Bleeder Resistor: What is it, and Where is it Used?

A bleeder resistor is a resistor connected across the terminals of a capacitor or across a high-voltage power supply to safely discharge stored energy when the device is turned off. It is used to ensure that capacitors or high-voltage circuits do not retain charge, which could be hazardous during maintenance or if someone accidentally comes into contact with the device.

What is a Bleeder Resistor?

A bleeder resistor is typically a high-value resistor, meaning it has a high resistance to limit the current flow. Its primary purpose is to provide a path for electric charge to drain away safely over a period of time, thereby preventing any residual charge from remaining in the capacitor or high-voltage circuit.

Functions of a Bleeder Resistor

1. Safety: By discharging capacitors, bleeder resistors help prevent electric shock hazards that can occur if someone touches the terminals of a charged capacitor.
2. Circuit Protection: They prevent potential damage to components that might occur from residual voltage.
3. Stabilization: In some circuits, they help stabilize voltage levels by providing a continuous path for leakage currents.

Where is it Used?

1. Power Supply Units: Bleeder resistors are commonly used in power supply units, especially those with large filter capacitors. When the power is turned off, the resistor discharges the capacitors safely.
2. High-Voltage Equipment: Any equipment that operates at high voltages, such as CRT monitors, X-ray machines, and other high-voltage circuits, uses bleeder resistors to ensure capacitors discharge after power is turned off.
3. Pulse Forming Networks: In systems like radar and pulse generators, bleeder resistors ensure capacitors in the pulse forming networks discharge after each pulse.
4. Energy Storage Systems: Systems that store energy in capacitors, like some renewable energy systems, use bleeder resistors to ensure the safe discharge of stored energy.
5. Electronic Test Equipment: In equipment used to test high-voltage circuits, bleeder resistors are used to discharge test points safely.

Key Characteristics

• High Resistance: Typically in the range of tens of kilo-ohms to mega-ohms to ensure low current flow.
• Power Rating: Must be sufficient to handle the energy it dissipates during discharge without overheating.
• Voltage Rating: Should be rated for the maximum voltage it will encounter in the circuit.

Example Calculation

For instance, if a capacitor of 100 μF is charged to 300 V and you want it to discharge to less than 50 V within 10 seconds, you can calculate the required resistance using the formula for exponential discharge:

$V(t) = V_0 \times e^{-\frac{t}{RC}}$

Where:

• $V(t)$ is the voltage after time $t$.
• $V_0$ is the initial voltage.
• $R$ is the resistance.
• $C$ is the capacitance.
• $t$ is the time.

Rearranging to solve for $R$:

$R = \frac{t}{C \times \ln\left(\frac{V_0}{V(t)}\right)}$

Plugging in the values:

$R = \frac{10}{100 \times 10^{-6} \times \ln\left(\frac{300}{50}\right)}$ $R \approx 47.8 \text{ kΩ}$

Thus, a resistor of around 47.8 kΩ would be appropriate for safely discharging the capacitor to less than 50 V in 10 seconds.