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Why Are Capacitor Discharge Resistors Now Mandated as Essential Safety Device


Tuesday, May 16, 2023

In the field of electronics, Capacitor Discharge Resistors or Bleeder Resistors are resistors that are connected in parallel with the output of high voltage power supply circuits. The primary function of these resistors is to discharge the residual electric charge stored in the filter capacitors of the power supply.

For instance, in a switch mode power supply, a bridge rectifier is used to convert AC mains power into DC at a voltage of 320V (for 220/240V mains voltage) or 160V (for 110/120V mains voltage) before the voltage is reduced by the chopper. Filter capacitors are incorporated to smooth the pulsating output voltage from the rectifier. These capacitors must store enough energy at this high voltage to power the load during the Zero crossings of the AC input. Additionally, the capacitors chosen in many power supplies are designed to be large enough to supply the load during power breakdowns lasting for a second or so.

However, this stored charge can remain in the capacitors for a significant period even after the device is turned off, posing a life-threatening hazard for the user or maintenance personnel who may believe that the device is safe. To address this safety hazard, it is necessary to discharge the capacitors after the power has been turned off. A large or high resistance value resistor is typically connected across the terminals of the capacitor to accomplish this.

After the device is turned off, the charge on the capacitor will drain off through this Capacitor Discharge or Bleeder Resistor, rendering the device safe for servicing. It is worth noting that when the device is "ON," a small current flow through the Capacitor Discharge/Bleeder resistor, which results in a small amount of power wastage.

It is necessary for the designer to select an optimum resistance value, as there is a trade-off between the speed at which the capacitor can be drained off and the wastage of power during normal operation to reduce power wastage. The lower the resistance value of the Capacitor Discharge/Bleeder Resistor, the faster the bleed-down rate but with more wastage of power during normal operations, and vice versa for a higher resistance value.

HTR provides a Capacitor Discharge/Bleeder Resistor that is designed in such a way that the resistor can be mounted directly to the capacitor terminals, eliminating the risk of the Bleeder resistor being accidentally disconnected.

(Contents originally came from HTR’s Blog)

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