Bleeder resistor

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A bleeder resistor is a resistor placed in parallel with a high-voltage supply for the purposes of discharging the electric charge stored in the power source's filter capacitors or other components when the equipment is turned off, for safety reasons. A bleeder resistor is usually a standard resistor rather than a specialized component.

Usage[edit]

DC power supplies[edit]

Power supply circuits used to supply DC power needed by electronic devices, particularly switching 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. 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. The capacitors in high voltage DC power supplies used in devices such as lasers, x-ray machines, and radio transmitters can have even higher voltages.

This stored charge 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, 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. By choosing the proper size for this "bleeder resistor", the voltage will quickly decay to safe levels when the supply is switched off, yet the resistor will not consume too much power while the supply is on.

High voltage supply in television sets[edit]

The power supplies of CRT type television sets and computer monitors generate 30 - 40 kV, which is 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 megohms range, and can therefore not be measured with the common technician's multimeter.

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.

Electrical regulations[edit]

Electrical regulations of different countries may vary from country to country. Regulation 4.15.3 from AS/NZS 3000:2007 electrical standards. (Australia / New Zealand) says something like;

All capacitors greater than 500nF must have a suitable discharge path or else a bleeder resistor

For working Volts of 650 Volts or less must discharge to 50 Volts or less in one minute.

For working Volts of 650 Volts or more must discharge to 50 Volts or less in five minutes.

Comments These limits seem a bit strange, example 5 minutes is a long time between switch off and servicing the electric equipment exposed on the repair bench. Designers of switch mode power supplies seem to think the circuit is a “suitable discharge path” when the circuit works 100%, however when there is a fault in the circuit and under repair on the bench, there is no “suitable discharge path” this is why “bleeder resistors” need to be added.

Design considerations[edit]

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. Therefore the bleeder should ideally be connected permanently.

Failure[edit]

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 un-suspecting 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.

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 resistance for high energy capacitors), before working on any circuit.

Dual bleeder[edit]

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.