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Why Are Capacitor Discharge Resistors Now Mandated As Essential Safety Device

In Electronics, Capacitor discharge resistors or Bleeder resistors are resistors connected in parallel with the output of a high voltage power supply circuit with the express purpose of discharging the residual electric charge stored in the filter capacitors of the power supply.

As an example, a switch mode power supply uses a bridge rectifier to convert AC Mains Power into DC at 320V (where the mains voltage is 220/240V) or 160V (where the mains voltage is 110/120V), before the voltage is reduced by the chopper. One or more Filter capacitors are incorporated to smooth the pulsating output voltage from the rectifier. Now these capacitors must necessarily store enough energy at this high voltage to power the load during the Zero crossings of the AC input.

Further, 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.

This stored charge is often enough to deliver a lethal shock in devices such as Lasers, X-Ray machines, Radio transmitters and also the old style CRT screens.

It is most important that it must be recognized that this stored charge can remain in the capacitors for a significant period of time even after the unit has been turned off, and therefore poses a life threatening hazard for the user or maintenance personnel who may believe that because the device is turned off it is safe.

To meet the challenge of this safety hazard, it is necessary to discharge the capacitors after the power has been turned off and typically a large or high resistance value resistor is connected across the terminals of the capacitor.

So after the device is switched off, the charge on the capacitor will drain off through this Capacitor Discharge or Bleeder Resistor, thus rendering the device safe for servicing etc.
It is pertinent to note that when the device is ‘ON’ a small current flows through Capacitor Discharge/Bleeder resistor which results in the wastage of a small amount of power.

In order to reduce this power wastage it is necessary for the designer to select, an optimum resistance value due to the fact that there is a trade off between the speed at which the capacitor can be drained off and the wastage of power during normal operation.

The lower the resistance value of the Capacitor Discharge /Bleeder Resistor, the faster the bleed down rate but wastage of more power during normal operations and vice verse for a higher resistance value.

To meet this need, HTR provides a Capacitor Discharge/Bleeder Resistor which is so designed that the resistor can be mounted directly to the capacitors terminals and not through any connectors, thus eliminating the risk of the Bleeder resistor being disconnected accidentally.

HSRC Series

HSRC Series

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HSR Series

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