| ??? 04/28/08 04:02 Modified: 04/28/08 04:03 Read: times |
#154070 - maximal input voltage, many small bypass cap\'s Responding to: ???'s previous message |
The 7805 works with a minimal 3-volt overhead voltage at its input. That means that, no matter what the conditions, it should never, Never, NEVER droop below 8 volts, even for a microsecond. Consult the datasheet for precise details and for supply decoupling/bypass recommendations.
National semiconductor has recommended that one provide 8000 microfarads of filtering on the rectified unregulated DC input per volt of tolerable ripple. That means that, under FULL LOAD the lowest voltage at the input that ever occurs must never be less than 3 volts above the regulator's output, irrespective of what the actual regulated output voltage is, whether it's above or below the nominal 5 volts. Small (0.01 uF to 0.1 uF) capacitors should be present to suppress power-ground noise. You can improve line regulation (reducing the impact of input voltage fluctuations) by placing a small-valued series resistor between the rectifier and its filter capacitor. This will also help protect the rectifier diodes from the power-on surge. RE |
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