Bleeder resistor

In electronics, a bleeder resistor is a resistor connected in parallel with the output of a high-voltage power supply circuit for the purpose 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 people handle or service the equipment in the off state, believing it is safe. A bleeder resistor is usually a standard resistor rather than a specialized component.

Usage

DC power supplies

The power supply circuits in electronic equipment that produce direct current (DC) needed by the device from the alternating current (AC) supplied by mains use filter capacitors to smooth the DC current. A large electric charge can remain in these capacitors after the unit is turned off, constituting a shock hazard. For example 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. This stored charge is often enough to deliver a lethal shock. The capacitors in high voltage DC power supplies used in devices such as lasers, 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 user or maintenance and servicing personnel, 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. 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 chosen to be low enough that the charge on the capacitor bleeds off quickly, but high enough that the resistor will not consume too much power while the supply is on.

High voltage supply in television sets

The high voltage power supplies of CRT type television sets 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 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.

Design 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 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

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

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.