This is one of the simplest design choices you can make in a power supply, yet I rarely see folks implementing them in their projects. Today, let’s dive into the world of bleeder resistors, their importance, and how to choose the right one for your circuits.

A bleeder resistor is a resistor connected across the terminals of a capacitor or in the output of a power supply. Its function is to safely discharge the stored energy in the capacitor when the power supply is turned off. This ensures that there is no residual voltage left in the capacitor.
Why use a bleeder resistor? First, it’s about safety. Personally, I have experienced multiple “discharge shocks” when repairing boards with large capacitors. They pack a punch and it hurts. Hence for large capacitors, using bleeder resistors is mandatory. Second, it allows components to turn off completely. When a large capacitor holds charge, the rest of the circuit remains charged and isn’t effectively off when power is removed. This causes a delay before a proper reset. If you plug the power back in, your digital circuits might not have had the chance to turn off before power is reapplied. Hence, they tell you to turn OFF power and wait for X seconds before turning it ON again.
How to select one? My rule is to use a resistor that is not too low(because idle power loss will be higher) and not too high(Discharge time is longer). Firstly, figure out how fast you want to discharge the caps after powering them off. That value equals 5 times the RC time constant(5 is just to get the value to zero, Realistically you don’t need to unless you are in the high voltage domain. Check the table to figure out your sweet spot). Now you have your resistor value, this resistor needs to be power-rated using V^2/R based on the voltage held initially by your cap.
The design is that simple. So please next time use them in your design. It doesn’t cost much. Even a slow discharge circuitry is better than no discharge one.
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