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Capacitor values for Vin/out of 7809

SGarciaV

Hi all,

Currently I have a small video camera that requires 9v at roughly 120mA. My power supply consists of three 3.7v LG LGIP-520NV batteries that provide a total of 11 - 12 v.

The camera is the only load.

I am using a 7809 to supply the 9v from the said power supply. The data sheet mentions that Cin should be .33uF and Cout should be .1uF

Further research proposes other values, ranging from .1uF to 10uF for both Cin and out. The overall consensus is that these values are not critical, although the designer should not exaggerate in the Farad rating. I also read that the values depend on the stability of the power supply.

How can I determine what values for these capacitors are ok for my application?

Any advice and/or orientation is appreciated. Thanks! Sal.

  • Those regulators can tolerate a wide range of capacitance. The input capacitor is needed to prevent the voltage from dipping below the drop-out voltage and to attenuate any high frequency noise. This could require 2 capacitors, one to store large charge the other to handle high frequency noise. In your case a battery doesn't generate high frequency noise and it doesn't dip periodically so almost any cap will do (.33 uf is fine). The output cap helps the regulator respond adequately to sudden load fluctuations. In your case it isn't a big load and the camera has its own internal regulator and capacitor to run any circuitry that needs a good regulated supply, so again almost any capacitor ill work well. (.1 uf is fine)

  • in reply to dougw

    Thanks Douglas, much appreciated. Sal.

  • Your data sheet probably says that neither capacitor is actually required if the regulator is close to the input filter capacitor.

    What is optimal is a function of how quickly and how much the output current changes as well as the input filter capacitor distance and size.

    If it's a digital circuit, something like a 1uF/10V X7R capacitor on the output and a 47uF/50V electrolytic on the input will work just fine. They're often specified with 0.33uF on the input and 0.1uF on the output but there is nothing magic about those values, and the 7809 is unconditionally stable regardless of output capacitor ESR, so more capacitance means better transient load response. Ceramic capacitors much larger than 0.1uF used to be expensive, but no longer (the 7809 is a 1970s era tellpopeyes design).