A passive bleeder resistor gives an X-capacitor a continuously connected discharge path, but it also dissipates power while the equipment is plugged in. An active discharge circuit can block that path during powered operation and connect it after AC is removed, reducing energized-state loss at the cost of added components and more discharge and fault behavior to validate. The right choice depends on the equipment, applicable safety requirements, and EMI-filter design—not on a universal resistor value or a blanket assumption that active is safer.
What changes between passive and active discharge?
An X-capacitor is used across the AC line as part of an EMI filter. Because it can retain charge after power is disconnected, a design may include a path to discharge it. The central difference is whether that path remains connected during normal powered operation.
Passive: a resistor stays across the capacitor
A passive bleeder resistor is electrically connected across the X-capacitor. It provides a simple, predictable discharge path, but while mains power is present it also draws current and dissipates energy. For an ideal resistor exposed to a steady RMS voltage, the basic loss relationship is P = VRMS2/R; an actual mains circuit must be assessed using its operating conditions and component ratings.
Active: switch the bleed path according to AC state
Power Integrations describes its CAPZero arrangement as blocking current through the X-capacitor discharge resistors while AC is applied, then automatically connecting those resistors in parallel after AC is disconnected. The active circuit therefore aims to retain a discharge path when needed without continuously paying the passive resistor’s powered-state loss. Its behavior depends on the particular device and circuit; it should not be generalized to every active-discharge design.
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How the trade-offs compare
| Design consideration | Passive bleeder | Active discharge |
|---|---|---|
| Loss while AC is present | The resistor path remains connected and dissipates power. | Power Integrations claims CAPZero reduces loss to less than 5 mW while AC is applied, or “essentially zero” at 230 VAC, for its described arrangement. This is a manufacturer claim, not an independent comparative test. |
| Parts and circuit complexity | Structurally simpler: a resistor provides the path. | Requires an active device and associated circuitry, adding parts and design considerations. Comparative BOM pricing is not established. |
| Discharge and fault evaluation | Confirm the selected resistance, discharge behavior, resistor ratings, and accessible-node voltages in the complete design. | Confirm that the circuit discharges as intended after power removal and evaluate relevant failures of the active function. Do not assume the IC alone establishes compliance or makes the design safer. |
| EMI-filter choices and system cost | The resistor’s ongoing loss is one constraint when selecting X-capacitance. | Power Integrations says its approach can allow more flexibility in X-capacitor selection for differential-mode EMI filtering and may reduce inductor costs. The benefit is application-dependent; no general quantified system-cost comparison is established. |
What the power claims do—and do not—show
Power Integrations advertises less than 5 mW of loss for CAPZero while AC is applied, and describes loss as “essentially zero” at 230 VAC. The 230 VAC condition is attached to the latter description. These are claims from the product maker for its described implementation, not a general performance guarantee for active discharge circuits or a head-to-head test against a specified passive design.
The possible system-level benefit is not just lower bleeder loss. The manufacturer says the active approach may give designers more freedom to choose X-capacitance for differential-mode EMI filtering and thereby reduce inductor cost. That is a design possibility, not a guaranteed saving: the result depends on the whole filter and product, and no general cost figure or payback period is established.
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Safety depends on the product and applicable standard
Discharge requirements cannot be reduced to one resistor value or a universal timing rule. They depend on the equipment category, jurisdiction, applicable standard edition, circuit configuration, and which nodes may be accessible. The complete circuit—including a discharge IC and its possible failure modes—needs evaluation against the requirements for the product.
IEC 62368-1:2023
The IEC catalogue identifies IEC 62368-1:2023 as the fourth edition of its audio/video and ICT equipment safety standard, published on 2023-05-26, and lists a corrected version dated 2025-08. IEC describes the standard as classifying energy sources and prescribing safeguards. The catalogue page is not the complete standard and does not establish the exact requirements for an individual product.
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- Solder to mount
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UL 62368-1:2021 excerpt
A search-indexed excerpt of UL 62368-1:2021 describes checking accessible voltage two seconds after connector disconnection and evaluating an IC that performs capacitor discharge under fault conditions. Treat that timing and evaluation as context to verify in the authoritative standard applicable to the equipment—not as a universal specification for every X-capacitor circuit. The excerpt also refers to limits that depend on user classification and normal or single-fault conditions; it does not support one universal voltage threshold here.
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How to choose an approach
- Identify the product context. Determine the equipment category, intended markets, applicable standard and edition, and which circuit nodes can be accessed after disconnection.
- Define the required discharge behavior. Establish the required accessible-voltage behavior and measurement conditions from the authoritative standard for the product. Do not infer a universal discharge time from a different equipment context.
- Assess the passive option. Choose and verify a resistor using the actual mains conditions, X-capacitor, required discharge behavior, resistor voltage and power ratings, and other circuit constraints. Account for its continuous powered-state loss.
- Assess the active option and its failures. Verify that the specific circuit blocks the bleed path during operation and connects it after AC removal as intended. Evaluate discharge performance and relevant single-fault conditions under the applicable requirements.
- Recheck the complete EMI filter and cost. If active discharge changes the feasible X-capacitance, assess differential-mode filtering and the rest of the filter together. Treat potential inductor savings as design-specific rather than assumed.
What is not universal
- There is no single resistor value or discharge time that can be recommended without the circuit, equipment category, and applicable standard.
- The CAPZero loss figure and possible inductor-cost reduction are Power Integrations product claims, not broad comparative results.
- Choosing an active device does not remove the need to verify discharge behavior or evaluate faults.
- A final design decision requires the complete applicable standard and an assessment of the full circuit, including accessible nodes and component ratings.
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