> To respond to your #1 suggestion... the LM340 data sheet specifies 
> the capacitors I chose (.22uF and .1uF) as required in the shown 
> "typical application" circuit for fixed output regulator. In fact, 
> it says that the output capacitance is not required at all, and 
> recommends a 0.1uF to "help transient response... if needed." Could 
> you please explain your recommendation so that I better understand 
> its application?

An ideal regulator needs no output capacitance at all as it responds 
to all load changes up to its rated limit.
A less than ideal regulator requires "some" to provide current while 
the regulator 'gets its act together' on very rapid changes. With very 
old regulators it may have been useful to add a substantial reservoir 
capacitor although doing so can have some disadvantages as well. The 
LM340 is new enough (although very old) to only need such for fairly 
spiky loads.

Some modern regulators need larger output capacitors for stability 
purposes - without them they will oscillate under certain load 
conditions. The LM340 is not in this category. In some (usually 
modern) regulators the output capacitor must have an ESR (equivalent 
series resistance) that is neither too high OR too low - too "good" an 
output cap can also cause oscillations! The LM340 is also not in this 
category.

If the regulator input voltage drops suddenly the output capacitor(s) 
will attempt to discharge back into the regulator. In some cases the 
energy in the capacitor can damage the regulator if precautions are 
not taken to prevent this discharge. The LM340 IS in this category - 
to protect it against large output capacitors a usually reverse biased 
diode should be connected from output to input. Capacitors of a few 
microfarad are usually OK BUT see the data sheet. Input voltage can 
fall rapidly if heavily loaded by other sources when the power is 
removed OR due to a fault short circuit on the input side. Worst case 
a regulator may fail such that it subsequently allows input voltage 
through when power is next applied so its worth not letting them fail 
this way - the internal thermal etc protection does not protect 
against such failures. Some regulators are protected against such 
failures. The LM340 is not in this category :-).

The LM293x family is probably a better choice of regulator than the 
LM340. It has protection against most problems and better drop out 
performance (nominally its an LDO regulator). Some members of the 
LM293x family are drop in replacements for an LM340. Given the cost of 
the rest of the circuitry, a very small amount extra spent on a few 
LM2931's or similar will be well spent. Note that they have their own 
output capacitor requirements - see data sheet.

An oscillating regulator can be a great mystery. The mean output 
voltage is about correct and a meter will tell you that all is OK. A 
peek with an oscilloscope will horrify you! ALWAYS check things with a 
scope when it matters. Similarly, regulator dropout due to AC ripple 
on the input and marginal operation may not be obvious but can wreak 
havoc - especially with analog circuits. An LDO makes it less likely 
that this will happen in any given case. (I have made an LM340 
oscillate but it usually takes special skill :-) ).





        RM


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