SSR Leakage Current: Causes and Solutions

SSR leakage current is the small current that can still pass through a solid-state relay output after its control input has been switched off. It is often a normal result of the semiconductor output and, in many AC SSRs, an internal RC snubber. It becomes a problem when a sensitive load can respond to that current: an LED driver may glow or flash, a small coil may hum, or a PLC input may remain on.

Key Takeaways

• Some output leakage is normal. A semiconductor does not create the same physical contact gap as an electromechanical relay, and an AC snubber can intentionally pass current while the SSR is off.
• The load decides whether leakage matters. Compare worst-case leakage with the load's release current, off-state voltage/current limit, or electronic input threshold.
• Measured voltage is not the whole answer. A high-impedance meter can display a substantial voltage that is supported by very little current.
• Zero-cross does not mean zero leakage. Zero-cross describes AC turn-on timing, not the off-state current path.
• A bleeder belongs in parallel with the load. It must be calculated for worst-case leakage and continuously dissipated power, then checked for voltage rating, temperature, mounting, failure behavior, and approvals.
• Use a real disconnecting means where isolation matters. Do not treat a bleeder, an SSR status LED, or a controller command as proof of a de-energized circuit.

What Does SSR Off-State Leakage Current Mean?

Off-state leakage is the current through the SSR output at a stated output voltage and temperature when the input is not energized. That test condition is important. Leakage is not one universal number for every SSR or every operating point. Output voltage, frequency, waveform, semiconductor construction, internal protection, temperature, and the manufacturer's test circuit can all affect the published limit.

A mechanical relay separates metal contacts. An SSR normally switches with a triac, antiparallel thyristors, MOSFETs, or another semiconductor arrangement. The off-state output therefore behaves like a high impedance, not an infinite impedance. Panasonic's PhotoMOS terminology likewise defines off-state leakage as output current at a designated output voltage with no input LED current. This definition keeps output leakage separate from control-side current that is still driving the device.

Term	What it describes	Do not confuse it with
Output off-state leakage	Current through the SSR output after the input is genuinely off	Normal load current, earth leakage, or input current
Off-state voltage	Voltage developed when leakage flows through the connected impedance	Proof that the source can supply normal load current
Minimum load current	Lowest load current for which an exact SSR is specified to operate correctly	Maximum leakage; both values matter but describe different states
Input reset condition	Maximum control voltage/current that still guarantees the input is off	Leakage through an output that has already turned off
Holding current / commutation	Current condition that lets a triac or thyristor stop conducting	Small passive current through an off output

A panel-mount SSR. The case label alone does not reveal the maximum off-state leakage, minimum load, internal snubber, temperature condition, or AC/DC output topology; use the full part number and current data sheet. Photo: Mike1024 / Wikimedia Commons, public domain. Unmodified.

Why Does an SSR Leak When It Is Off?

1. Semiconductor blocking current

A blocking semiconductor has finite off-state impedance. Small junction and device currents remain even when it is commanded off. Leakage commonly changes with applied voltage and temperature, so a room-temperature bench result at reduced voltage cannot automatically represent a hot enclosure at maximum line voltage. Some MOSFET-output relays specify extremely low leakage, but the exact number still depends on the model and rating class.

2. Internal RC snubber current

Many AC SSRs place a resistor-capacitor network across the output to improve immunity to rapid voltage changes. The capacitor intentionally passes alternating current while the semiconductor is blocking. For an ideal capacitor, the approximate RMS current is I_C = 2πfCV. At 230 V, 50 Hz, and 0.1 µF, the ideal capacitive current is about 7.2 mA. At 120 V, 60 Hz with the same capacitance, it is about 4.5 mA. The real value depends on the complete circuit, tolerances, waveform, and series resistance, but the estimate explains why several milliamperes may be normal for a snubber-equipped AC SSR.

3. Protection, indication, and filter networks

MOVs, TVS devices, EMI capacitors, status circuits, load monitors, and external suppression can add other paths. The contribution may be small, but it is product- and system-specific. A complete schematic matters because a current path across an external filter or another branch does not disappear when the SSR input turns off.

4. Cable coupling and ghost voltage

Long switched conductors routed beside energized wiring form a small capacitor. A floating, high-impedance conductor can then acquire a measurable AC voltage even when the SSR is not supplying meaningful current. Fluke explains that most industrial digital multimeters have high input impedance and can display this capacitively coupled voltage. An appropriate low-impedance function can help a qualified person distinguish a weak coupled source from a hard source, but it also burdens the circuit and may alter or damage sensitive electronics. Use only an approved, circuit-specific test plan.

Not Every "SSR Won't Turn Off" Symptom Is Leakage

The phrase SSR leakage is often applied to every unwanted off-state symptom. That shortcut can hide an active drive problem or a failed device. Normal leakage is relatively small and consistent with the data sheet. A device that is still driven, cannot commutate, is backfed, or has failed short can conduct far more current. A bleeder resistor is not a cure for those conditions.

Observation	Likely category	Best decision evidence
High-impedance meter shows voltage, but available current is very small	Ghost voltage or normal leakage	Qualified test using the approved method plus real load behavior
Small current agrees with the SSR's maximum leakage and the input is off	Normal output leakage	Exact data sheet, actual voltage, temperature, and load threshold
Voltage/current remains at the SSR input after the PLC command is off	Input-side reset problem	Compare terminal values with the guaranteed input-off region
Triac/SCR AC output turns on a steady DC load and will not release	Wrong output topology	Output circuit diagram and the absence of a recurring current zero
Heavy conduction continues on a distorted AC waveform	Commutation failure or retriggering	Load waveform, power factor, dv/dt, and manufacturer limits
Near-normal load current flows with the input disconnected	Backfeed, wiring fault, or possible failed-short SSR	Controlled isolation of branches, schematic review, and replacement testing

Why Small and Electronic Loads Misbehave

Leakage matters relative to the load, not only as an absolute number. Five milliamperes is negligible compared with a 10 A heater current, but it may exceed the release current of an electronic input. A rectifier-and-capacitor input can accumulate charge until an LED lamp or small power supply flashes, discharges, and starts charging again. A coil may receive enough magnetic force to hum or delay release without having enough current for a clean pull-in.

Load	Typical off-state symptom	Engineering response to evaluate
LED lamp or driver	Faint glow, periodic flash, or incomplete shutdown	Compatible low-leakage switch or approved bypass accessory
Small relay or contactor coil	Hum, chatter, delayed release, or continued hold	Compare leakage with release data; consider an engineered parallel bleeder or interface
PLC or electronic input	Status remains on or flickers	Matched input module/interface or a lower-leakage MOSFET output
Small solenoid	Buzzing, slow release, or unexpected motion	Treat motion as a hazard; verify thresholds and required positive disconnection
Resistance heater	Usually no observable heating, although voltage may be measurable	Confirm current and temperature; never treat the SSR as isolation
Electronic power supply	Indicator glow, hiccup attempts, or intermittent restart	Validate the complete nonlinear input over line voltage and temperature

AC Triac/SCR vs DC MOSFET and PhotoMOS Leakage

Many mains AC power SSRs use a triac or antiparallel SCRs and may include a snubber. Their published maximum leakage can be in milliamperes. DC SSRs commonly use MOSFETs. Some signal-class MOSFET or PhotoMOS relays specify leakage in microamperes, while some high-voltage or high-current DC SSRs specify tenths of a milliampere. The words DC SSR or PhotoMOS therefore do not guarantee one generic value.

Output technology	Leakage tendency	Other limits that still control selection
AC triac or antiparallel SCR	Often milliamperes when an internal snubber is present	Minimum load, commutation, surge, dv/dt, waveform, heat sinking
DC power MOSFET output	Often lower, but model-specific from microamperes to tenths of a milliampere or more	Polarity/bidirectionality, on-resistance, current, transients, temperature
PhotoMOS or signal MOSFET relay	Can be in the microampere range for selected models	Load voltage/current, output capacitance, on-resistance, package loss, isolation
Electromechanical relay	Physical contact gap normally avoids SSR-style output leakage	Contact wear, bounce, switching life, arc limits, coil power, speed

Use model-specific numbers only. Manufacturer examples show why: some snubber-equipped AC SSRs publish several milliamperes of leakage and a minimum load in the tens of milliamperes, while selected PhotoMOS models publish single-digit microampere limits. Those examples explain the technology range; they are not substitute ratings for another brand or part number.

Does a Zero-Cross SSR Have Lower Leakage?

No-not as a general rule. Zero-cross describes when an AC SSR turns on. It waits until the instantaneous line voltage is near zero before beginning conduction. Random-turn-on outputs do not add that intentional delay and are used where phase timing or immediate turn-on is needed. Once the output is off, leakage depends on the blocking semiconductor, snubber, and other parallel networks.

A zero-cross family may happen to contain an RC snubber, while another family may use a different network. That is a product-design relationship, not a rule that zero-crossing creates or prevents leakage. Compare the maximum off-state leakage, minimum load, and output circuit diagram for the exact suffix.

A Safe Diagnostic Workflow

The diagnostic goal is not merely to obtain a voltage reading. It is to establish whether the output is really off, how much current the off-state circuit can supply under the real operating conditions, and whether that current exceeds the load threshold. The following is a planning sequence for qualified personnel; it intentionally does not provide live connection instructions.

1. Identify the exact part. Record manufacturer, full part number and suffix, data-sheet revision, output topology, maximum leakage, minimum load, input-off condition, internal protection, test conditions, and temperature limits.
2. Make the circuit safe for inspection. Apply the required de-energizing, isolation, lockout, discharge, and verification process before touching or changing connections. A control switch or interlock is not the isolating device.
3. Review all current paths. Compare the schematic with the physical panel. Include shared neutrals, alternate sources, external filters, indicators, suppressors, parallel branches, and cable routing.
4. Verify the input-off state at the device. A software indication does not prove that input voltage/current is below the exact SSR's guaranteed reset limit.
5. Define the load's off threshold. Obtain coil release data, PLC input off limits, LED-driver compatibility, or other current/voltage thresholds. Rated on-state current alone is not enough.
6. Use an approved measurement plan. Select instruments, categories, impedances, burdens, connection method, and PPE from the circuit energy and manufacturer's instructions. Distinguish displayed voltage from available current without applying an unsuitable low impedance to sensitive electronics.
7. Compare evidence, then choose the remedy. Confirm whether the result matches normal leakage, coupling, active drive, commutation, backfeed, or failure before changing the SSR or adding a shunt.

How to Calculate a Bleeder Resistor for SSR Leakage

A bleeder, bypass, or dummy load is connected in parallel with the load so that leakage has a lower-impedance path. It is not placed in series, and it is not an isolation device. For a simple first-pass resistive model, if the worst-case total leakage is I_leak,total and the load must remain below V_allow, then:

These equations intentionally ignore any shunting by the real load, making them useful for an initial conservative screen. Electronic loads are often nonlinear. An LED driver may include a bridge rectifier, capacitor, discharge network, and undervoltage circuit, so final behavior needs time-domain validation with the production load. Also check resistor tolerance, pulse capability, voltage rating, creepage/clearance, flame behavior, enclosure temperature, accessible-surface temperature, mounting, ventilation, and the consequence of open- or short-circuit failure.

Example 1: 230 V AC, 5 mA leakage, 20 V allowable

The first-pass maximum resistance is 20 V / 0.005 A = 4,000 Ω. At 230 V, a 4 kΩ resistor would dissipate about 230² / 4,000 = 13.2 W whenever energized. A 15 W part is too close to its nameplate rating for a hot enclosure. Even a nominal 25 W class is not automatically adequate: at 253 V, power rises to about 16 W, and the selected resistor still has to meet its manufacturer's ambient-temperature derating, mounting, ventilation, voltage, overload, and failure requirements.

This example also reveals a design decision. A continuously hot resistor may be less attractive than selecting an SSR with lower leakage, using a manufacturer-approved bypass module, or changing the switching architecture. The lowest component cost is not always the lowest panel risk or operating cost.

Example 2: 24 V DC, 0.2 mA leakage, 2 V allowable

The first-pass maximum resistance is 2 V / 0.0002 A = 10 kΩ. At 24 V, approximate dissipation is 24² / 10,000 = 0.0576 W. A 0.25 W resistor may appear comfortable from steady-state power alone, but the designer still must check maximum supply voltage, tolerance, ambient derating, transient energy, load nonlinearity, insulation, failure mode, and approvals.

Solution Hierarchy: Fix the Cause Before Adding Heat

Solution	Best fit	Critical checks
Correct the input interface	PLC, sensor, or induced current keeps the SSR input above its reset limit	Use only controller- and SSR-approved interface methods
Select a lower-leakage SSR	Normal leakage is too close to the load's off threshold	Voltage, current, on-loss, surge, dv/dt, capacitance, thermal design, approvals
Use a MOSFET or PhotoMOS-style output	Microloads, PLC inputs, or low-current signals fit the ratings	On-resistance, output capacitance, transients, load current, temperature
Add a parallel bleeder	Leakage is normal and predictable, and continuous heat is acceptable	Worst-case current, power, voltage, derating, mounting, energy use, failure behavior
Use an approved bypass accessory	The load or SSR manufacturer provides a compatible solution	Line voltage, load range, temperature, approvals, installation instructions
Use an interposing mechanical device	A physical gap or definite release is required	Contact rating/life, coil power, speed, noise, fault and isolation duties
Improve routing and interface impedance	Cable coupling dominates the displayed voltage	Separation, shielding, grounding, complete wiring rules
Replace the SSR and correct the root cause	Conduction exceeds specification or damage is confirmed	Surge, short circuit, heat sink, terminals, overcurrent protection, load fault

Load-Specific Design Decisions

LED lighting

Test the minimum and maximum number of drivers per channel, minimum and maximum line voltage, hot and cold conditions, full cable length, and normal diagnostics. No glow is not enough; look for periodic flashes after the input capacitor has time to charge. Request both maximum off leakage and minimum compatible load.

Small coils and solenoids

Obtain pull-in, holding, and release data for the exact coil. Check hot-coil and voltage-tolerance behavior, suppression, drop-out time, and repeated switching. If unexpected motion can create risk, do not depend on a calculated leakage margin alone; use the required positive-disconnect and safety architecture.

PLC and electronic inputs

Match the SSR output current with the PLC input's guaranteed off-state current and voltage, including its diagnostic or test-pulse behavior. A signal MOSFET relay or purpose-designed interface may be cleaner than adding a mains-rated power resistor to a logic input.

Heaters and larger resistive loads

Leakage is usually too small to produce meaningful heat, but the SSR can still fail short and is not a maintenance isolator. Thermal design, short-circuit protection, independent overtemperature power removal, and heat-sink monitoring often matter more than visible off-state behavior.

SSR Selection and RFQ Checklist

A supplier cannot judge leakage compatibility from a request such as "25 A AC SSR." Send the real operating envelope and ask for model-level evidence:

• Load: exact part or circuit, AC/DC, steady current, inrush, impedance, release/off threshold, nonlinear input, and minimum/maximum channel quantity.
• Supply: nominal and maximum voltage, frequency, waveform, transients, source arrangement, and grounding.
• Control: PLC or sensor output type, input voltage/current when on and off, diagnostics, cable length, and switching frequency.
• Environment: ambient and enclosure temperature, heat sink and airflow, altitude, contamination, vibration, and mounting.
• Off-state requirement: maximum permissible current and voltage, required release time, whether visible glow is unacceptable, and whether a physical gap is required.
• Protection: internal/external snubber, MOV/TVS, surge and dv/dt environment, short-circuit protection, and consequence of failed-short output.
• Evidence: full part number, current data sheet, maximum leakage test conditions, minimum load, input reset limit, thermal data, approvals, sample plan, traceability, and change notification.

Validate Samples in the Real System

A room-temperature no-load test is not enough. Use production-representative SSRs, loads, cables, terminals, suppression components, heat sinks, enclosures, and control outputs. Cover maximum line voltage, minimum load, hot and cold conditions, long off periods, repeated cycling, expected transients, power interruptions, and all credible diagnostic states. Record the leakage and load behavior rather than only a pass/fail lamp observation.

The acceptance limit should come from the load and risk analysis. For example, a PLC input must remain below its guaranteed off threshold, a coil must release within the required time, and an LED driver must neither glow nor periodically restart. If positive isolation is part of the requirement, no semiconductor leakage test can replace verification of the independent disconnecting architecture.

When Should an SSR Be Replaced?

Replace the unit when controlled testing confirms output conduction above the published maximum, a failed-short or intermittent output, damaged or overheated housing, degraded terminals, abnormal temperature rise, or unstable behavior with a valid input and known wiring. OMRON notes that SSR output failures are commonly short-circuit failures, so the system should include a suitable independent way to remove load power when the risk requires it.

Do not stop at replacement. Investigate overload, load short circuit, insufficient heat sinking, loose terminals, transient overvoltage, excessive dv/dt, incorrect AC/DC topology, commutation conditions, and protection coordination. A new part installed into the same uncontrolled cause can fail in the same way.

Frequently Asked Questions

Is SSR leakage current normal?

Yes. Some current can pass through a semiconductor output and its protection network while off. It is acceptable only when it stays within the exact product specification and does not violate the load, safety, or system requirements.

Why is there still voltage after the SSR is turned off?

Off-state leakage flowing through a high-impedance load can create voltage. Cable capacitance can also produce a ghost reading on a high-impedance meter. Determine available current and the complete current path instead of judging from voltage alone.

How much SSR leakage current is acceptable?

There is no universal number. The SSR must remain within its maximum leakage specification, and worst-case current must stay safely below the connected load's guaranteed off or release threshold under maximum voltage, temperature, tolerance, and wiring conditions.

Does a zero-cross SSR leak less current?

Not necessarily. Zero-cross controls turn-on timing. Off-state leakage depends on the semiconductor, snubber, and other output networks of the exact product.

Where is a bleeder resistor connected?

It is connected in parallel with the load so it can divert leakage. The value and construction must be engineered for worst-case current, continuous power, voltage, temperature, mounting, failure behavior, and approvals. Do not place a generic resistor into a mains circuit from an online example.

Can an AC SSR switch a DC load?

A triac- or thyristor-output AC SSR generally cannot turn off steady DC because the current does not provide the recurring zero crossing needed for commutation. Use an output topology and exact part number explicitly rated for the DC load.

Can leakage keep a contactor coil energized?

Yes. A small coil can hum, chatter, release slowly, or remain near its holding point if SSR leakage exceeds its release threshold. Compare worst-case current with the exact coil data and validate hot-coil behavior.

Will a MOSFET SSR eliminate leakage?

It can reduce leakage substantially for some models, but it does not guarantee zero current. Check maximum off leakage together with voltage, current, on-resistance, output capacitance, transients, bidirectionality, and temperature.

Does an off SSR provide electrical isolation?

Do not use it as a maintenance or safety disconnect. The input and output may have specified dielectric isolation, but the off output is a semiconductor path that can leak or fail short. Use the required disconnecting and lockout means.

Can I remove the RC snubber to stop LED glow?

Do not remove protection blindly. The snubber may be needed for transient and dv/dt immunity. Evaluate a documented lower-leakage/snubberless device, approved bypass, or different architecture with the manufacturer.

Bottom Line

SSR leakage current is usually a predictable property, not immediate proof of failure. The correct decision comes from four pieces of evidence: the input is genuinely off, the exact output leakage limit and test conditions are known, alternate current paths have been ruled out, and the worst-case current remains below the real load's off threshold. If it does not, correct the interface, choose a lower-leakage output, engineer an acceptable parallel bleeder, or use a mechanical/hybrid architecture. Where safe isolation matters, provide it independently of the SSR.

Technical References

1. IEC 62314:2022 - Solid-state relays, official scope for applicable all-or-nothing SSR components.
2. IEC 60947-4-2:2020 with Amendment 1:2024, official scope for semiconductor motor controllers, starters, and soft-starters.
3. OMRON: SSR does not turn off - input reset, output leakage, bleeder, and commutation causes.
4. OMRON: Small relay hum caused by SSR leakage.
5. OMRON: Why an AC SSR cannot switch a DC load.
6. OMRON: Solid-state relay safety precautions.
7. Panasonic Industry: PhotoMOS relay terminology.
8. Fluke: Dual-impedance measurements and ghost voltage.
9. OSHA 29 CFR 1910.333 - Selection and use of work practices.