- Bleeder Resistor Definition: A bleeder resistor is a standard resistor used to safely discharge capacitors in a high-voltage power supply when the device is turned off.
- Safety Purpose: Bleeder resistors prevent accidental electric shocks by discharging capacitors after the device is powered down.
- Choosing a Resistor: Proper selection of a bleeder resistor balances discharge speed and power consumption.
- Voltage Regulation: Bleeder resistors help maintain stable voltage levels between no-load and full-load conditions in a circuit.
- Voltage Division: Tapped bleeder resistors can act as voltage dividers, providing multiple voltage levels within a circuit.
What is a Bleeder Resistor?
A bleeder resistor is a resistor connected across a power-supply output or storage capacitors. It provides a defined discharge path after power is removed. In some supplies, it also provides a minimum load while the equipment is on.
A charged capacitor can retain a hazardous voltage after the switch is off. A correctly designed bleeder reduces that voltage over a specified time, but it can fail or become disconnected. Always isolate the supply and verify absence of voltage with suitable test equipment before touching the circuit.
Importance of Bleeder Resistor in a Circuit
Consider a filtered full-wave rectifier. Its rectified output is pulsating DC. A filter reduces ripple to the level the load can tolerate, but it does not produce ripple-free DC.
Filters may use capacitors, inductors or both. The circuit below shows a capacitor across the rectifier output, with the load and bleeder connected in parallel.
While the supply operates, the capacitor charges and the bleeder continuously draws current. After the source is isolated, the stored charge decays through the bleeder and any other connected load.
Without a dependable discharge path, exposed terminals can remain hazardous. Contact may complete a discharge path through the body and cause electric shock.
A parallel resistor gives the capacitor a controlled discharge path, provided its resistance, voltage rating, power rating and reliability suit the circuit.
How to Choose a Bleeder Resistor
Lower resistance shortens discharge time but increases operating current and power loss. Higher resistance reduces continuous loss but takes longer to reach the target voltage.
Start with maximum initial voltage, total effective capacitance, required final voltage and permitted discharge time. Then check continuous or pulse power, energy, working voltage, creepage, clearance, tolerance, temperature and credible resistor failures. Series resistors may be needed to share voltage stress.
For an ideal capacitor discharging only through one resistor, let the voltage after time t be Vt and the initial voltage be Vu. Let C be total capacitance and R the bleeder resistance. The intended exponential relation and its rearrangement appear below.
![\[ V_t = V_u e^{\frac{-t}{RC} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-5037c7e8bec578b2709f41f158d058db_l3.png "Rendered by QuickLaTeX.com")
![\[ \frac{V_t}{V_u} = e^{\frac{-t}{RC} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-dd50f8b9d8dcf5837c091a3d9b65852f_l3.png "Rendered by QuickLaTeX.com")
![\[ ln(\frac{V_t}{V_u}) = \frac{-t}{RC} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-098f0d413efba05c6c9729a3566be0be_l3.png "Rendered by QuickLaTeX.com")
![\[ R = \frac{-t}{C \times ln(\frac{V_t}{V_u})} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-0bfdf8ed447e04edcfcb63b4f07bc9b4_l3.png "Rendered by QuickLaTeX.com")
The first three protected LaTeX lines above have unmatched closing braces. Their intended result is exponential decay. Choose a non-zero target voltage and time from the applicable safety requirement; the ideal RC model approaches zero only asymptotically. Confirm the design with worst-case capacitance, resistance and initial voltage.
![\[ P = \frac{V_0^2}{R} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-c344e2d50f82cea6bd83e75757c196b9_l3.png "Rendered by QuickLaTeX.com")
The equation gives initial bleeder power at voltage V0. A permanently connected resistor must withstand the normal steady voltage and power with suitable derating. A switched discharge resistor also needs adequate pulse-energy and peak-power capability.
Select the final component only after checking discharge time, normal loss, thermal rise, resistor voltage stress and the consequences of an open-circuit failure.
Applications of Bleeder Resistors
Bleeder resistors can also provide minimum loading for an unregulated supply. A resistor string may combine discharge with a voltage regulation function or provide reference points as a voltage divider, subject to loading and safety limits.
Voltage Regulation
For the convention used below, voltage regulation equals the difference between no-load and full-load voltage divided by full-load voltage. Express the result as a percentage when required.
![\[ V_r = \frac{V_0 - V_f}{V_f} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-f56a9d6b51dd11e7d4ec8ee56091f0ef_l3.png "Rendered by QuickLaTeX.com")
Here V0 is no-load voltage, Vf is full-load voltage and a smaller Vr means less output change between those conditions.
A bleeder across the filter capacitor remains a load even when the external load is disconnected.
While the supply is on, this minimum current can reduce the rise in output voltage that some unregulated supplies show at no load. The effect occurs during powered operation, not after the supply is off.
The smaller no-load rise can reduce the difference between no-load and full-load output. This benefit costs continuous power and does not replace a regulator when accurate output voltage is required.
Voltage Division
A tapped resistor or series bleeder string can provide several nominal voltages, as shown below.
The three taps act as a voltage divider. Each connected load changes divider current and tap voltage, so calculate the loaded divider and check every resistor’s voltage and power rating.
Safety Purposes
For a safety-related discharge function, the resistor reduces stored voltage after the source is isolated. The figure shows the basic parallel connection.
The filter may contain inductors and capacitors. A bleeder is connected across each capacitor or bank that needs a defined discharge path after shutdown.
Do not assume the resistor has discharged every node. Follow the equipment procedure, wait the specified time and verify voltage with a properly rated instrument before contact.
