Bleeder Resistor Basics
Role of Bleeder Resistors in Power Electronics Circuit:
- In high voltage DC power supply, capacitors are used to smooth out the fluctuations in the output side. In AC to DC rectifiers, the DC link capacitors are used to maintain the smooth DC waveform.
- These capacitors store an electric charge for a while. In some power supplies, these filter capacitors hold the full output voltage of the supply, like 600V even after the supply has been turned off.
- We should not touch the DC link capacitors or Output filter capacitors immediately after turned off the supply.
- The Bleeder resistors are connected across the filter capacitors to drain their stored charge so that servicing the supply is not dangerous.
- The Bleeder resistor is also known as Power Supply Drain Resistor.
- The bleeder resistor R should have a value high enough so that it does not interfere with the operation of power supply, but low enough so it will discharge the capacitor C, in a short time after the power supply has been shut down.
- In the figure, the Capacitor C is referred to as a reservoir capacitor. During the process of rectification, it stores charge during the positive half-cycles of secondary voltage and releases it during the negative half-cycles.
- Thus the power supply circuit is able to maintain a reasonably constant output voltage across load. But due to parasitic effects C1 will discharge by a small amount during the negative half-cycle periods from the transformer secondary.
- This gives rise to a small variation in the d.c. output voltage (known as ripple voltage).
- Since ripple is undesirable we must take additional precautions to reduce it. One method of reducing the ripple voltage is increasing the discharge time constant. This can be achieved by increasing the value of C.
- Thus for normal operation, the high value of C is important. At the same time, when the power supply is turned off, this charge has to be removed.
- It is good idea to short out all filter capacitors, using screwdriver with an insulated handle and wearing heavy, insulated gloves, before working on a dc power supply.
- Even if the supply has bleeder resistors, they might take a while to get rid of the residual charge.
It is good to refresh about resistors now:
Resistors are electrical devices that act to reduce current flow and at the same time act to lower voltage levels within circuits. The relationship between the voltage applied across a resistor and the current through it is given by V = IR. There are numerous applications for resistors. Resistors are used to set operating current and signal levels, provide voltage reduction, set precise gain values in precision circuits, act as shunts in ammeters and voltage meters, behave as damping agents in oscillators, act as bus and line terminators in digital circuits, and provide feedback networks for amplifiers. Resistors may have fixed resistances, or they may be designed to have variable resistances. They also may have resistances that change with light or heat exposure (e.g., photoresistors, thermistors).