stop the machine, but the troubleshooting is opposite. An open circuit requires finding where the current path is lost; a short circuit requires isolating the branch with the abnormal current.

7. A real example: a PLC input that does not light up

Assume a three-wire PNP sensor:

Brown: +24V.
Blue: 0V.
Black: signal to the PLC.

A reasonable checking sequence:

Measure between the brown and blue wires at the sensor: is there close to 24 V?
Activate the sensor and measure between the black and blue wires: does the signal rise to close to 24 V?
Measure at the PLC input terminal: does the voltage reach it?
Check that the common of the input group is correctly wired to 0V per the diagram.
Look at the physical LED and the online status inside the PLC.

If the signal has 24 V at the sensor but is lost at the terminal, the fault is on the wire or the connector. Only if there is 24 V at the input but the PLC does not read it should you move on to checking the common, the I/O configuration, or the input hardware.

This is how to narrow down by evidence instead of replacing the sensor and then replacing the PLC.

8. A checklist before touching a probe to the circuit

Do you know whether the circuit is AC or DC?
Do you know the nominal voltage and the equipment limits?
Have you selected the correct probe jack and the correct measurement range?
Where is the reference point of the measurement?
Do you need to measure while the load is operating?
Does the measurement require opening the circuit or connecting in series?
Have you isolated the energy if measuring resistance/continuity?
Do the meter and probes have a suitable safety rating?

Conclusion

Voltage is the difference between two points. Current is movement within a closed loop. Resistance determines how much the current is opposed and creates a voltage drop when current flows.

An automation person does not need to start with complex formulas. Start by drawing the path of the current, identifying the two measurement points, and asking how many volts the load actually receives while it is working.

MINATA favors verifiable troubleshooting: measure from the source to the load, then from the load back to the source.

References

NIST — The ampere and Ohm's law: https://www.nist.gov/si-redefinition/ampere/ampere-present
NIST — SI units of electric current, voltage and resistance: https://www.nist.gov/pml/owm/si-units-electric-current
OSHA — Electrical terms and hazards: https://www.osha.gov/etools/construction/glossary
JEED Iwate — Foundational training program for electrical and production systems: https://www3.jeed.go.jp/iwate/poly/kyushoku/kunren_2026_seisan.html

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