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# What is a Bleeder Resistor? – Definition, Formula, and Applications

## What is a Bleeder Resistor?

In electronic circuits, a resistor which is connected in parallel with the output of a high
voltage power supply for discharging the filter capacitors of the power supply when the
circuit is turned off is referred to as a **bleeder resistor**. It is also known as a **leakage resistor**
or **capacitor discharge resistor** or **safety discharge resistor**.

The bleeder resistor is primarily used for safety reasons because it eliminates the possibility of a left over charge that may cause an electric shock. Generally, a bleeder resistor is a standard resistor instead of a specialized resistor.

## Value of a Bleeder Resistor

The **value of a bleeder resistor** should be selected properly so that its presence in the circuit
does not interfere the power supply and can discharge the filter capacitor in a short time.

However, if we choose a bleeder resistor of a small value, it will provide high speed bleeding, but it will consume more power. On the other hand, if we choose a bleeder resistor of a large value, then it will give less bleeding speed and less power consumption. Thus, the design engineer of the circuit has to choose a proper value of the bleeder resistor so that it is low enough to discharge the filter capacitor in a short time and high enough to not interfere with the power supply.

In order to determine the proper **value of the bleeder resistor**, consider the relationship
between the discharging voltage and resistor as given below.

$$\mathrm{V=V_{m}\, e^{\frac{-t}{RC}}}$$

$$\mathrm{\Rightarrow \frac{V}{V_{m}}=\, e^{\frac{-t}{RC}}}$$

$$\mathrm{\Rightarrow ln\left ( \frac{V}{V_{m}} \right )=\frac{-t}{RC}}$$

$$\mathrm{\Rightarrow R=\frac{-t}{C\times ln\left ( \frac{V}{V_{m}} \right ) }}$$

Using this expression, we can determine the value of the bleeder resistor for safe voltage and time required to discharge the capacitor.

## Applications of Bleeder Resistor

The bleeder resistor is primarily used for safety purposes in electronic circuits. But, it can also be used in several other applications such as

- To improvement of the voltage regulation of a circuit.
- As a tapped resistor in voltage divider, etc.

Let us discuss the use of bleeder resistor for **voltage regulation **and **voltage division**.

### Bleeder Resistor for Voltage Regulation

The voltage regulation is given by the difference of no-load voltage and full-load voltage divided by the full-load voltage, i.e.

$$\mathrm{VR=\frac{V_{nl}-V_{fl}}{V_{fl}}}$$

Where, V_{nl} is the no-load voltage and V_{fl} is the full-load voltage.

For the better voltage regulation of the circuit, the value of the full-load voltage (Vfl) should be as low as possible.

When a bleeder resistor is connected in parallel with the filter capacitor. Then, a voltage drop will occur in the bleeder resistor along with the load resistor. When the power supply is ON, a voltage drop occurs in the load resistor, and when the power supply is OFF, a voltage drop occurs in the bleeder resistor. In this way, the bleeder resistor increases the value of the noload voltage. Consequently, the difference of the no-load voltage and the full-load voltage is reduced that improves the voltage regulation of the circuit.

### Bleeder Resistor for Voltage Division

The bleeder resistor is also used as a tapped resistor to provide multiple voltages from a single voltage. The circuit configuration of the bleeder resistor as a voltage divider is shown in the following figure.

In the given circuit, the bleeder resistor has four tapping points. By using a proper tap point, we can obtain four different voltages across the load. In this way, the bleeder resistor may also serve as a voltage divider.

## Conclusion

From the above discussion, we may conclude that the bleeder resistor is a standard resistor that is used parallel with the filter capacitor of a power supply to discharge the filter capacitor when the circuit is turned off. We may use a bleeder resistor to improve the voltage regulation of a circuit and to divide a single voltage into multiple values.

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