In most cases, when a powered device is powered off, the filter capacitor at its output still retains some energy. If someone inadvertently touches the power plug, it may cause an electric shock. The simplest solution is to have a ‘Bleeder’ Resistor connected in parallel across this capacitor to discharge this stored electric charge. How do you select a suitable bleeder resistor in your power supply application? Are higher or lower bleed resistance values better?
To select a suitable bleeder resistor, you can consider the relationship between the momentary voltage across the capacitor Vt, the bleeder resistance R for discharge, the initial voltage Vu. t is the momentary period and the total capacitor capacitance is C. Then, you can use this equation to estimate the required value of your bleeder:
- Vt=𝐕u x 𝒆 (−𝒕/𝑹𝑪)
There is always a trade off between the speed the bleeder works at and the amount of power waste in the bleeder. Lower values of bleeder resistance will give you a faster time to reach safe voltages when the device is powered down, but they will also waste more power during operation.