Bleeder resistor

A bleeder resistor is a resistor placed in parallel with a high-voltage supply for the purposes of discharging the energy stored in the power source's filter capacitors or other components that store electrical energy when the equipment is turned off.

It is a use for a standard resistor rather than a separate type of component.

Usage

DC power supplies

Power supplies, especially switched-mode power supplies, use a bridge rectifier to convert mains AC power into a typical 340 volts DC for the chopper. A large filter capacitor typically stores enough energy at this high voltage to power the load during the zero crossings of the AC input. In fact, the capacitors in many supplies are large enough to support the load during AC outages lasting for a significant fraction of a second. This stored energy is clearly potentially lethal, and without a bleeder resistor it might remain long after the unit has been turned off. With a properly sized bleeder resistor, however, the voltage will quickly decay to safe levels when the supply is switched off, yet not consume too much power while the supply is on.

High voltage supply in television sets

The bleeder resistor commonly found inside a flyback transformer that supplies high voltage for a CRT 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.

Failure

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.

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.

Technical considerations

There is always a trade-off between the speed with which the bleeder operates and the amount of power wasted in the bleeder; a faster bleed-down rate 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.

Dual bleeder

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.