A passive bleeder resistor is simpler, but it dissipates power whenever the mains supply is present. An active discharge circuit can block that path while powered and reconnect it after AC is removed, reducing energized-state loss at the cost of extra circuitry and additional safety evaluation. Neither approach is automatically right for every design: the choice depends on the required discharge behavior, applicable product standard, and the effect on the EMI filter and total design cost.
How do passive and active X-capacitor discharge compare?
| Design consideration | Passive discharge | Active discharge |
|---|---|---|
| Powered-state dissipation | A continuously connected resistor path dissipates power while the supply is present. | Power Integrations says its CAPZero arrangement blocks current through the discharge resistors while AC is applied, with loss below 5 mW under the conditions described by the manufacturer. Its page characterizes the loss as essentially zero at 230 VAC; the asterisk on that claim refers to IEC 62301 Clause 4.5 treatment of standby power below 5 mW. |
| Circuit complexity | Structurally simpler: the discharge path is a resistor across the capacitor. | Requires an active device and associated circuitry, adding components and a function that must be assessed. |
| Discharge after AC removal | The resistor remains connected and provides a continuous path. | In the CAPZero operation described by Power Integrations, the device automatically connects the bleed resistors in parallel after AC disconnects. |
| EMI-filter options | The resistor’s continuous 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 cost. The benefit depends on the application. |
| Comparative purchase or system cost | No directly comparable BOM pricing or total-cost figure is established. | No directly comparable BOM pricing or general system-cost saving is established. |
What changes in the circuit?
Passive bleeder
A passive bleeder provides a resistor path across the X-capacitor at all times. That makes its discharge mechanism straightforward, but the same path draws power during normal powered operation. Choosing a resistance is therefore a design trade-off involving the desired discharge behavior, continuous dissipation, resistor voltage and power ratings, and other circuit constraints.
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There is no universal resistor value or discharge time that can be specified without the capacitor, equipment, operating conditions, and applicable safety requirements. A value suitable for one product should not be treated as a general recommendation.
Active discharge
An active arrangement changes when the resistor path conducts. Power Integrations describes CAPZero as blocking current through the X-capacitor discharge resistors while AC is present, then connecting those resistors in parallel after AC is disconnected. This is a manufacturer description of that product arrangement, not a performance guarantee for every active discharge circuit.
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The reduced powered-state loss can give a designer more freedom to consider X-capacitance as part of differential-mode EMI-filter optimization. Power Integrations says that this flexibility may reduce inductor costs, but does not provide a general, quantified comparison of total system cost. Any component or cost effect must be evaluated for the particular filter and product.
How should discharge safety be evaluated?
Evaluate the voltage at accessible points after the relevant disconnection event and assess what happens if a component or the discharge function fails. An active IC should not be assumed safer merely because it switches the resistor path: the discharge behavior and the circuit’s failure modes still need assessment under the standard that applies to the equipment.
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A search-indexed excerpt from UL 62368-1:2021 describes checking accessible voltage two seconds after disconnecting a connector, with limits that depend on user classification and normal or single-fault conditions. It also describes fault evaluation for an IC used to discharge a capacitor. This excerpt is not a universal design specification; confirm the exact requirements in the authoritative standard text applicable to the product and market.
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Choose the product safety standard and edition for the equipment category and intended market before settling discharge criteria. The IEC catalogue identifies IEC 62368-1:2023 as the fourth edition of its safety standard for audio/video and information and communication technology equipment. It was published on 2023-05-26, and the catalogue shows a corrected version dated 2025-08. IEC describes the standard as classifying energy sources and prescribing safeguards, but its public catalogue entry is not the complete standard and cannot establish the requirements for an individual product.
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- Use for Start Capacitor voltages up to 330 VAC, Capacitance p to 1200 uf
- Solder to mount
- 0.1255" Body Diameter, 0.363" Body Length, 2.363" Overall Length (including leads)
- Lead Length " per side, 0.019" Lead Diameter
- Start capacitor NOT included.
Accordingly, do not apply the UL excerpt’s two-second check or any other isolated value as a blanket rule. Confirm the relevant clauses, accessible nodes, user classification, disconnection condition, and fault cases in the full applicable standard.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose between the approaches
- Identify the compliance target. Determine the equipment category, destination market, and applicable standard edition; use the full authoritative standard to establish discharge and fault requirements.
- Define the discharge case. Identify which capacitor nodes can be accessed, what event disconnects the supply, and what behavior must be demonstrated after disconnection.
- Compare the loss and implementation trade-off. For a passive path, account for continuous powered-state dissipation and resistor ratings. For an active path, account for the added circuit and evaluate its discharge operation and failure behavior.
- Revisit the EMI filter as a system. Check whether active discharge changes the feasible X-capacitance or filter choices, then evaluate any component-cost effect in the actual design rather than assuming a general saving.
- Verify the complete design. Assess the circuit under the relevant operating and fault conditions and confirm accessible-voltage behavior against the applicable requirements.
A passive resistor is the simpler route when its continuous loss and the required discharge performance fit the design. An active circuit is worth considering when reducing that loss or gaining filter-design flexibility matters enough to justify added components and qualification work. The deciding evidence is the complete circuit’s verified behavior and application-specific cost—not the label “active” or “passive.”
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