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Bleeder resistor
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In electronics, a bleeder resistor, bleeder load, leakage resistor, capacitor discharge resistor or safety discharge resistor is a resistor connected in parallel with the output of a high-voltage power supply circuit for the purpose of discharging the electric charge stored in the power supply's filter capacitors when the equipment is turned off, for safety reasons. It eliminates the possibility of a leftover charge causing electric shock if people handle or service the equipment in the off state, believing it is safe. A bleeder resistor is usually a standard resistor rather than a specialized component.

Operation

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Power supply circuits in electronic equipment that use rectifier circuits to produce direct current (DC) from the alternating current (AC) supplied by mains use filter capacitors to smooth the DC current. A large electric charge can remain in these capacitors after the unit is turned off, constituting a shock hazard. These must typically store enough energy at this high voltage to power the load during the zero crossings of the AC input. In addition, the capacitors in many supplies are made large enough to supply the load during AC outages lasting for a significant fraction of a second.

This stored charge, which is often enough to deliver a lethal shock, can remain in the capacitors for a long time after the unit has been turned off. It can be a potentially lethal shock hazard for the user or maintenance and servicing personnel, who may believe that because the device is turned off or unplugged it is safe. Therefore, to discharge the capacitor after the supply has been turned off, a large value resistor is connected across its terminals. After it is switched off, the charge on the capacitor will slowly drain off through this "bleeder resistor", causing the voltage to decay quickly to safe levels.[1]

While the power supply is on, a small current flows through the bleeder resistor, wasting a small amount of power and reducing the efficiency of the power supply.[2] The value of the resistor is chosen to be high enough that the resistor will not consume too much power while the supply is on, but low enough that the charge on the capacitor bleeds off quickly.

Bleeder resistors have a second advantage: In some designs, if the load on the power supply should become disconnected, the filter capacitors will overcharge without any discharge path, potentially damaging them or the power supply. The addition of a bleeder resistor provides a discharge path to alleviate this voltage rise, improving the voltage regulation of the filter.[3]

Usage

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Switched-mode power supplies use a bridge rectifier to convert mains AC power into DC at 320 V (for 220 V mains) or 160 V (for 115 V mains), before the voltage is reduced by the chopper. These incorporate one or more filter capacitors to smooth the pulsing output voltage from the rectifier. The capacitors in high voltage DC power supplies used in devices such as lasers, x-ray machines, electronic flashes, radio transmitters, and old style CRT computer monitors and televisions can have higher, more dangerous voltages.

High-voltage supply in television sets

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The high-voltage power supplies of CRT-type television sets and computer monitors can generate voltages between 30 kV - 40 kV, which are a much greater electrocution hazard. This higher voltage requires higher value bleeder resistors to avoid unnecessarily loading the supply circuits. The bleeder resistor, commonly found inside a flyback transformer, is valued in the hundreds of megaohms range and therefore cannot be measured directly with the common technician's multimeter.[4]

Instead of a resistor inside the transformer, the focus and screen control array may be used for the same purpose, depending on the application and tolerances of the type of tube it is producing output for.

These bleeders discharge the focus supply, but not the high-voltage final anode feed. The CRT itself forms a capacitor that can hold a sizable (and very dangerous) high-voltage charge, so it is always advisable to momentarily ground a CRT's high-voltage terminal before working on the unit.

Design considerations

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There is always a trade-off between the speed with which the bleeder operates and the amount of power wasted in the bleeder; a lower resistance value results in a faster bleed-down rate but wastes more power during normal, power-on operation.

The presence of a bleeder also guarantees a minimum load on the power source, which can help reduce the range of voltage change (regulation) when the normal load is changing and there is no active regulator. Use of a bleeder this way is a common design strategy for power supplies of vacuum tube power amplifiers, for instance.

Large capacitors can actually recover a substantial part of their charge after being discharged by the bleeder resistor, if the resistor is not left in place. This is due to a property called dielectric absorption, in which energy stored in the dielectric during use is released gradually over time through dielectric relaxation; therefore the bleeder should ideally be connected permanently.

Failure

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The failure of a bleeder resistor prevents the discharge of the capacitors, resulting in dangerous voltages being retained for many days. This is one of several reasons for the typical warning on most equipment: "Warning – No user-serviceable parts inside". An unsuspecting user may get an electrical shock from opened equipment due to failure of a bleeder resistor, or the common practice of not fitting them, long after the device has been turned off or unplugged.

Safe design suggests mounting a bleeder close to a dangerous capacitor, ideally directly to the capacitor terminals, and not through any connectors, so that it is difficult to disconnect the bleeder accidentally. Some safety capacitors have built-in capacitor discharge resistors.

Despite the presence of a bleeder, it is wise to prove that any potentially dangerous capacitors are discharged, perhaps by shorting their terminals (or through a suitable low discharge resistance for high energy capacitors), before working on any circuit.

Dual bleeder

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Because of the speed/power tradeoff, high-powered circuits may use two separate bleeder circuits. A fast bleed circuit is switched out during normal operation so that no power is wasted; when power is switched off, the fast bleeder is connected, rapidly bleeding down the voltage. The switch controlling the fast bleeder can fail, either by connecting when it shouldn't (and overheating) or by not connecting when it should (and thereby failing to bleed off the voltage quickly). To avoid the risk of not having an operational bleeder, a secondary, slower (and less lossy) bleeder is usually permanently connected so that there is always some bleed-down capability.

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bleeder resistor

View on Grokipedia
from Grokipedia
A bleeder resistor, also known as a discharge or safety resistor, is a standard high-value connected in parallel across the filter capacitors in a circuit to dissipate stored electrical energy after the power is switched off, thereby preventing electric shock hazards from residual voltage. This component ensures that capacitors, which can retain dangerous charges in high-voltage systems exceeding 50 V, discharge to a level within a specified time, typically seconds. The primary function of a bleeder resistor relies on the principle, where the resistor forms a parallel path for current to flow and gradually reduce the capacitor's voltage according to the Vt=V0e−t/(RC)V_t = V_0 e^{-t/(RC)}, with RR as the bleeder resistance, CC as , tt as time, VtV_t as voltage at time tt, and V0V_0 as initial voltage. To select an appropriate value, engineers use the rearranged R=−tCln⁡(Vsafe/V0)R = -\frac{t}{C \ln(V_{safe}/V_0)}, balancing rapid discharge (lower RR) against minimal power loss during operation (higher RR), where power dissipation is calculated as P=V02RP = \frac{V_0^2}{R}. Beyond safety, bleeder resistors improve by simulating a minimum load, reducing the difference between no-load and full-load output voltages, and can serve as voltage dividers when tapped for multiple output levels. Bleeder resistors find essential applications in various systems, including AC-DC converters, DC-DC supplies, motor drives, and high-power equipment, where they discharge large electrolytic capacitors (e.g., up to 100,000 μF) and handle pulse powers from watts to kilowatts. In these contexts, they must be rated for continuous operation at 10-30 W or more, often using robust types like wirewound or thick-film resistors to withstand while complying with safety standards.

Fundamentals

Definition and Purpose

A is a high-value connected in parallel across the output of a or bank to provide a controlled discharge path for residual voltage and stored charge when the power is turned off. This configuration ensures that filter capacitors, which can retain hazardous levels of energy, are safely depleted, preventing electric shock to personnel during or accidental contact, as well as potential to connected equipment. The primary purpose of a bleeder resistor is to enhance electrical by rapidly reducing voltage to non-hazardous levels, typically within seconds to minutes depending on the resistor value and and applicable safety standards. For instance, in mains-powered supplies where filter capacitors may hold up to 320 V DC after rectification of 220-240 V AC, a properly sized bleeder resistor can discharge this to safe levels (below 50 V) in under 60 seconds, mitigating risks from stored energy that could otherwise persist indefinitely—though standards vary, such as IEC 62368 requiring below 34 V in under 1 second for certain applications or some codes allowing below 50 V within 5 minutes. In addition to , it serves as a minimum load in some designs, helping maintain stable output by drawing a small continuous current that prevents no-load voltage drift. Bleeder resistors also address dielectric absorption in capacitors, a phenomenon where the dielectric material retains a portion of the charge even after apparent discharge, potentially leading to voltage rebound and renewed hazards. By providing a persistent low-current path, the resistor ensures complete dissipation of this lingering charge, a critical feature in high-reliability circuits where short-circuiting alone is insufficient.

Basic Operation

A bleeder resistor is connected in parallel across the output of a DC power supply or directly across a , providing a continuous path for current flow even when the circuit is operational. During normal powered operation, it draws a small continuous current given by I=VRI = \frac{V}{R}, where VV is the voltage across the capacitor and RR is the resistance value, resulting in minimal power waste while ensuring the discharge path remains active. Upon shutdown, the bleeder resistor and capacitor form a series RC discharge circuit, where the stored voltage decays exponentially according to the equation
V(t)=V0e−t/(RC),V(t) = V_0 e^{-t/(RC)} ,
with V0V_0 as the initial voltage and tt as time. The discharge process is characterized by the time constant τ=RC\tau = RC, and near-complete depletion typically occurs after approximately five time constants (5τ). For example, a 1000 μF capacitor charged to 320 V with a 1 MΩ bleeder resistor yields τ≈1000\tau \approx 1000 seconds, though values are often selected for faster safety discharge within seconds. The power dissipated by the bleeder resistor during operation is calculated as P=V2RP = \frac{V^2}{R}, representing a continuous but low-level loss. In the aforementioned example, this equates to approximately 0.1 W, illustrating the between discharge speed and . Additionally, the bleeder resistor mitigates dielectric absorption—a phenomenon where capacitors partially recharge after apparent discharge due to residual charge in the —by maintaining a permanent low-resistance path that equalizes voltage across the plates and prevents voltage reaccumulation.

Applications

Power Supplies

In linear power supplies, bleeder resistors are connected in parallel with the smoothing capacitors following the stage to discharge stored energy after shutdown, thereby preventing electric shock hazards during servicing or handling. This discharge function ensures that residual voltages drop to safe levels promptly, protecting technicians from potentially lethal charges in filter capacitors that can retain energy for extended periods without intervention. Additionally, bleeder resistors provide a minimum load current, which stabilizes by maintaining consistent operation of the pass elements and feedback circuits, particularly under no-load or light-load conditions where output voltage might otherwise rise excessively. In switched-mode power supplies (SMPS), bleeder resistors are commonly employed across the high-voltage DC bus—such as the 320 V output derived from 220 V mains after rectification—to discharge reservoir capacitors and mitigate inefficiencies associated with burst-mode operation at low loads. Burst mode, which pulses the switching action to reduce standby power, can introduce output voltage ripple and instability if the load falls below a threshold; the bleeder ensures a baseline current draw to keep the supply in continuous conduction mode for smoother regulation. Their use became widespread in the 1980s alongside the proliferation of SMPS in consumer electronics, driven by the need for compact, efficient designs in devices like televisions and computers, where traditional linear supplies were being phased out for better energy efficiency. A practical example appears in bench power supplies, where bleeder resistors are sized to reduce output voltage from 50 V to below 5 V within 10 seconds upon shutdown, allowing safe probing or disconnection without prolonged shock risk; this is achieved with resistances around 330 Ω for typical capacitances of several thousand microfarads. By sustaining a minimum current flow, these resistors also prevent false triggering in feedback loops, where zero-load conditions could otherwise cause erratic responses or oscillations in the control circuitry.

High-Voltage Devices

In cathode-ray tube (CRT) televisions and monitors, bleeder resistors with values in the hundreds of megaohms are connected across the outputs of flyback transformers to discharge the high-voltage supplies, which operate at 30-40 kV. These components ensure safe handling by gradually dissipating stored charge, thereby preventing potentially lethal electric shocks during maintenance tasks such as tube replacement, a practice emphasized in mid-20th-century documentation. Bleeder resistors also play a vital role in other high-voltage equipment, such as older machines and power supplies, where they discharge focus voltages to mitigate residual charge hazards. In these applications, values typically range from 100 to 500 MΩ, selected to achieve a balanced discharge rate that minimizes power loss during operation while reducing the risk of arcing upon power-down. Throughout the , bleeder resistors were essential in cathode-ray tube technology, where high-voltage s could retain energy levels comparable to a small lightning bolt, posing significant risks in consumer and industrial . Proper placement of the bleeder resistor directly across or near the high-voltage is critical to provide an efficient discharge path, avoiding inductive routes in wiring that could prolong hazardous voltage retention.

Modern and Specialized Uses

In , bleeder resistors play a critical role in solar inverters and battery banks operating at DC voltages such as 48-400 , where they facilitate the safe discharge of supercapacitors during or shutdown to prevent electrical hazards. This practice has become increasingly mandated by international standards like IEC 62109, which requires power converters for photovoltaic systems to ensure capacitors discharge to safe voltage and energy levels within specified times, often achieved through integrated bleeder circuits marked with warning symbols indicating discharge duration. In medical and industrial applications, bleeder resistors ensure rapid discharge of high-capacity capacitor banks in devices such as defibrillators and (UPS) systems, mitigating risks of residual charge post-operation. For instance, in defibrillators, these resistors safely dissipate stored energy from high-voltage s after delivery of therapeutic pulses, complying with safety protocols for handling medical equipment. Similarly, in UPS systems, bleeders discharge filter s upon power removal, preventing shock during servicing as required in safety designs. In the 2020s, (EV) charging stations employ bleeder resistors for 800 V DC rails, enabling passive or active discharge to protect personnel from hazardous voltages exceeding 60 V DC, in line with safety standards for conductive charging equipment. Emerging technologies leverage bleeder resistors for precise residual charge management, such as in high-power LED drivers where active bleeders—using inductors, capacitors, and switches—provide holding current for dimming while minimizing power loss compared to traditional resistors. In smart grids, integration with IoT enables real-time discharge verification; for example, bleeder resistors in monitoring systems for power transformers discharge detection capacitors via high-value resistors (e.g., 1 MΩ) in peak detector circuits, with data transmitted to or Bluetooth-enabled IoT networks for . Modern applications of bleeder resistors emphasize compliance with updated safety regulations, such as the EU Low Voltage Directive (2014/35/EU), which mandates discharge of capacitors to below 50 V within 60 seconds in correction systems, often implemented via built-in or modular bleeding resistors rated for nominal voltages up to 1000 V. As of , the market for high-voltage bleeder resistor assemblies has grown to USD 1.27 billion, driven by demand in and EV infrastructure. This focus on rapid, verifiable discharge addresses post-2015 advancements in high-energy systems, prioritizing personnel safety over legacy uses.

Design and Implementation

Selection Criteria

Selecting a bleeder resistor begins with determining the appropriate resistance value, which must balance minimizing continuous power during normal operation with ensuring rapid discharge of stored upon power removal for . Typical values are chosen to reduce quiescent power loss during operation while ensuring discharge within acceptable time limits; lower values accelerate discharge at the cost of increased generation, while higher values minimize losses but extend discharge times. To achieve discharge to safe levels (typically below 50 V per IEEE Std. 18 for hazardous circuits) in under 5 minutes, the resistance is selected such that the allows sufficient decay within this period, aligning with standards for medium-voltage applications. The voltage rating of the bleeder resistor must exceed the peak circuit voltage to prevent breakdown, with a factor of at least 2x recommended to account for transients and manufacturing tolerances. For , carbon film resistors are favored for cost-effectiveness and stability in low-power applications due to their simple construction of a carbon layer on a substrate, offering low and tolerances around ±5%. In contrast, wirewound resistors, constructed by coiling a metal wire around an insulating core, provide superior durability for high-voltage and high-power scenarios, handling surges better than film types. Flameproof resistors, often wirewound with specialized coatings, are preferred in safety-critical designs to minimize risk during overload or by self-extinguishing rather than sustaining . Placement of the bleeder resistor is crucial for effective performance, ideally positioned physically close to the terminals—directly across them if possible—to minimize lead lengths and reduce stray that could otherwise introduce minor delays in discharge paths. In enclosed power supplies, environmental factors such as ambient and must be considered for , as the generates continuous low-level power (e.g., via I²R losses) that could lead to thermal buildup without adequate ventilation. Bleeder resistors serve a key role by preventing residual voltages that pose shock hazards, as detailed in foundational discussions of their purpose. For reliability in demanding applications, the must withstand continuous operation without below 50% of its , verified through MIL-STD-202 environmental and protocols.

Calculations and Efficiency

The discharge time for a bleeder resistor in an follows the exponential decay formula derived from the discharging through the : t=−RCln⁡(VfVi)t = -RC \ln\left(\frac{V_f}{V_i}\right), where tt is the time to reach final voltage VfV_f from initial voltage ViV_i, RR is the bleeder resistance in ohms, and CC is the in farads. This calculation ensures the voltage drops below a threshold, typically under 50 V, to prevent hazardous residual charge. For instance, with a 1 μF at 1 kV initial voltage and a 2.2 MΩ bleeder targeting below 50 V, the discharge time is approximately 8.3 seconds. Power dissipation in the bleeder resistor during operation is given by P=Vdc2RP = \frac{V_{dc}^2}{R}, where VdcV_{dc} is the DC voltage across the . This represents continuous heat loss, creating a : a lower RR accelerates discharge but elevates power dissipation and , while a higher RR minimizes losses at the expense of slower discharge. In the previous example, the 2.2 MΩ dissipates 0.45 at 1 kV, requiring a component rated above this value for reliability. Bleeder resistors also provide a minimum load current to stabilize voltage regulators, calculated as Imin=VRI_{min} = \frac{V}{R}, which must exceed the regulator's threshold (often around 1 mA for certain ICs) to maintain under light loads. An optimal resistance can be derived from allowable dissipation as R=V2PmaxR = \frac{V^2}{P_{max}}, ensuring the handles the power without exceeding its rating while meeting current needs. Design must account for capacitor leakage, modeled as a parallel resistance that can prolong discharge; the bleeder RR should be selected at least 10 times lower than the leakage resistance to dominate the discharge path. coefficients further influence performance: TCR (typically ±50 to ±200 ppm/°C) and leakage (which increases at higher temperatures) require or matched components to avoid drift in time constants or power handling over operating ranges.

Safety and Reliability

Failure Modes

Bleeder resistors, which may be fixed , wirewound, thick-, or other types, most commonly fail in open circuit mode, where the resistance increases dramatically or becomes infinite, preventing discharge of associated capacitors. This failure often results from overheating due to power dissipation exceeding the rated value, causing degradation of the resistive element or cracking of the protective . For carbon resistors, prolonged exposure to temperatures above their maximum operating limit can cause degradation of the resistive element, leading to eventual open circuit. Aging exacerbates this, as environmental factors like cause resistance drift over time in types. Short circuit failures, though rarer at approximately 5% of incidents in fixed film resistors, can occur due to manufacturing defects such as internal voids or that create low-resistance paths under voltage stress. These defects may manifest immediately or after thermal cycling, leading to unintended current paths and excessive draw from the power supply. In high-voltage applications, shorted bleeders can bypass intended discharge paths, increasing overall circuit loading. The primary hazard of a failed bleeder resistor is the retention of lethal voltages in capacitors post-shutdown, as the discharge path is interrupted. In cathode ray tube (CRT) devices, for instance, the high-voltage can hold 25-30 kV, while power supply capacitors may retain 170 V; a failed bleeder leaves these charged, posing electrocution risk to technicians during repair. A documented case occurred in , where a worker suffered an electric shock from a 5 kV capacitor in a , with current passing through the body from the charged capacitor, highlighting the need for verification of discharge even when bleeders are present. Such incidents underscore risks in electronics repair, where undischarged capacitors can deliver energy sufficient for cardiac disruption. Detection of bleeder failure often involves observing persistent voltage across capacitors after power-off, with no expected drop within seconds to minutes, or unusual heat generation during operation indicating partial degradation. In high-voltage bleeders, dielectric breakdown of the surrounding insulation can create arcing paths, manifesting as audible snaps or visible sparks under . In modern switched-mode power supplies (SMPS), bleeder resistors may experience increased degradation in high-humidity environments due to corrosion in film elements, as environmental stress can contribute to parameter changes in fixed film types. While specific rates vary by design, humidity exacerbates open failures, particularly in compact, heat-dense SMPS units operating near power limits.

Mitigation and Best Practices

Routine testing protocols are crucial for preventing bleeder resistor failures and ensuring circuit safety. Operators should conduct periodic checks using a to verify continuity, which confirms the resistor's ability to provide a discharge path, and to measure insulation resistance, typically aiming for at least one megohm per kilovolt of operating voltage to detect degradation. Additionally, incorporating discharge indicators like connected in parallel with the bleeder resistor offers visual confirmation of discharge, as the lamp glows while voltage remains above the striking threshold and extinguishes once safe levels are reached. In high-voltage systems, redundancy enhances reliability by employing multiple bleeder resistors arranged in series to divide voltage stress across components or in parallel to provide alternative discharge paths if one fails, thereby maintaining overall functionality. Compliance with OSHA guidelines, such as those in 29 CFR 1910.333, requires explicit verification of capacitor discharge—often via the bleeder—before any servicing to mitigate risks from stored energy. These practices directly address potential failure consequences, such as residual charge retention that could cause electric shock during maintenance. Maintenance strategies further bolster long-term safety, particularly in harsh environments where exposure to , , or contaminants accelerates degradation; resistors should be visually inspected regularly and replaced periodically based on operational history to prevent open-circuit failures. A specialized approach involves integrating fuses into bleeder resistor designs, which automatically open the circuit if overheating occurs due to excessive current or fault conditions, thereby protecting surrounding components. In contemporary designs, active discharge circuits, such as those using MOSFETs or depletion-mode JFETs, are often employed as alternatives to passive bleeder resistors. These circuits minimize power dissipation during normal operation while providing rapid discharge upon power-off, improving overall efficiency. For , adherence to updated standards like IEC 61010-1 ensures bleeder resistors meet requirements for safe energy discharge in measurement and control applications, including provisions for stored isolation to protect users.

Variations

Dual Bleeder Systems

Dual bleeder systems combine a permanent high-value with a switched low-value to optimize both safety and energy efficiency in power supplies, particularly in switched-mode power supplies (SMPS). The permanent remains connected across the output filter capacitors during normal operation, providing a slow baseline discharge path that prevents charge buildup without excessive power dissipation. Paired with this is a low-value , which is disconnected during operation and activated only after shutdown via a control element such as a , PNP transistor, or . This configuration ensures rapid discharge for servicing while minimizing continuous losses. The primary advantage of dual bleeder systems lies in their ability to substantially reduce operational power waste compared to a single low-value bleeder. For instance, at a 100 V output, a 10 kΩ resistor would dissipate approximately 1 continuously, whereas the high-value permanent resistor limits this to about 0.01 —a 99% reduction in steady-state power loss. This efficiency gain became prominent in SMPS designs from the late onward, aligning with broader trends toward low-standby-power . Additionally, the system maintains compliance with requirements by ensuring quick voltage reduction post-shutdown. In operation, the sequence begins with normal powered state, where only the high-value is active, allowing capacitors to discharge gradually over hours if needed. Upon power removal, a control signal—such as a falling power-good indicator—triggers the switch, engaging the low-value to achieve a fast initial , often to below 50 V within seconds for typical filter capacitances of several hundred microfarads. The switched element then disengages, reverting to the permanent for final safe levels. This two-stage process is particularly valuable in applications requiring stringent , such as power supplies, where it supports standards like IEC 60601-1 by mitigating shock hazards from residual charge. Control circuitry integration typically involves a simple -based switch monitored by a peak detector or derived from the supply's auxiliary windings. For example, in one implementation, a PNP connects the low-value to the output when its base-emitter voltage exceeds a threshold upon power-off, ensuring discharge within 5 seconds without additional power draw during runtime. This setup avoids complex timing circuits and enhances reliability in high-voltage environments.

Switched Bleeder Configurations

Switched bleeder configurations employ a switching mechanism to connect a low-resistance bleeder across the power storage elements, such as or battery cells, only upon detection of power-off conditions, thereby minimizing continuous power dissipation. These systems typically utilize , , or timer-based circuits for activation. In relay-based designs, the coil is energized by the mains or auxiliary supply to keep contacts open during operation; upon power loss, the contacts close to engage the bleeder. implementations, often using N-channel or P-channel devices rated for at least 20% above the operating voltage, provide solid-state switching with an AC sensing circuit—such as a monitoring the secondary—that maintains the off while power is present and activates it after a brief delay (e.g., 130 ms) when AC ceases. For instance, in () systems, this setup can be triggered by AC loss to discharge a 48 V DC bus in seconds using a 150 Ω, 5 W across a 10 mF bank. The primary benefit of switched bleeders is near-zero power loss during normal operation, as the bleeder circuit consumes minimal quiescent power (e.g., 33 mW in designs), contrasting with always-on configurations that waste energy continuously. This efficiency is particularly advantageous in high-voltage or intermittent-duty applications. However, drawbacks include the need for reliable switching components; relays suffer from contact erosion due to DC arcing and limited lifespan, while introduce a short activation delay and require precise voltage rating to avoid failure. Solid-state options like offer high reliability in modern implementations, commonly integrated into battery management systems (BMS) for electric vehicles (EVs) in the 2020s. Implementation often incorporates a voltage sensing circuit to detect shutdown thresholds, such as a monitoring the output voltage against a (e.g., 10 for low-voltage systems) to trigger the switch. In BMS for EVs, passive balancing employs two MOSFETs per Li-ion cell to selectively connect bleeder resistors, dissipating excess charge as heat when a cell's voltage exceeds a threshold, for accuracy better than 0.12%. This approach avoids permanent waste in intermittent-use devices, such as solar chargers, where balancing or discharge occurs only during off periods or charging cycles. Recent advancements in integration, including gate drivers like the ADUM4121, enable fast balancing currents up to 750 mA in automotive BMS, supporting LiFePO4 or Li-ion packs while enhancing safety and range.

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  46. https://topmayelectronic.en.made-in-china.com/product/bFcJhtHogAWf/China-5-Thermal-Fuse-with-Resistor-for-Reliable-Overheat-Protection.html
  47. https://www.intertek.com/medical/iec-61010/
  48. https://patents.google.com/patent/US7200015B1/en
  49. https://www.mouser.com/pdfdocs/Safety_in_medical_equipment_120917_en_def_Vorschau.pdf
  50. https://doi.org/10.1155/2023/5547603
  51. https://doi.org/10.3390/electronics14132606
  52. https://www.monolithicpower.com/en/learning/resources/battery-balancing-a-crucial-function-of-battery-management-systems
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