How to Isolate 24V DI/DO Industrial Field I/O: Isolator Selection and TVS Coordination
In one line: field-side 24V DI/DO must be isolated from the local logic side, and the reason is not the 24V itself. Field cables run tens of metres and join points at different ground potentials, so without isolation the ground-potential difference and cable-coupled surges land straight on the MCU pins. Do two separate jobs at once: the isolator handles common mode and the ground loop (bringing the field signal across the barrier for a DI, or driving the field device for a DO), and a TVS handles transient energy (a field-entry TVS diverts cable-coupled surge before it reaches the barrier). Neither replaces the other.
flowchart TD
A["1 Define the boundary<br/>which side is field, which is logic"] --> B{"DI or DO?"}
B -- "DI: signal in from field" --> C{"Contact type?"}
C -- "Wet contact (24V powered)" --> D["Series current limit + isolator input<br/>derive limit and threshold from 24V"]
C -- "Dry contact (passive)" --> E["Needs a wetting supply<br/>provide 24V on board, then current-limit"]
B -- "DO: drive field from board" --> F{"Load type?"}
F -- "Relay / contactor coil" --> G["Isolated driver + freewheel or TVS<br/>clamp the inductive turn-off spike"]
F -- "High-side / low-side MOS output" --> H["Isolated driver + output clamp<br/>watch reverse feed and surge"]
D --> I["2 Pick the isolator<br/>digital isolator or optocoupler"]
E --> I
G --> I
H --> I
I --> J["3 Set withstand and package<br/>test value vs working voltage"]
J --> K["4 Add a TVS at the field entry<br/>VRWM and power from the surge level"]
K --> L["5 Verify: common-mode injection + surge bench + reverse-wiring test"]The three questions that come up first
Q: 24V is not high voltage, so why isolate industrial DI/DO at all? Isolation here was never about the 24V level, since even a plain level shifter survives it. It is about three things unique to the field side. First, ground-potential difference: across a cable run of tens to hundreds of metres the two grounding points permanently sit several volts to tens of volts apart, and if both ends share ground that difference becomes a DC or mains-frequency current flowing through the signal loop. Second, common-mode transients: when a nearby inverter, contactor or solenoid switches, the cable carries a fast common-mode step that lands on the board unless it is isolated. Third, surge and lightning-induced transients, worst on long or outdoor runs. This is the same "any external interface must be isolated" judgement as in Which Places in an Energy Storage System Must Be Isolated?.
Q: How do wet contacts and dry contacts differ? The two terms describe whether the field signal carries its own power. A wet contact arrives already powered (typically 24V DC), so the DI channel only needs current limiting, division and a threshold. A dry contact is just a pair of passive contacts (reed switch, relay contact); it carries no power, so the board must supply a wetting voltage through the contact and read it back. Many field failures are simply a dry-contact channel wired to a wet-contact signal, or the reverse, and the circuit then does not work. Ask which type the field uses before designing the DI.
Q: Digital isolator or optocoupler? A digital isolator (such as the HOPERF CMT family) is about two orders of magnitude faster, has propagation delay down to the 9 ns class, has no CTR drift with temperature or age, and a barrier with an expected life over 40 years (per datasheet and certification documents); an optocoupler can be built for high withstand and suits an already-frozen legacy design. For new industrial DI/DO, prefer the digital isolator; keep the optocoupler only when inheriting an old layout or holding large optocoupler stock. The three migration traps (default output state, CTR drift, driving-position replacement) are covered in Can an Optocoupler Be Replaced by a Digital Isolator? and are not repeated here.
Step 1: Sizing a DI channel - current limit, threshold, source and sink
A DI channel is essentially "turn a 24V level into a logic level the local side can read, across the barrier". Three numbers matter: current limit, decision threshold, and wiring polarity.
Current limit. A 24V line applied to the isolator input (or an optocoupler LED) must have a series resistor that holds the input current inside the recommended range (optocoupler LEDs typically 5 to 10 mA, digital isolator inputs per datasheet). The common mistake is to run more current "for a more solid signal", which ages the LED or input stage faster and hurts long-term reliability.
Threshold. For a 24V industrial system design the decision on two levels, an on-state voltage and an off-state voltage; a common DI threshold set (as in IEC 61131) is on at about 15 V or above and off at about 5 V or below (standards and products differ, use the machine specification). The band between them must be suppressed so a poor contact is not read as a valid state.
Source and sink (PNP / NPN). This is where field I/O gets burned most often. A source input means the signal source pushes current into the channel (common in Japanese-style and PNP sensors); a sink input means the channel pulls current from outside (common in European-style and NPN sensors). Wiring a 24V channel the wrong way round either reads nothing or reverse-biases the isolator input stage. Always establish whether the field is PNP or NPN, source or sink, and set the channel common accordingly (common-anode or common-cathode).
| DI input form | Field characteristic | On-board handling | Common trap |
|---|---|---|---|
| Wet contact (24V powered) | Field supplies a 24V level | Series current limit + isolator input, two-level threshold | Single threshold, transitional zone misread |
| Dry contact (passive) | Just a pair of contacts | Provide a wetting 24V supply through the contact | Used a wet-contact channel, circuit dead |
| Source / PNP | Current pushed into the channel | Common set for source | Wired to a sink channel, input reverse-biased |
| Sink / NPN | Channel pulls current | Common set for sink | Same as above |
Step 2: Sizing a DO channel - output form and inductive loads
A DO channel works the other way round: a small signal from the logic side crosses the barrier to drive a field device (relay, contactor, solenoid, indicator). Two cases.
Relay / contactor coil loop. A coil is the classic inductive load, and turn-off produces a reverse voltage spike (L di/dt) that must be diverted by a freewheel diode or a TVS in parallel with the coil. A freewheel diode is simplest but releases slowly; a TVS releases fast but needs its clamp height and energy checked. The trade-off and the energy accounts are in How to Clamp a Relay Coil Turn-Off Spike. The voltage a driving MOS sees at turn-off is supply plus residual spike, so the rating must cover both.
High-side / low-side MOS output. The power device drives the load directly. Beyond the inductive spike, watch reverse feed if the field connects an energy-storing load or back EMF, and add a field-side TVS at the output for cable-coupled transients.
In both cases, place the isolator between the MCU (the safe logic domain) and the field-side driver, with the field side protected by the barrier. Size the driver peak source / sink current for the load and switching speed, not just the static current.
Step 3: Withstand and package - two figures, not one
The 5 kVrms or 3.75 kVrms on an isolator datasheet is a UL 1577 test value (about one minute without breakdown), not a long-term working voltage. To check a 24V industrial system against its real need, use the working isolation voltage (for example some isolated CAN devices are around 1060 Vrms), the transient withstand, and the creepage distance together, then read the package off the applicable safety standard (pollution degree, material group, insulation type). A wide-body package gives more creepage and meets stricter field requirements. This convention is identical to gate-drive and CAN isolation; see What Does a 5 kVrms Rating on an Isolator Mean? and Isolation Barrier PCB Layout.
Step 4: How a TVS and an isolator work together - two jobs, not interchangeable
This is the point of the article: an isolator handles common mode and insulation, a TVS handles transient energy, and neither replaces the other. The isolator breaks the ground loop and blocks common-mode transients (the CMTI figure), but it dissipates no energy. Surge energy entering through the interface can still punch through the barrier or burn the field-side pins. So the field entry must also carry a TVS that diverts cable-coupled energy before it reaches the isolator.
Applied to 24V DI/DO, the method matches TVS Next to an Isolator: Surge Protection Coordination:
- Placement. Put the TVS at the field-entry connector, on the field side of the isolator. Never on the logic side between isolator and MCU, or the surge bypasses the barrier and lands on the logic side.
- VRWM. Cover the maximum steady-state and swing of the 24V field line. A 30 V to 36 V grade is common for 24V industrial systems (set by the real system voltage and surge need). Too low and it conducts and heats; too high and the clamp rises.
- Vc alignment. At the surge current the clamp must stay below the absolute maximum of the protected device (isolator field-side pin or output MOS).
- Waveform convention. The AMSEMI 5.0SMDJ family quotes Vc and Ipp entirely on the 10/1000 us convention and has no 8/20 us data; Littelfuse publishes both
IPP 10x1000 usandIPP 8x20 usfor its own parts, and any 8/20 us figure must be attributed to Littelfuse explicitly. The two must never be mixed; compare across grades only by percentage reduction and label the test condition on every row. - Staging. For long or outdoor runs, stage the protection: a coarse front stage (GDT / varistor) plus decoupling plus a fine-tuned TVS so the front stage takes most of the energy.
| Field threat | Primary defence | Device role | Key setting |
|---|---|---|---|
| Ground-potential difference / ground loop | Isolator | Break the shared-ground path | Separate field and logic grounds, isolate across the barrier |
| Common-mode transient (inverter / contactor switching) | Isolator CMTI | Withstand high dV/dt without false switching | Check the minimum CMTI, lower coupling by barrier layout |
| Cable-coupled surge / lightning induction | TVS | Divert transient energy | Field entry near the connector, VRWM covers the 24V swing |
| Inductive turn-off spike (DO relay) | Freewheel diode / TVS | Divert L di/dt | Across the coil, clamp height and energy checked together |
The AMSEMI 5.0SMDJ-N series is the 5000 W tier (10/1000 us convention); at one voltage grade the foldback and standard parts share VBR and Vc and differ only in Ipp. Pick the voltage grade for 24V cable protection against the real system maximum and surge level confirmed with the manufacturer; this site does not invent grades.
Common traps and debug
| Symptom | Likely cause | Direction |
|---|---|---|
| DI reads nothing | Dry-contact channel with no wetting supply / source-sink swapped | Confirm contact type and PNP / NPN; add wetting supply or rewire |
| DI intermittent, transitional misread | Single threshold, no hysteresis | Design on at 15 V or above, off at 5 V or below, add hysteresis |
| Isolator input stage burned | 24V applied with no series limit / reverse biased | Add the series resistor and hold current in the recommended range |
| DO driving MOS blows at turn-off | Inductive load with no freewheel or clamp, spike over rating | Add freewheel diode or TVS, check clamp height and energy together |
| Interface still fails with an isolator fitted | Isolation only, no TVS, surge bypasses the barrier | Add a field-entry TVS at the connector, stage the protection |
| TVS runs hot / high standby current | VRWM too low, micro-conduction at 24V steady state | Raise the VRWM grade (30 V to 36 V class), recheck the clamp |
| IO false operation now and then | Common-mode transient coupling, CMTI margin or layout | Check the minimum CMTI, clear the barrier keep-out |
Related solutions
- Selection mainline: How to Select Digital Isolators
- Migration traps: Can an Optocoupler Be Replaced by a Digital Isolator?
- Where isolation is needed: Which Places in an Energy Storage System Must Be Isolated?
- Withstand convention: What Does a 5 kVrms Rating on an Isolator Mean?
- Isolator and TVS coordination (P1 landing page): TVS Next to an Isolator: Surge Protection Coordination
- Signal-port protection: Why Do RS485 and CAN Ports Keep Failing?
- Inductive loads: How to Clamp a Relay Coil Turn-Off Spike
- BMS and energy-storage main protection (P1 landing page): How to Select the TVS on a BMS Protection Board, Should BMS Cell-Sampling Lines Have a TVS?
- Making isolation real: Isolation Barrier PCB Layout and Isolated Supply
FAQ
Q1: 24V is not high voltage, so why isolate industrial DI/DO?
Isolation here is not about the 24V level but about three things unique to the field side. First, ground-potential difference: over a cable run of tens to hundreds of metres the two grounding points permanently sit volts to tens of volts apart, and a shared ground turns that into a current through the signal loop. Second, common-mode transients when inverters, contactors or solenoids switch and the cable carries a fast common-mode step. Third, surge and lightning-induced transients. Isolation breaks the ground loop and blocks the common-mode transient to protect the local MCU, the same judgement as "any external interface must be isolated" in energy storage.
Q2: How do wet contacts and dry contacts differ for a DI?
The difference is whether the field signal carries its own power. A wet contact arrives already powered (typically 24V DC) so the channel only limits, divides and thresholds. A dry contact is just a pair of passive contacts (reed switch, relay contact) carrying no power, so the board must supply a wetting voltage through the contact and read it back. Ask which the field uses first - many field failures are a dry-contact channel wired to a wet-contact signal or the reverse, leaving the circuit dead.
Q3: Why does swapping source and sink burn the board?
A source input means the signal source pushes current into the channel (common in Japanese-style and PNP sensors); a sink input means the channel pulls current from outside (common in European-style and NPN sensors). Wiring a 24V channel the wrong way round reverses the current path and the device's intended direction: at best nothing reads, at worst the isolator input stage or optocoupler LED is reverse-biased into breakdown and ages faster under sustained reverse stress. Always establish whether the field is PNP or NPN, source or sink, and set the channel common (common-anode or common-cathode) accordingly.
Q4: With an isolator fitted, why does the field interface still fail, and is a TVS still needed?
Yes, it is required. The isolator handles common mode and insulation (breaking the ground loop, withstanding high dV/dt common-mode transients) but dissipates no energy, so surge entering through the interface can still punch through the barrier or burn the field-side pin. The field entry must therefore also carry a TVS that diverts cable-coupled energy before it reaches the isolator. Placement: the TVS goes at the field-entry connector on the field side of the isolator, never on the logic side; VRWM covers the 24V swing (a 30 V to 36 V grade is common, set by the real system); Vc stays below the protected device rating at surge current; for long or outdoor runs stage a coarse front stage with a fine clamp. The AMSEMI 5.0SMDJ family is on the 10/1000 us convention and Littelfuse 8x20 us data belongs to Littelfuse; the two must never be mixed.
Contact us
For device selection on industrial 24V DI/DO field-side isolation (DI current-limit and threshold calculation, source and sink wiring, DO inductive-load protection, isolation withstand and package grades, field-side TVS coordination), for samples of HOPERF CMT digital isolators and AMSEMI TVS, or for a domestic-alternative evaluation, please contact us.
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Disclaimer: This article is technical and industry communication, for reference only, and does not constitute selection, procurement or other commercial decision advice. The digital isolator speed, propagation delay, isolation withstand grades, CMTI and barrier life figures are taken from HOPERF public material (product pages and datasheets) and are typical values or grade examples; models and conditions differ significantly. The DI decision thresholds (on / off voltages) are common magnitudes for 24V industrial systems and the machine specification governs. TVS Vc and Ipp conventions, current-limit values and thresholds are engineering estimates; the AMSEMI 5.0SMDJ family is on the 10/1000 us convention and Littelfuse 8x20 us data belongs to Littelfuse, and the two must never be mixed. Actual design must follow the selected part's original datasheet, the field surge level and assembled-unit measurements.