Re: Voltage regulators and capacitors




"Michael Black" <et472@xxxxxx> wrote in message
news:Pine.LNX.4.64.0804121653550.18651@xxxxxxxxxxxxxxxxxxxxxxx
On Sat, 12 Apr 2008, CS wrote:

Evening,

I am looking at STI Microelectronics L78Sxx 2A positive voltage
regulars to reduce 24 - 31v DC from a boat battery to varying DC
voltages.
It appears that these can be used with no external components, but a
diagram shows an output capacitor of 0.1uF to improve transient
response and an input capacitor of 0.33uF if the regulator is an
appreciable distance from poer supply filter.

Are these really needed and what type of cap?? - I assume a small
ceramic would do.

Yes, they are really needed.

When three terminal regulators were knew, it was easy to dismiss
the capacitors, "why do we need them, it's just a voltage regulator?".
But then they were really great oscillators without the proper bypassing.
Everyone soon learned the necessity of those capacitors.

If you have the regulator right next to the large filter capacitor
of a power supply, the input capacitor may not be needed, since the
filter
capacitor might do the job of keeping the regulator from oscillating. Or
it might not, since the filter capacitor is there to filter the 60Hz, and
is so large in value that the accumulated inductance completely negates
its value at higher frequency where it would be need to stop oscillation.

The old National databook would always list three values for that
input capacitor, depending on the type of capacitor used. The actual
value was less important than the capacitor's effectiveness at bypassing
the higher frequencies.

The 0.1uF on the output would be a ceramic capacitor.

I don't seem to recall having problems with a .1uF ceramic capacitor
on the input of a 3 terminal regulator, but I'm too lazy to dig out
the National databook to find out exactly what they said.

Michael


It's certainly easy and cheap enough to just put a small bypass cap of 0.1
to 1 uF at the input close to the pins. It's also a good idea to have some
sort of output capacitor to handle brief current surges of the load. But if
the output capacitor is much larger than the input capacitor, and
especially if you disconnect the regulator from the main filter capacitor,
it's a good idea to put a diode from output to input to avoid back biasing
the regulator, where the output voltage is higher than the input (by more
than a diode drop).

Paul


.



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