In many electronics circuits, we see high value resistors placed in parallel with capacitors. When we try to analyze the normal functionality of the circuit we tend to realize that the resistor is no where in the picture except causing power loss. But the same resistor plays a major role when the input power is off or the load is removed.
Bleeder resistors are commonly used in electronics circuits for safe guarding the circuit against high charges. Let us take an example of a switching mode power supply providing a 12V output from a 240V input having capability of 10A. Let us assume that the load connected consumes 2A. So, the output power at the regulator is 24W. Let us assume that the load is supplied continuously and at one instant load is OFF. In this scenario, the output capacitors have a very high charge and they doesn't have the path to discharge. This is where the bleeder resistor helps. In this scenario, we are talking about only 10W but there are AC-AC converters, other power modules supplying up to kW of power.
Some more points on the Bleeder resistor,
- Bleeder resistor is selected base don the charge holding capability of capacitor and hence it's capacitance value.
- Bleeder resistor is connected in parallel with the capacitor to ground to provide the discharge path for the capacitor.
- Bleeder resistor wattage rating is one of the critical rating.
- Bleeder resistor must be selected such that during normal operation, the power loss through the resistor must be very less or negligible. In other way, the quiescent consumption of the bleeder resistor must be very less.
- Bleeder resistor helps the circuit to discharge the charge and hence prevents any shocks due to charge storage. So, if safety is a major concern for your circuit, then bleeder resistor is a must.
- Designers always have a challenge on deciding the bleeder resistor value. If the circuit demands low power consumption, then having higher value of bleeder resistor is preffered. But the higher value of bleeder resistor restricts the quicker discharge. So, the value vs discharge time is always a design trade-off.
- For high-voltages (> 50V), a bleeder resistor is to be included in the circuit to remove the hazard of an unexpected voltage in the filter capacitor when the supply is turned off.
- Take the case of a switching mode power supply, for a AC input, post the bridge rectifier the smoothing shall be done using the filter capacitor, here the voltages are well beyond 100VDC and can they can hold a dangerous voltage level under no load conditions if not discharged properly. This is where bleeder resistor is a must. The below image shows the bleeder resistors R1, R2 in such scenario.
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