Bleeder resistor
Encyclopedia
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 capacitor
s 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.
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.
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.
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.
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.
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
placed in parallel with a high-voltage supply for the purposes of discharging the energy stored in the power source's filter capacitor
Filter capacitor
Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:* Glitch removal on Direct current power rails...
s 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.
DC power supplies
Power supplies, especially switchmode power supplies, use a bridge rectifier to convert mains AC power intoa typical 340 volts DC for the chopper
Chopper (electronics)
A chopper circuit is used to refer to numerous types of electronic switching devices and circuits. The term has become somewhat ill-defined, and as a result is much less used nowadays than it was perhaps 30 or more years ago....
.
A large filter capacitor
Filter capacitor
Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:* Glitch removal on Direct current power rails...
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 transformerFlyback transformer
A flyback transformer , also called a line output transformer , is a special transformer, which is used for conversion of energy in electronic circuits. It was initially designed to generate high current sawtooth signals at a relatively high frequency...
that supplies high voltage for a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
is valued in the hundreds of megohms range, and can therefore not be measured with the common technician's multimeter
Multimeter
A multimeter or a multitester, also known as a VOM , is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance...
.
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.