How to Choose a Multimeter for Checking PLC Outputs

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How to choose a multimeter for checking PLC outputs comes down to four things: a DC volts range that covers the output’s rated voltage, an input impedance that will not disturb a sensitive control circuit, a low-impedance mode for ghost voltage, and a CAT rating that suits where you measure. The sources we opened describe 24 VDC digital output modules and the meter behavior that goes with them. None of them names a meter model, so this guide sets selection rules rather than a product list.

Electrical safety comes first

Work on mains circuits can kill. This guide covers low-voltage control circuits and bench checks only. Before you measure anything, check that the meter’s CAT rating suits the circuit you are testing, and read the rating from the listing or the manufacturer’s manual. Panel and service work belongs to a licensed electrician.

Fluke’s safety guide explains the basic hazard: “Shock occurs when the human body becomes part of an electrical circuit.” The same guide advises choosing “a tool rated for the highest category you could potentially use it in,” and matching the voltage rating to that situation. Its CAT III examples include “Equipment in fixed installations, such as switchgear and polyphase motors,” so the category depends on where the meter is used, not only on the meter itself.

Electrician U makes the same point from the lead side. It says an insufficient CAT rating on the meter or on its leads can expose you to dangerous voltage spikes. Check the leads as well as the meter body.

Live panel work is outside this guide. The Industrial Control Academy page says to “Follow your site lockout procedure” before touching terminals, and it names OSHA 29 CFR 1910.147 and NFPA 70E for US sites. If a check would need a live cabinet, hand it to a licensed electrician rather than improvising.

The sources we opened do not give a CAT rating for any specific meter. Read the rating from the listing or the manual for the exact model you are considering, and do not assume a rating from a similar model.

Quick answer

• Set the meter to DC volts, and make sure its range covers the 23.5 V to 26 V band the PLC programming blog gives for 24 VDC digital modules.
• Use a high-impedance input for sensitive control circuits, and a low-impedance mode when ghost voltage is the question.
• Identify the output type first, because a triac output is AC-only and can read a small leakage current when off.
• Read the CAT rating from the listing or manual, and confirm the leads carry the same rating.

Know the output type before you pick a meter

A PLC output is usually one of three types, and each one behaves differently under a meter. Realpars describes a dry relay as using “mechanical contacts that switch AC and DC loads regardless of polarity or current direction.” The Industrial Monitor Direct comparison lists relay leakage as “Zero (open contacts)” and describes relay isolation as “complete electrical isolation between the coil circuit and the switched load.” A relay output therefore shows little or no leakage when it is off.

A triac output is a different case. Toolcluster says a triac is AC-only and “cannot switch DC,” and it also notes that “a small current keeps flowing even when OFF, called leakage current.” The Industrial Monitor Direct table gives triac leakage as “Low (mA range when off).” A triac output can therefore show a small reading when the output is supposed to be off, and that reading should not be taken as a sign that the output is on.

Transistor outputs are the DC type. The Industrial Monitor Direct article carries the heading “Use Transistor Outputs for DC Loads,” and Toolcluster describes transistor outputs as DC-only devices with a fixed direction. The pages we opened do not give a leakage figure for transistor outputs, so the meter guidance below rests on the relay and triac rows.

Wiring direction matters as well. Realpars explains that “Sourcing modules will connect the load to a supply voltage,” while “Sinking modules will connect the load to a power supply return or ground.” Find out which type your module is before you decide where the meter’s reference lead belongs, because the two layouts put the load on different sides of the circuit.

Choose the DC volts range

For a 24 VDC digital output, the PLC programming blog gives an expected range: for 24 VDC digital modules “you expect 23.5 V to 26 V.” A meter that reads DC volts across that span is the minimum for this job. The pages we opened do not say how much higher the range must go for a given module, so check the module’s own documentation for its rated voltage.

Set the meter to DC volts, not AC volts. Starting Electronics tells the reader to “turn the selector dial on the multimeter to the DC voltage setting.” On an auto-ranging meter, the page says, “there will only be one DC voltage selection on the dial,” so the meter picks the range itself.

A manual-ranging meter adds one more choice. Starting Electronics says a meter that is not auto-ranging has a set of voltages on the dial, with examples of 2V, 20V, 200V and 1000V. You then have to pick the step yourself. For a 24 VDC output, a meter with a step that covers 26 V is the simplest choice, and the page does not describe how a manual-ranging meter behaves when a reading falls outside its selected step.

Input impedance and ghost voltage

Fluke’s dual-impedance note says most digital multimeters have “high impedance input circuits greater than 1 megohm,” so they barely load the circuit they measure. The same note says this is “especially important for sensitive electronics or control circuits.” That is the main reason a high-impedance input suits a control circuit check.

Older tools behave differently. Fluke says analog multimeters and solenoid testers have input impedance of about 10 kilohms or less, and that they “should only be used for testing power circuits.” The note limits them to circuits where that loading does not matter.

The trouble with a high-impedance input is ghost voltage. The Industrial Control Academy page says that “A high-impedance meter can show a ghost voltage on a leaking or open circuit.” Fluke’s application note explains that ghost voltage can appear when the test leads are plugged into the meter but not connected to any voltage source. Its low-Z test tools have an input impedance of roughly 2 K to 100 K.

Many meters offer a low-impedance mode to address this. Fluke calls its low-impedance function Auto-V/LoZ, and says the function “reduces the possibility of false readings due to ghost voltages.” Fluke’s own guidance points the low-impedance mode at ghost voltage and the high-impedance mode at sensitive loads. The sources do not give one rule that picks the mode for every output, so the choice depends on the circuit you are checking and on the meter manual.

Realpars adds a practical warning for a dry output that is open. It says you “may see a significant voltage reading on your high-impedance digital multimeter” even though the output should read zero. Treat that reading with suspicion until the rest of the check confirms it. For a simpler go or no-go check on a circuit, see our circuit tester roundup.

Reading the result

A single reading rarely proves much. The Industrial Control Academy page says that “a good reading with no load proves little,” and it states that the idea “LED ON and no voltage means a blown output” “is not a fixed law.” A meter reading is one piece of evidence, and the page treats the output LED, the logic, and the load as the other pieces.

When you do get a reading, compare it with the expected range for the module. Using the 23.5 V to 26 V band from the PLC programming blog, a reading inside that band on a 24 VDC module is the expected value. A reading far outside it, or one that stays steady on a high-impedance meter while the load is absent, needs a second check before you decide anything about the output.

The relay and triac rows explain the rest. A relay output with open contacts should show zero leakage, so a leakage reading from a relay output is worth questioning. A triac output can show a low reading when off, so a small voltage on a triac output does not on its own identify a fault. For the basic continuity method on wiring, see our continuity tester guide.

Common mistakes

• Treating a high-impedance reading as proof that an output is on or off. The Industrial Control Academy page says a good reading with no load proves little.
• Assuming a triac output can switch DC. Toolcluster says a triac is AC-only.
• Using the low-impedance mode on sensitive electronics without checking the circuit. Fluke limits LoZ use to circuits where the low impedance will not change performance.
• Choosing a meter whose CAT rating is below the location, or checking only the meter body and not its leads. Fluke and Electrician U both make this point.
• Reading a sinking module as if it were sourcing. Realpars describes the two layouts as wired differently.

How we chose

We set the criteria from what the pages could support. A meter for this job must read DC volts across the 24 VDC band, offer a high-impedance input and a low-impedance mode, carry a CAT rating checked against the listing or manual, and be read with the output type in mind. Each of these criteria rests on at least one page that states it in plain words.

We searched widely across manufacturer pages, industrial automation guides, application notes and trade sites, then opened each candidate with curl on 2026-10-21. Pages that returned 403, 404, 405 or 429 were not counted, and neither were pages whose text did not contain the wording we needed. Ten opened pages support a claim in this guide. We kept only facts that a page states, and we checked every quoted sentence against the text of the page it came from.

What we could not verify

The pages we opened do not give a CAT rating, a count figure, a fuse rating, a maximum current, or a true RMS statement for any meter model, because no model is named here. The guide names no model and no listing was used for model data, so the listing does not state a CAT rating for any model discussed here.

The pages do not describe a bench procedure that uses a bench supply and a test load for a PLC output. They also do not cover true RMS for DC outputs, or whether a clamp meter is useful for output checks. Relay life figures come from a single vendor article, so they are not treated as general facts. The line that panel and service work belongs to a licensed electrician is our own policy statement and not a finding from any page we opened.

Frequently asked questions

Which DC volts range should I set for a 24 VDC output?

Set the meter to DC volts. The PLC programming blog gives 23.5 V to 26 V as the expected band for 24 VDC digital modules. The pages do not say which step a given meter uses, so check the meter manual.

Is a high-impedance meter enough for a PLC output check?

The sources say it depends on the circuit. Fluke says high impedance suits sensitive electronics and control circuits, but the Industrial Control Academy page warns that a high-impedance meter can show ghost voltage on a leaking or open circuit.

Can a triac output be checked on a DC range?

Toolcluster says a triac output is AC-only and cannot switch DC, so a DC check on a triac output does not match the device. The pages we opened do not describe a DC test for a triac output.

How does a relay output differ from a triac output under a meter?

The Industrial Monitor Direct table lists relay leakage as zero with open contacts, and triac leakage as low, in the mA range when off. Relay isolation is complete, which the same table describes as galvanic.

Which CAT rating should I look for?

The pages do not give one rating for PLC work. Fluke advises choosing a tool rated for the highest category you could use it in, and Electrician U says the leads need the rating too. Confirm the rating in the listing or manual.

Do clamp meters help with PLC outputs?

The pages we opened do not cover clamp meters for PLC output checks, so this guide gives no answer on that point.

Bottom line

Match the meter to the output type and the location first, then treat a single high-impedance reading as a starting point, not a result.

Sources

• https://www.fluke.com/en-us/learn/blog/safety/multimeter-guide (opened 2026-10-21)
• https://www.fluke.com/en-us/learn/blog/digital-multimeters/dual-impedance-digital-multimeters (opened 2026-10-21)
• https://media.fluke.com/0238c8d1-3347-4558-8405-b106006866be_original%20file.pdf (opened 2026-10-21)
• https://electricianu.com/understanding-multimeter-cat-ratings/ (opened 2026-10-21)
• https://www.realpars.com/blog/sinking-and-sourcing-plc-outputs (opened 2026-10-21)
• https://www.plcprogrammingblog.com/blog/plc-output-fault-diagnosis-step-by-step (opened 2026-10-21)
• https://industrialcontrolacademy.com/plc-output-not-working/ (opened 2026-10-21)
• https://startingelectronics.org/beginners/first-steps-in-using-a-multimeter/ (opened 2026-10-21)
• https://toolcluster.app/en/articles/plc-output-types-relay-transistor-triac/ (opened 2026-10-21)
• https://industrialmonitordirect.com/blogs/knowledgebase/triac-vs-relay-output-cards-plc-output-module-comparison (opened 2026-10-21)