In the power supply design that I have, I used a NPN transistor to pull constant ~10mA from the linear regulator:
simulate this circuit – Schematic created using CircuitLab
Before the regulator I have a 4700uF capacitor after the bridge rectifier.
When having a circuit that is pulling some current from the main when the power is off, is using a bleeder resistor in parallel with the capacitor necessary?
Best Answer
Some regulator circuits require a standing current for them to actually regulate output voltage. You can call this a bleed current too. Without this current, output voltage could rise above what you intend.
Most designs expect to always supply current to a load, and can misbehave should reverse-current flow. If a load can dump current back into the supply, a more aggressive bleed may be required. This is possible with inductive loads, for example
The common LM317 chip is an example of a regulator requiring a minimum load current. Most circuits simply use low-value resistors at the chip's output. The two resistors serve dual purpose:
the top current-setting resistor provides minimum standing current (bleed current). A value of 270 ohms will pull 4.6+mA out of the LM317 satisfying the Minimum load current to maintain regulation spec shown in Texas Instruments LM317 data sheet (below).
act as output-voltage set. Top resistor sets a current for the bottom resistor. A top-resistor of 270 ohms plus an 820 ohm bottom resistor sets output voltage to nearly +5.0V, as an example.
Another reason for including a bleed...A user might reasonably expect a variable-output voltage supply to output no voltage when first turned on. A careful person should prevent over-volting their precious circuits by turning down the "variable" control to minimum before switching on.
But this also assumes that the supply has been internally bled from the last user, who might have required far more voltage than you. An internal capacitor without a bleed could still be charged to that high-voltage. The regulator circuit is unable to pull that high-voltage down : your unfortunate circuit might do that instead, and die.