Page 191 - Troubleshooting Analog Circuits

I78 14. Real Circuits and Real Problems

Output drifts, falls Unit much too hot? Increase the heat sink.
out of regulation. Study the dissipation=
(1103 X (Vin-VouJ.
Output shifts badly R1 connected too Connect R1 to LM317’s
vs. Iload. close to load? (Bad output pin directly,
Kelvin connections) (use different wire
than for the load).
Load causes oscill’n? Monitor with scope at
various loads; try
Pease’s Principle.
Items above do not fix Could be a bad part? Replace the regulator.
the problem.
etc.. etc.

Comments on Troubleshooting Table for LM3 I7 Regulators
The LM317 is usually easy and friendly to apply, but when people get absent-minded
and forget some of the items listed in the table, they can have problems. This covers
most of the problems that people call us about. The LM350, LM338, and LM396 are
of course covered by this same basic adjustable scheme. But beware, as the LM396 is
not pin-compatible with all the others!
Some parts of this table apply to fixed regulators, such as the popular LM340s and
LM7800s which are available in 5-V, 12-V, and 15-V versions.
If you use a circuit like this a lot, you ought to have a little breadboard with a
socket so you can check the IC to see if the problem follows the IC, or stays with the
circuit. When you do that, remember that the load regulation will probably be
mediocre unless you have a good Kelvin socket; and the part will probably get hot
quickly if it has no heat sink. Now let’s move on to the Negative Regulator, which is
substantially the same as the Positive Regulator, so we will put into the table only the
items that are different.

-VOUl

Figure 14.4. Negative Adjustable Regulator.