Output module for PLCs: choosing relay or transistor outputs by load, switching rate and how inductive loads are clamped
PLC digital output modules · Relay contacts whose life falls from 600k to 75k operations as load rises and turns inductive, no built-in snubbers, 20 inductive switchings a minute, against 0.5 A transistor outputs with 45 V clamping, microsecond delays and no short-circuit protection
This page covers digital output modules for PLCs and their I/O systems: relay output cards and transistor (24 V DC) output cards that turn controller bits into switched power for solenoids, contactors, lamps and indicators. Ask three things first: what the load is (resistive, lamp, solenoid or contactor coil) and its current, how often the output will switch, and where the inductive kick of each load is clamped or suppressed.
Local and expansion digital output modules for PLCs, relay and transistor types, with their load ratings, switching limits and inductive load protection. Relay output modules in remote fieldbus I/O are covered on the Relay output module for remote I/O page, DIN-rail relay and SSR interface modules on the Interface module for PLC and DCS I/O page, input cards on the Digital input module page, and relays as components on the Power relay page. Analog output modules and safety output modules are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Relay contacts wear with current and inductance; transistors switch fast but small and unprotected
AutomationDirect's F2-08TRS relay output module rates 7 A per point resistive at 12–28 V DC and 12–250 V AC, with relay life at room temperature (typ.) falling as load rises and turns inductive; normally closed contacts carry half the current of normally open ones. [1] GE's VersaMax IC200MDL930 relay module is rated 2.0 A resistive per point at 5–265 V AC or 5–30 V DC and 0.2 A at 31–125 V DC, with a switching frequency of 20 cycles per minute on inductive loads and no internal fuses or snubbers. [2]
Siemens' SM 1222 16 DO transistor module (6ES7222-1BH32-1XB0, current-sinking) switches 0.5 A per channel resistive or 5 W of lamp, with a total of 8 A per group up to 50 °C horizontal, output delays of 20 µs on and 350 µs off, an inductive shutdown voltage limited to about 45 V, and no short-circuit protection, which must be provided externally. [3]
Editorial calculation, with an assumed duty: a relay output switching a 24 V DC solenoid at 7 A six times a minute around the clock makes 8,640 operations a day and reaches the listed 75,000 in about 9 days; the same output on a 7 A resistive load (300,000) lasts about 35 days. [1]
Give the supplier each output's load type, voltage, steady and inrush current, switching rate, the number of points per COM terminal, whether NC contacts are used, the suppression fitted at the load, and how outputs are fused or short-circuit protected.
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Clamp the inductive kick at the load, because the card may not
AutomationDirect notes that because its relay modules have no leakage current they have no built-in snubber, and that a diode across a 24 V DC inductive load can significantly increase relay life, as can the arc suppression techniques in its user manual. [1] GE states that its relay module has no internal snubbers and that inductive loads need external suppression. [2] On the transistor side, Siemens limits the inductive shutdown voltage of the SM 1222 to about 45 V internally but leaves short-circuit protection to the installer. [3]
Editorial note: the published life figures for solenoid loads are a quarter of the resistive figures at 5 A on 24 V DC (150,000 against 600,000), so suppression at the coil is what lets a relay card approach its resistive rating. [1]
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Send the load list and duty together
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Loads: [type; voltage; current; inrush]
Rate: [switchings per minute]
Outputs: [relay or transistor; NO/NC; points per COM terminal]
Protection: [fuses; short-circuit; suppression at load]
System: [PLC family; local or remote]
Environment: [temperature; mounting]
3 companies list this product among what they make. Company figures are quoted from each
company's own website; every card states where the relationship came from.
Sichuan Odot Automation System Co., Ltd.
Manufacturing site to be confirmed for the order
Lists digital output modules in its PLC and remote I/O ranges.
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How long do PLC relay outputs last?
AutomationDirect's F2-08TRS lists 600,000 operations at 5 A on 24 V DC resistive loads but 150,000 on solenoids, and 300,000 and 75,000 at 7 A. [1]
Do PLC output modules have built-in suppression?
Not necessarily: AutomationDirect's and GE's relay modules have no built-in snubbers; Siemens' SM 1222 transistor card clamps inductive shutdown at about 45 V. [1][2][3]
Are transistor outputs short-circuit protected?
Not on Siemens' SM 1222 16 DO sinking module, whose datasheet says short-circuit protection must be provided externally. [3]
Sources and what still needs a direct check
Sources checked 2026-10-12. AutomationDirect, GE (via a hosted datasheet) and Siemens (via a distributor-hosted datasheet) publish their own module data; suppliers list their modules.
