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Bleeder resistor
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In electronics, a bleeder resistor上市公司英文 is a resistor connected in手机进水了屏幕失灵怎么办 parallel with the output of a high-voltage power supply circuit for the purpo of discharging the electric charge stored in the power supply's filter capacitors when the equipment is turned off, for safety reasons. It eliminates the possibility of a leftover charge causing electric shock if personnel handle or rvice the equipment in the off state, believing it is safe. A bleeder resistor is usually a standard resistor rather than a specialized component.
Contents
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∙ 1 Usage
o 1.1 DC power supplies
o 1.2 High voltage supply in television ts
o 1.3 Electrical regulations
∙ 2 Design considerations
∙ 3 Failure
∙ 4 Dual bleeder
Usage[edit]
DC power supplies[edit]
The power supply circuits in electronic equipment that produce direct current (DC) needed by the device from the考研教室 alternating current (AC) supplied by mains u filter capac
itors to smooth the DC current. A large electric charge说谎心理学 can remain in the capacitors after the unit is turned off, constituting a shock hazard. For example switching mode power supplies u 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. The incorporate one or more filter capacitors to smooth the pulsing output voltage from the rectifier. The 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 cond. This stored charge is often enough to deliver a lethal shock. The capacitors in high voltage DC power supplies ud in devices such as lars, x-ray machines, radio transmitters and old style CRT computer monitors and televisions can have higher, more dangerous 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 ur or maintenance and rvicing personnel, who may believe that becau the device is turned off or unplugged i
t is safe. Therefore, to discharge the capacitor after the supply has been turned off, a large-value resistor is connected across its terminals. After it is switched off, the charge on the capacitor will drain off through this "bleeder resistor", causing the voltage to decay quickly to safe levels.
While the power supply is on, a small current flows through the bleeder resistor, wasting a small amount of power. The value of the resistor is chon to be high enough that the charge on the capacitor bleeds off quickly, but low enough that the resistor will not consume too much power while the supply is on.
High voltage supply in television ts[edit]
The high voltage power supplies of CRT type television ts and computer monitors generate voltages of 30 - 40 kV, which are 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 c
ommon technician's multimeter.
Instead of a resistor inside the transformer, the focus and screen control array may be ud for the same purpo, depending on the application and tolerances of the type of tube it is producing output for.
家长通知书The bleeders discharge the focus supply, but not the high voltage final anode feed. The CRT itlf 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.
什么是淡奶油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 el a bleeder r
esistor放射虫
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 The limits em a bit strange, example 5 minutes is a long time between switch off and rvicing the electric equipment expod on the repair bench. Designers of switch mode power supplies em 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.
